SEPTEMBER 24 -26 / ROME. ITALY
Department of Physics
Sapienza Università di Roma
BOOK OF
ABSTRACTS
Conference Program
2014
Session Chair
Session
Session
Session
Session
Session
Session
1
2
3
4
5
6
P. Calvani - R. Raimondi
S. Lupi - M. Grilli
M. Putti - P. Carretta
D. Daghero
L. Benfatto - C. Aruta
J. Lorenzana
Program tear sheet is
available at the end
of the present booklet.
2
}
Wednesday 24/9
8h30
Registration
and Posting
Thursday 25/9
Friday 26/9
Session 3
Superconductors I
Session 5
Strongly correlated systems
Session 1
Two-dimensional Electron Gas I
8h50
Fanfarillo
8h50
Mizuguchi
9h30
Welcome
9h10
Caglieris
9h30
Oles
9h40
Triscone
9h30
Profeta
9h50
Ciuchi
10h20
Radovic
9h50
Sanna
10h10 Ortenzi
10h40
Dreda
10h10 Bossoni
10h30 Brzezicki
11h00
Coffè break
10h30 Coffè break
10h50 Coffè break
11h30
Ortolani
11h00 Perucchi
11h20 Bertacco
11h50
Di Gennaro
11h20 Mirri
11h40 Joseph
12h10
Rubano
11h40 Cantoni
12h00 Manca
12h30
Bucheli
12h00 Boeri
12h20 Mitrano
12h50
End of Session 1
12h40 End of Session 3
12h40 End of Session 5
Lunch and Poster Session
Lunch and Poster Session II
Lunch
Session 2
Session 4
Two-dimensional Electron Gas II Superconductors II
Session 6 Magnetic
oxides and Multiferroics
14h30
Golden
14h00 Nicoletti
13h40 Bowen
15h10
Di Sante
14h40 Di Castro
14h00 Méchin
15h30
Perfetto
15h00 Orgiani
14h20 Giovannetti
15h50
Autore
15h20 De Filippis
14h40 De Luca
16h10
Coffè break
15h40 Giannetti
15h00 Barone
16h40
Citro
16h00 Cilento
15h20 Malavasi
17h00
Piatti
16h20 Peng
15h40 End of Session 6
17h20
Leveratto
16h40 End of Session 4
17h40
Cappelluti
18h00
Ludbrook
17h10 Guided Visit
18h20
End of Session 2
20h00 Conference dinner
16h00 Conference closure
Party and Poster Session
19h45
End of Poster Session
3
SITE
QR
CODE
WEB
Scientific Secretariat
Fernanda Lupinacci
Department of Physics
Sapienza Università di Roma
Administrative Secretariat/Organization
Patrizia Pasolini
Mariachiara Fant
Symposia s.r.l.
Webmaster/Graphic design
Fulvio Medici
Department of Physics
Sapienza Università di Roma
Chair
Paolo Calvani
Josè Lorenzana
Stefano Lupi
Scientific Committee
Carmela Aruta
Riccardo Bertacco
Pietro Carretta
Dario Daghero
José Lorenzana
Stefano Lupi
Gino Maritato
Daniele Marrè
Sandro Massidda
Canio Noce
Giancarlo Panaccione
Giampiero Pepe
Silvia Picozzi
Marina Putti
Presentations
All ORAL presentations will be held at the Department of Physics in
AULA AMALDI (Marconi Building, first floor).
The POSTER session will take place at the first floor of the Marconi Building
near Aula Amaldi (see the map at the end of the present booklet).
All Oral and Poster contributions are reported in the present booklet.
Location
Marconi Building
Aula Amaldi
Minerva’s statue
6
Invited speakers
THURSDAY 25 - 12h00
Lilia Boeri
Institute for Theoretical and Computational Physics
TU Graz, Austria
WEDNESDAY 24 - 14h30
Mark Golden
Van der Waals-Zeeman Institute for Experimental
Physics Institute of Physics, University of Amsterdam,
The Netherlands
FRIDAY 26 - 8h50
Yoshikazu Mizuguchi
Electrical and Electronic Engineering Tokyo
Metropolitan University, Japan
THURSDAY 25 - 14h00
Daniele Nicoletti
Max Planck Institute for the Structure and Dynamics of
Matter and University of Hamburg, Germany
WEDNESDAY 24 - 9h40
Jean-Marc Triscone
Department of Condensed Matter Physics,
University of Geneva, Switzerland
7
Wednesday 24/9
Aula Amaldi
Department of Physics
Session 1
Two-dimensional
Electron Gas I
Session 2
Two-dimensional
Electron Gas II
wed 24
9.40
10.20
2-dimensional Superconductivity at the LaAlO3/SrTiO3 Interface
D. Li1, A. Fête1, W, Liu1, D. Stornaiuolo1,3, S. Gariglio1, M. Gabay2, and
J.-M. Triscone1
1. DPMC, University of Geneva, 24 quai E.-Ansermet, CH-1211 Geneva 4, Switzerland
2. Laboratoire de Physique des Solides, Université de Paris-Sud, 91405 Orsay, Cedex,
France
3. Now at Università degli Studi di Napoli Federico II and CNR-SPIN Napoli, Italy
The interface between LaAlO3 and SrTiO3, two good band insulators, which was
found in 2004 to be conducting [1], and, in some doping range, superconducting
with a maximum critical temperature of about 200 mK [2] is attracting of lot of
attention. The electron gas has a thickness of a few nanometers at low temperatures and a low electronic density, typically 5 1013 electrons/cm2. Being naturally
sandwiched between two insulators, it is ideal for performing electric field effect
experiments that allow the carrier density to be tuned and the phase diagram of the
system to be determined [3].
I will discuss in this presentation superconductivity, the phase diagram of the system and the link with doped bulk SrTiO3 [2,3]; spin orbit [4,5] and an approach that
allows superconducting coupling between different gases to be studied [6]. First
experiments demonstrating coupling of two superconducting interfacial gases will
be shown.
[1] A. Ohtomo, H. Y. Hwang, Nature 427, 423 (2004).
[2] N. Reyren, S. Thiel, A. D. Caviglia, L. Fitting Kourkoutis, G. Hammerl, C. Richter, C.
W. Schneider, T. Kopp, A.-S. Ruetschi, D. Jaccard, M. Gabay, D. A. Muller, J.-M. Triscone and J. Mannhart, Science 317, 1196 (2007).
[3] A. Caviglia, S. Gariglio, N. Reyren, D. Jaccard, T. Schneider, M. Gabay, S. Thiel, G.
Hammerl, J. Mannhart, and J.-M. Triscone, Nature 456, 624 (2008).
[4] A.D. Caviglia, M. Gabay, S. Gariglio, N. Reyren, C. Cancellieri, and J.-M. Triscone,
Physical Review 104, 126803 (2010).
[5] A. Fête, S. Gariglio, A. D. Caviglia, M. Gabay, J.-M. Triscone, Physical Review B
(RC) 86, 201105 (2012).
[6] D.Li, S. Gariglio, C. Cancellieri, A. Fête, J.-M. Triscone, APL Materials 2, 012102
(2014).
Session 1 - Two-dimensional Electron Gas I
99
wed 24
10.20
10.40
Spectroscopy Views of Low Dimensional Electron Gas (LDEGs) at
STO surface and LAO/STO interface: Final depiction
M. Radovic1,2, N. C. Plumb1, T. Schmitt1, M. Shi1, G. Landolt1, J. H. Dil3,
M. Salluzzo3, L. Patthey2, J. Mesot1,4,5
1. Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
2. SwissFEL, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
3. Institute of Condensed Matter Physics, Ecole Polytechnique Fédérale de
Lausanne,CH-1015 Lausanne, Switzerland
4. CNR-SPIN, Complesso Monte S. Angelo,Via Cinthia, I-80126 Napoli, Italy
5. Laboratory for Solid State Physics, ETH Zürich, CH-8093 Zürich, Switzerland
The interfaces between transition metal oxides show astonishing properties: tunable insulator-superconductor-metal transitions, large magnetoresistance, coexisting nano-domains of ferromagnetism and superconductivity, etc. More recently,
2DEG was discovered also at the (001) surface of SrTiO3 (STO) single crystals.
Therefore, a complete understanding of the LDEGs becomes a basic step for a
clear interpretation of the electronic properties of interfaces obtained by combining
STO and other oxides. Here we show, using angle-resolved photoemission spectroscopy (ARPES) in UV and Soft X-ray ranges, spin- angle-resolved photoemission spectroscopy (SARPES) and Resonant Inelastic X ray scattering (RIXS), a
comprehensive depiction of the LDEGs electronic structure at STO surface and
LAO/STO interface.
1. A. F. Santander-Syro, F. Fortuna, C. Bareille, T. C. Rodel, G. Landolt, N. C. Plumb,
2.
3.
4.
5.
10
J. H. Dil, and M. Radovic, Giant spin splitting of the two-dimensional electron gas at
the surface of SrTiO3, under review (2014).
N. C. Plumb, M. Kobayashi, M. Salluzzo, E. Razzoli, C. Matt, V. N. Strocov, K.-J.
Zhou, C. Monney, T. Schmitt, M. Shi, J. Mesot, L. Patthey, M. Radovic, From the
SrTiO3 Surface to the LAlO3/SrTiO3 Interface: How thickness is critical, (2013).
N. C. Plumb, M. Salluzzo, E. Razzoli, M. Månsson, M. Falub, J. Krempasky, C.Matt,
J. Chang, J. Minar, J. Braun, H. Ebert, B. Delley, K.-J. Zhou, C. Monney, T.Schmitt,
M. Shi, J. Mesot, C. Quitmann, L. Patthey, M. Radovic, Mixed dimensionality of confined conducting electrons in the surface region of SrTiO3, under review (2014).
R. Di Capua, M. Radovic, G. M. De Luca, I. Maggio-Aprile, F. Miletto Granozio, N.
C. Plumb, Z. Ristic, U. Scotti di Uccio, R. Vaglio, and M. Salluzzo, Observation of a
Two-Dimensional Electron Gas at the Surface of Annealed SrTiO3 Single Crystals by
Scanning Tunneling Spectroscopy, PHYSICAL REVIEW B 86, 155425 (2012).
Kejin Zhou, Milan Radovic, Justine Schlappa, Vladimir Strocov, Ruggero Frison,
Joel Mesot, Luc Patthey, and Thorsten Schmitt, Localized vs. delocalized character
of charge carriers in LaAlO3/ SrTiO3 superlattices, PHYSICAL REVIEW B Rapid
Communication 83, 201402(R)(2011).
Session 1 - Two-dimensional Electron Gas I
wed 24
10.40
11.00
Interface states and band offset profiling of LaAlO3/SrTiO3
conductive heterojunctions probed by element-specific resonant
spectroscopies
G. Drera1, G. Salvinelli1, A. Giampietri1, M. Huijben2, A. Brinkman2, and
L. Sangaletti1
1. I-LAMP and Dipartimento di Matematica e Fisica, Università Cattolica, Via dei
Musei 41, 25121 Brescia, Italy
2. Faculty of Science and Technology and MESA+ Institute for Nanotechnology,
University of Twente, 7500 AE Enschede, The Netherlands
Evidences of a conducting interface between the band insulators SrTiO3 (STO) and
LaAlO3 (LAO) have opened up the possibility of using complex oxide materials
in two-dimensional electron gas applications [1]. In the LAO-STO junction, the
mechanism causing the interface conductivity was first believed to be a charge
transfer from the LAO surface to the uppermost STO layer at the interface, as a
consequence of the diverging electrical potential due to the polar nature of LAO.
Such kind of electronic reconstruction is expected to deeply modify the conduction
band minima (CBM) and valence band maxima (VBM) heterojunction profiles.
An experimental probe of the electronic strucure is thus fundamental in order to
discriminate possible alternative doping mechanism, such as cation interdiffusion,
oxigen vacancies, lattice distorsions, La-Al stoichometry and surface defects.
The spectral shape of Ti3+ conductive states has been successfully probed by resonant photoemission at Ti L2,3 edge [3], which can also be extended to directly
measure the interface band structure (Resonant ARPES [2]). However, in order to
track the VBM and CBM dispersion across the junction a depth sensitive probe
must be used, such as photoelectron spectroscopy carried out with different photon
energies.
In this work, a detailed study of in-gap states and core levels in conductive and insulating LAO-STO is reported [4], carried out with a large set of electron spectroscopy and absorption techniques (XPS, ResPES and X-ray absorption spectroscopy, resonant ARPES, X-ray photoelectron diffraction). A detailed analysis of both
core-level and valence band photoemission data measured on the heterojunctions
and on pristine oxides allowed us to measure the VBM and CBM profiles, and thus
to estimate the spatial extension of the Ti 3d1 states in the buried interface. A deep
notch at the CBM profile at the conductive interface is observed, proving the quasi2D distribution of the electron gas.
1 - M. Huijben et al., Nature Mater. 5, 556 (2006)
2 - G. Berner et al, Phys. Rev. Lett. 110, 247601 (2013)
3 - G. Drera et al., App. Phys. Lett. 98, 052907 (2011)
4 - G. Drera et al., Phys. Rev. B 87, 075435 (2013)
11
Session 1 - Two-dimensional Electron Gas I 11
wed 24
11.30
11.50
Hardening of the ferroelectric phonon in LaAlO3/SrTiO3
heterostructures
A. Nucara1, L.Baldassarre2, M.Ortolani1, F.Miletto Granozio3 and P. Calvani1
1. CNR-SPIN and Dipartimento di Fisica, Università di Roma “La Sapienza”, P.le A.
Moro 2, 00185 Roma, Italy
2. 2Center for Life NanoScience@Sapienza, Istituto Italiano di Tecnologia, Viale
Regina Elena 291, 00185 Roma, Italy
3. 3CNR-SPIN and Dipartimento di Scienze Fisiche, Università degli Studi Federico II
Complesso Universitario di Monte Sant’Angelo via Cinthia 80126, Napoli, Italy
The interface between SrTiO3 (STO) and LaAlO3 (LAO) is known to host highmobile electrons, despite the large value of the gap for both oxides: this occurrence
promotes the LAO/STO interface as an evident example of 2-dimensional electron
gas (2DEG) (1). The lattice and the charge dynamics at the interface are still
undisclosed: the presence of plasmonic modes of the 2DEG as well as the formation of mobile lattice distortions in the STO substrate are both foreseen but not
observed experimentally. We measured the infrared (IR) reflectivity of LAO/STO
heterostructures grown by laser ablation, with and without the 2DEG, vs. temperature. We found that the IR-active transverse phonon TO1 peaked at 90 cm-1 at 300
K and responsible for the incipient ferroelectricity of STO, is systematically blueshifted by the presence of the 2DEG at all temperatures. We explain this behavior
by the radiative excitation of charge modes at the STO surface, analogous to the
plasmons observed at the surface of graphene and in topological insulators (2, 3).
The polarization field produced in STO by
these excited plasmons behaves in a similar way to that produced by the application
of an external dc field (4). Here, however,
a spontaneous hindering of incipient ferroelectricity is first observed.
The frequency of the TO1 phonon vs. temperature. Blue dots: sample with 2DEG , red
squares: without 2DEG.
(1) C. Cantoni et al. Adv. Mater. 24, 3952, (2012)
(2) Ju, L. et al. Nature Nanotech. 6, 630, (2011)
(3) P. Di Pietro et al. Nature Nanotech. 8, 556, (2013)
(4) I. A. Akimov et al. Phys. Rev. Letts, 84, 4625, (2000)
12
Session 1 - Two-dimensional Electron Gas I
wed 24
11.50
12.10
Addressing the Origin of Conductivity in Two Dimensional
Electron Gases at Oxide Interfaces
E. Di Gennaro, A. Khare, M. Can, A. Safeen, A. Sambri, U. Scotti di Uccio,
F. Miletto Granozio
CNR-SPIN and Physics Department, University of Naples “Federico II”
Oxide interfaces exhibit sometimes functionalities that are totally new with respect
to those of their constituent materials [1]. A prototypical example of this emergent
behaviour is provided by the 2-dimensional electron gas (2DEG) first found at the
LaAlO3/SrTiO3 interface. In this work, we investigate the formation of 2DEG in
different epitaxial interfaces, LaAlO3/SrTiO3, LaGaO3/SrTiO3 [2], NdGaO3/SrTiO3
[3] and Al2O3/SrTiO3 [4], comparing their properties with those of their amorphous
counterparts, which also exhibit a highly mobile electron gas [4,5].
A number of experiments have been designed and performed with the specific
aim of highlighting similarities and differences between two kind of interfaces.
Experimental methods include transport under electric/magnetic field, wavelengthdependent photoconductivity and different photon-based spectroscopies.
The amorphous counterparts show a number of non-trivial similarities in their electronic properties on comparing with crystalline samples. The experimental evidences emerging from this work support nevertheless a different origin of carriers
in both the systems and are in agreement with an electrostatic mechanism being
responsible for the conductivity of crystalline interfaces.
[1] F. Miletto Granozio et al., MRS Bullettin 38, 1 (2013)
[2] E. Di Gennaro, F. Miletto Granozio et al, Adv. Opt. Mat. 1, 834 (2013).
[3] P. Perna, F. Miletto Granozio et al., Appl. Phys. Lett.97, 259901 (2010)
[4] Y. Chen et al, Adv. Mater. 26, 1462 (2014)
[5] Y. Chen et al., Nano Lett. 11, 3774 (2011)
[6] C. Cantoni, F. Miletto Granozio, et al., Adv. Mater. 24, 3952 (2012)
Corresponding author: emiliano@na.infn.it
13
Session 1 - Two-dimensional Electron Gas I 13
wed 24
12.10
12.30
Oxide-interface charge carrier dynamics investigated by
Second Harmonic Generation
A. Rubano,1 T. Günter,2 M. Fiebig,2 F. Miletto Granozio,1 U. Scotti di Uccio,1
L. Marrucci,1 D. Paparo.1
1. Dipartimento di Fisica, Federico II and CNR-SPIN, Complesso Universitario di
Monte S.Angelo, Via Cintia, Napoli.
2. ETH Zürich, Department of Materials, Vladimir-Prelog-Weg 4, CH-8093 Zürich
The technology for growing ultrathin oxide films or hetero-structures is nowadays
approaching the same level of atomic control as in the case of semiconductors. Yet,
in contrast to semiconductors, strong electron correlations lead to novel and sometimes exotic states at the interface. One of the most relevant examples of this, is the
formation of a two-dimensional electron gas (2DEG) at the interface between two
textbook band insulators, LaAlO3 (LAO) and SrTiO3 (STO). In spite of an intense
research effort, the physical origin of the 2DEG is still controversially discussed.
One particular aspect was quite under-investigated so far: the interfacial charge
carrier dynamics. The challenge is to merge ultrafast time-resolution and interfaceonly sensitivity together. Optical Second Harmonic Generation (SHG) possesses
both qualities and it is thus the ideal tool for achieving this goal. The technique is
based on the induction of light waves of frequency 2 ω by incident waves of frequency ω. The process couples to the symmetry breaking at the interface, being
the generation from the centrosymmetric bulk material suppressed: it is therefore
completely background-free. SHG is a multifunctional tool. It displays: (i) spectral resolution (Spectroscopy), (ii) in-plane lateral resolution (Imaging), (iii) phase
resolution (Phase-reference) and (iv) time resolution (Pump&Probe). The time
resolution can be as short as an optical pulse duration (10-100 fs). One example of
Pump&Probe measurement is given in the figure below. It clearly shows that the
SHG signal is strongly affected by excitation of the sample with an UV-pulse (350
nm) at time t=0, and that the charge
carrier dynamics depends qualitatively, and not only quantitatively,
on the number n of LAO over-layers:
for n=0,1 the SHG increases above
its equilibrium value, for all other n
the opposite happens. These results
are put together with measurements
of transient absorption dynamics,*
in order to understand the relationship between free charges, localized
charges and the electric fields and polarizations these populations are creating in the material system.
* Yamada, PRL 111, 047403, 2013
Corresponding author:
A. Rubano; e-mail address: rubano@fisica.unina.it
14
Session 1 - Two-dimensional Electron Gas I
Pseudo-gap in tunneling spectra as a signature of
inhomogeneous superconductivity in oxide interfaces
wed 24
12.30
12.50
D. Bucheli, S. Caprara, and M. Grilli
Department of Physics, Sapienza University of Rome, Piazzale Aldo Moro 2, Rome Italy
The observation of a two-dimensional metallic state at the interface of two insulating oxides, and the subsequent demonstration of its gate-tunable metal-tosuperconductor transition, has attracted much attention in the last decade. Numerous experiments indicate that the electron gas is inhomogeneous: magnetometry,
tunneling, and piezo-force spectroscopy experiments report submicrometric inhomogeneities. Further, transport measurements reveal a large width of the superconducting transition, suggesting charge inhomogeneity, and, at lower carrier density,
a saturation to a plateau with finite resistance; a clear signature of the percolating
character of the metal-to-superconductor transition. Several theoretical works report that the temperature dependence of the resistivity and superfluid stiffness is
well explained in a framework of superconducting islands (with randomly distributed local critical temperatures) embedded in a metallic background [1-2]. Into this
picture about the inhomogeneous nature of the interfacial electron gas arguably enter the recent superconductor-insulator-metal tunnel spectroscopy measurements at
the LaAlO3/SrTiO3 interface, that report a pseudo-gap phase with finite resistance
but superconductor-like density of states.
We recently showed [3] that this pseudo-gap phase can be explained by the inhomogeneous character of the electron gas. In line with our previous work, we consider the interface to consist of a metallic background embedding superconducting
islands. The superconductivity within each island is BCS-like, and the local critical
temperatures are randomly distributed, some of them necessarily exceeding the critical temperature for global percolation to the zero resistance state. Consequently,
tunneling spectra display a suppression of the density of states and coherence peaks
already well above the percolative transition. We stipulate that the main features
of the critical temperature distribution are structurally determined and therefore do
not depend on temperature. Inside most regions with a finite critical temperature,
standard BCS coherence is established, while a smaller fraction only displays incoherent pairing, likely associated with the size of these islands being smaller than
the superconducting coherence length. Attempts to fit the temperature dependence
of the spectra lead to the conclusion that a substantial part of the paired regions at
low temperature are likely formed by proximity effect in the metallic matrix and
were not foreseen at higher temperature.
[1] D. Bucheli, S. Caprara, C. Castellani, and M. Grilli, New J. Phys. 15, 023014 (2013).
[2] S. Caprara, et al. , Phys. Rev. B 88, 020504(R) (2013).
[3] D.Bucheli, S.Caprara, and M. Grilli, arXiv:1405.4666.
Corresponding author: Daniel Bucheli. Current address: Piazzale Aldo Moro 2, I-00185 Rome.
email: danielbucheli@hotmail.com
15
Session 1 - Two-dimensional Electron Gas I 15
wed 24
14.30
15.10
Surprises at the surfaces of topological insulators*
Mark S. Golden
Van der Waals-Zeeman Institute, University of Amsterdam
In an ideal three-dimensional topological insulator (TI), the bulk of the crystal
is fully insulating, yet the system possesses 2D edge states that are protected
by time reversal symmetry. In plain language we are simply dealing with surface states, something we know from the physics of regular metals and semiconductors. However, these topologically protected surface states are special in a
number of crucial aspects, making them the focus of an enormous research effort,
world-wide.
After a simple introduction into topological insulators, I will describe the efforts
required to generate an insulating bulk in single crystals of the Bi-based family
of 3D TI’s. The main part of the talk will deal with what we can learn from these
systems using the surface sensitive techniques scanning tunnelling spectroscopy
(STM) and angle-resolved photoemission (ARPES). I’ll then go on to describe
the interplay between electric field related band bending near the surfaces of
these compounds and photo-doping effects: put differently, I will describe how
to write tiny letters - small enough to fit on a human hair - on topologically protected paper.
In closing, I’ll relate some data from the same sort of surface-related techniques,
but now applied to the proposed topological Kondo insulators SmB6 and YbB6.
* This work was done in collaboration with: Emmanouil Frantzeskakis, Nick de
Jong, Berend Zwartsenberg, Anne de Visser, Yingkai Huang, Dong Wu, Erik van
Heumen, Yu Pan, Tran Van Bay, Pieter Pronk, Freek Massee, Shyama Varier Ramankutty, Erik Slooten & Huib Luigjes, and is funded by the FOM foundation, NWO
and the EU.
Mark S. Golden Van der Waals-Zeeman Institute | IoP | University of Amsterdam |
Sciencepark 904, 1098 XH | Amsterdam
M.S.Golden@uva.nl | Tel.: +31 20 525 6363 (lab and mobile number)
16
Session 2 - Two-dimensional Electron Gas II
Engineering Topological Properties in Rocksalt
Chalcogenides and the Role of Disorder in the Spectral
Features of Topological Crystalline Insulator Alloys
wed 24
15.10
15.30
D. Di Sante1,2, P. Barone1, E. Plekhanov1, S. Ciuchi2,3 and S. Picozzi1
1. CNR-SPIN, Via Vetoio, L’Aquila, Italy
2. University of L’Aquila, Department of Physical and Chemical Sciences, Via Vetoio,
L’Aquila, Italy
3. CNR-ISC, Via dei Taurini, Rome, Italy
By means of a comprehensive theoretical investigation, we show that external pressure [1] and strain [2] can induce topological phase transitions in IV–VI semiconducting chalcogenides with a rocksalt structure. We show that the band inversions
at high-symmetry points in the Brillouin zone that are related by mirror symmetry
are brought about by an “asymmetric” hybridization between cation and anion sp
orbitals [1,2]. By working out the microscopic conditions to be fulfilled in order
to maximize this hybridization, we identify materials in the rocksalt chalcogenide
class that are prone to undergo a topological phase transition induced by pressure
and/or alloying. Furthermore, the role of disorder in the relativistic spectral properties of disordered topological crystalline insulators is investigated by means of
hybrid functional DFT+CPA calculations [3]. Bulk Dirac cone is protected against
disorder effects induced by alloying as a consequence of a vanishing self-energy
at the Fermi level.
[1] P. Barone, T. Rauch, D. Di Sante, J. Henk, I. Mertig and S. Picozzi, Phys. Rev. B 88,
045207 (2013)
[2] P. Barone, D. Di Sante and S. Picozzi, Phys. Status Solidi RRL, No 12, 1102 (2013)
[3] D. Di Sante, P. Barone, E. Plekhanov, S. Ciuchi and S. Picozzi, (to be published)
Corresponding author: Domenico Di Sante: email: domenico.disante@aquila.infn.it
Session 2 - Two-dimensional Electron Gas II
17
17
wed 24
15.30
15.50
Dynamics of the Topological Phase Transition in Driven Systems
Hosting Majorana Fermions
Enrico Perfetto
Dipartimento di Fisica Universita’ di Roma Tor Vergata, Via della Ricerca Scientica 1,
00133 Roma
Controlling the dynamics of Majorana fermions subject to time-varying driving fields is of fundamental importance for the practical realization of topological quantum computing. Here we address the question of how it is possible to
dynamically generate and maintain the topological phase in cold-atom nanowires
after the temporal variation of the Hamiltonian parameters. We show that for a sudden quench the system can never relax toward a state exhibiting fully developed
Majorana fermions, independently of the initial and final Hamiltonians. Only for
sufficiently slow protocols the system behaves adiabatically, and the topological
phase can be reached [1].
[1] E. Perfetto, Phys. Rev. Lett. 110, 087001 (2013)
18
Session 2 - Two-dimensional Electron Gas II
Dirac plasmonics of Topological Insulators
wed 24
15.50
16.10
M. Autore1, F. D’Apuzzo2, S. Lupi3
1. Dipartimento di Fisica, Università di Roma Sapienza, P.le Aldo Moro 2, I-00185
Roma, Italy
2. Istituto Italiano di Tecnologia and Dipartimento di Fisica, Università di Roma
Sapienza, P.le Aldo Moro 2, I-00185 Roma, Italy
3. CNR-IOM and Dipartimento di Fisica, Università di Roma Sapienza, P.le Aldo Moro
2, I-00185 Roma, Italy
Recently, a new developement in the field of plasmonics has been achieved by
means of engineering plasmonic structures in Topological Insulators (TIs). Indeed,
their peculiar properties such as 2D intrinsic transport carried out by Dirac fermions, very high surface density (n ≥ 1013 cm-2) compared to the typical values for
metallic surfaces, backscattering protection and robustness of the topological phase
at room T, make them perfect candidates to develope plasmon based technology.
We performed a study of plasmonic excitations on topological insulator Bi2Se3based devices in the THz range, using Fourier Transform Infrared (FT-IR) spectroscopy. We investigated resonances dispersion in thin film samples patterned with
several shapes by means of Electron Beam Lithography (EBL) in collaboration
with the IFN in Rome. A first study was performed on Bi2Se3 micro-ribbon arrays,
enabling to trace the energy-momentum dispersion for plasmonic excitations, in
good agreement with the relation expected for 2D gases ωp k1/2. Most importantly,
we found a very good quantitative agreement with that predicted for Dirac plasmons. Then we investigated devices patterned with microring arrays, which are interesting because they exhibit two distinct resonances (bonding and antibonding),
arising from the hybridization between disk and anti-dot resonances. The measured
extinction coefficient has been compared to ab initio analytical model for plasmonic resonances, through a collaboration with the Institute of Photonic Sciences
in Barcelona. The comparison gives an impressive agreement, given the fact that
the model has no free parameter, thus opening the way for further prediction and
tailoring of plasmonic devices in the THz range. Finally, we studied the excitations
on Bi2Se3 micro-disk arrays and we intend to perform transmission measurements
under the effect of a strong magnetic field (up to 30 T), expecting to observe and
charachterize the resonance splitting tyipical for magnetoplasmons.
Moreover, we studied the quantum phase transition (QPT) from a topological insulator to a conventional band insulator exploiting the plasmonic properties of
micro-ribbon arrays of (Bi1-xInx)2Se3. Under the effect of In-Bi substitution, this
material undergoes a QPT around x=0.05 as recently observed by means of ARPES
and time domain spectroscopy. The study of plasmon dispersion and width as a
function of doping represents a first study of the behavior of collective excitations
through the QPT.
Correspondence to M. Autore, marta.autore@roma1.infn.it, Dipartimento di Fisica Università
Sapienza, Piazz.le Aldo Moro 5, 00185 Roma
Session 2 - Two-dimensional Electron Gas II
19
19
wed 24
16.40
17.00
Non-equilibrium transport properties of a topological corner
junction
R. Citro, F. Romano
Dipartimento di Fisica “E.R. Caianiello” and Unit Spin-CNR
Università degli Studi di Salerno, Via Giovanni Paolo II, 132-84084 Fisciano (Sa)
We study the transport properties of a four-terminal corner junction made by etching a two-dimensional topological insulator to form a quantum point contact (QPC).
The QPC geometry enables inter-boundary tunneling processes with spin-flipping
and spin-preserving allowing the coupling among states with different helicity[1,2],
while the tight confinement in the QPC region activates charging effects leading
to the Coulomb blockade physics. Peculiar signatures of these effects are theoretically investigated using a scattering field theory modified to take into account the
electron-electron interaction within a self-consistent mean-field approach[3]. The
current-voltage characteristics and the current fluctuations (noise) are derived beyond the linear response regime. Universal aspects of the thermal noise of a corner
junction made of helical matter are also discussed.
[1]F. Romeo, R. Citro, D. Ferraro, M. Sassetti, “Electrical switching and interferometry of massive Dirac particles in topological insulator constrictions”, Phys. Rev. B 86,
165418 (2012)
[2]D. Ferraro, G. Dolcetto, R. Citro, F. Romeo, M. Sassetti, Phys. Rev. B 87, 245419
(2013)
[3] F. Romeo and R. Citro, in preparation
20
Session 2 - Two-dimensional Electron Gas II
wed 24
17.00
17.20
Field-effect investigation of the low-temperature transport properties
of multilayer graphene by a novel polymer electrolyte solution
K. Sharda1, A. Sola1, E. Piatti1, M. Tortello1, D. Daghero1, M. Bruna2, A. C. Ferrari2,
R. S. Gonnelli1
1. Politecnico di Torino, Corso Duca degli Abruzzi, 24, Torino-10129, Italy
2. Cambridge Graphene Centre, University of Cambridge, JJ Thomson Avenue, Cambridge, UK
The use of polymer electrolyte solutions (PES) for electrochemical gating [1] the
surface of various materials, e.g. insulators, low-dimensionality carbon-based materials and even metals, is becoming popular. This technique produces very high
electric fields (~100 MV/cm) at the surface, which in turn give rise to record changes of the surface charge density (up to more than 1014 cm-2) that can even drive the
superconducting phase transition of the material [2].
Through this technique we recently succeeded in injecting surface charge densities
up to more than 4×1015 cm-2 in metallic thin films of Au, Ag and Cu [3]. Relative
variations of resistance up to 8%, 1.9% and 1.6% at room temperature in the three
metals [4], respectively, and up to a remarkable 10% at 4 K in Au films [3] were
observed.
Here we present the results of PES gating experiments applied to multi-layer
graphene devices (mainly from 3 to 5 layers) fabricated at the Cambridge Graphene Center. Surface charge densities exceeding 3×1014 cm-2 (at a gate voltage VG =
± 2.5 V) have been measured by double-step chronocoulometry, inferred from the
behavior of the Dirac curves as a function of VG and compared to Hall effect measurements. In particular, the dependence of the sheet resistance on VG for 3-layer
graphene devices gated by different electrolyte solutions clearly indicates that a
wider range of surface charge carrier density can be controlled by our PES if compared with previous results [5]. In these devices, transport measurements were also
performed down to 2.7 K and even at the highest injected charge density no phase
transition was observed. Instead, a continuous enhancement of the metallic behavior and a low-temperature logarithmic upturn of the resistance, strongly dependent
on the injected charge, has been observed. Since Kondo-effect and electron-electron interaction contributions can be ruled out on the basis of Raman measurements
and magnetic-field dependence of the resistance, this low-temperature behavior is
explained in terms of weak localization due to localized defects at sample surface.
The subsequent application of the standard weak localization theory [6] allowed
us to determine the characteristic elastic and inelastic/phase coherence scattering
times as a function of induced surface doping in 4- and 5-layer graphene devices.
[1] J. T. Ye et al., Nature Mat., 9 (2009) 125-128.
[2] J. T. Ye et al., Science, 338 (2012) 1193-6.
[3] D. Daghero et al. Phys. Rev. Lett., 108 (2012) 066807.
[4] M. Tortello et al., App. Surf. Sci., 269 (2013) 17-22.
[5] J. T. Ye et al., PNAS, 108, 32 (2011) 13002.
[6] G. Bergmann et al., Phys. Rev. B, 28 (1983) 2914.
Session 2 - Two-dimensional Electron Gas II
21
21
wed 24
17.20
17.40
Quantum transport in ZnO based heterostructures
A. Leveratto1, L. Pellegrino1, I. Pallecchi1, E. Bellingeri & D. Marré1, A. Jost2,
U. Zeitler2
1. CNR-SPIN corso Perrone 24, 16152 Genova, Italy and Dipartimento di Fisica, Via
Dodecaneso 33, 16146 Genova, Italy
2. High Field Magnet Laboratory, Institute for Molecules and Materials, Radboud University Nijmegen, 6525 ED Nijmegen, The Netherlands
In recent years, 2-dimensional electron gases (2DEG) in oxide heterostructures
have attracted a lot of attention for electrical device applications such as high electron mobility transistors (HEMTs) and quantum spin transport devices. Among the
heterostructures investigated only ZnO based multilayers unambiguously showed
the presence of a 2DEG at the interface[1,2]. Such heterostructures resemble to
GaAs or GaN based systems; at the heterointerfaces (Mg,Zn)O/ZnO a strong builtin potential arises from macroscopic polarization mismatch and bands align because of different bandgaps in the two layers (0.6 eV), causing the formation of a
potential well at the interface where a 2DEGs is confined. With respect to GaAs or
GaN heterostructures, being ZnO a multifunctional materials with interesting optomagneto-electronic properties, the realization of ZnO based quantum devices lead
to significant technological developments: ZnMgO/ZnO have been studied more
and more intensively because of its potential advantages in the applications as
HEMTs and ultraviolet optoelectronic devices and more recently for its property of
presenting a strong electronic correlation. In this contribution we present realization and characterization in high magnetic field of ZnO/MgxZn1-xO (x=0.5; 0.15)
heterostructures: samples have been deposited by pulsed laser deposition on ZnO
single crystal on Zn polar side; the optimization of substrate surface and chemical
termination and of deposition parameters are crucial to obtain high quality films.
High field characterization of electric and thermal transport properties has been
performed at HFML (High Field Magnet Laboratory - Nijmegen): quantum phenomena (IQHE and SdHO) were observed and surprisingly visible up to 20K. Here
we will focus on thermoelectric power experiments that showed for the first time
in an oxide based 2DEG quantum oscillations in the Seebeck and Nernst thermoelectric coefficients. A deep analysis of the thermal and electric transport has been
performed and we will show how Varlamov’s formula[3] for 2D systems (recently
successfully applied in the interpretation of analogous mesurements in graphene)
well explain our observed quantum Nernst-Ettingshausen oscillations.
[1]A. Tsukazaki, A. Ohtomo, T. Kita, Y. Ohno, H. Ohno, and M. Kawasaki, Science
315,1388 (2007).
[2]A. Tsukazaki, A. Ohtomo, M. Kawasaki, S. Akasaka, H. Yuji, K. Tamura, K.Nakahara,
T. Tanabe, A. Kamisawa, T. Gokmen, J. Shavani, M. Shayegan PRB78, 233308 (2008).
[3]I. A. Luk’yanchuk, A. A. Varlamov, and A. V. Kavokin PRL107, 016601 (2011)
Corresponding author: Alessandro Leveratto, CNR-SPIN & Physics Department Genova,
alessandro.leveratto@spin.cnr.it
22
Session 2 - Two-dimensional Electron Gas II
wed 24
17.40
18.00
Local strain and bandgap engineering in layered MoS2
A. Castellano-Gomez1, E. Cappelluti2,3,*, R. Roldán2, , M. Buscema1, F. Guinea2,
H.S.J. van der Zant1, and G.A. Steele1
1. Kavli Institute of Nanoscience, Delft University of Technology, Delft, The Netherlands
2. Instituto de Ciencia de Materiales de Madrid, CSIC, Madrid, Spain
3. Institute for Complex Systems, CNR, Rome, Italy
Graphene has represented in the past 10 years the most promising new material
for a new generation of electronic devices and for the investigation of fundamental physics. The effective employment of graphene-based materials in low-energy
electronics is however hindered by the difficulty of opening a bandgap without
affecting the mobility and the electronic properties. New actors have however recently entered on the stage, in particular metal-transition dichalcogenides, as MoS2
and similar compounds. Also these materials, like graphene, can be exfoliated to
reach atomically thickness. In advantage, they present an intrinsic bandgap whose
nature and size results to be highly sensitive to the number of layers and to external
conditions (strain, pressure, electric fields, etc.).
In this talk we demonstrate the possibility to control at a local scale the electronic
and optical properties by means of local strain. Lattice corrugations are induced in
few-layer MoS2 samples by means of controlled delamination on an elastic substrate [1]. Local strain is monitored through the phonon resonance energies in Raman spectroscopy, and the local direct bandgap is measured by photoluminescence.
The direct correlation between these features proves the feasibility to control the
bandgap at a local scale by means a suitable pattern of strain.
To understand these results, we generalize a proper tight-binding model for MoS2
[2] under non-uniform conditions, accounting for the local modulation of the hopping integrals. Such analysis suggests a possible change between direct- to indirect-bandgap upon strain at a local scale, and a possible “funnel” effect [3] in large
wrinkles.
[1] A. Castellanos-Gomez, R. Roldán, E. Cappelluti, M.Buscema, F. Guinea, H.S.J. van
der Zant, and G.A. Steel, Nano Letters 13, 5361 (2013).
[2] E. Cappelluti, R. Roldán, J.A. Silva-Guillén, P. Ordejón, and F. Guinea, F., Phys. Rev.
B 88, 075409 (2013).
[3] J. Feng, X. Qian, C.-W. Huang, and J. Li, , Nat. Photon. 6, 866 (2012).
Corresponding author: Instituto dei Sistemi Complessi, CNR, v. dei Taurini 19, 00185 Rome,
Italy. Ph.: +39-06-49937453; fax: +309-06-49937440;
email: emmanuele.cappelluti@roma1.infn.it
23
Session 2 - Two-dimensional Electron Gas II 23
wed 24
18.00
18.20
Strong enhancement of electron-phonon coupling
in Li-Graphene and the emergence of superconductivity
B. M. Ludbrook, G. Levy, C. N. Veenstra, M. Schneider, M. Zonno, A. Damascelli
Department of Physics & Astronomy, University of British Columbia, Vancouver, Canada
Quantum Matter Institute, University of British Columbia, Vancouver, Canada
The study of two-dimensional graphene, originally derived from bulk graphite, is
a power-ful example of our ability to control and study the rich physics that exists
in low dimen-sional systems. Bulk graphite intercalated with certain alkali species
exhibits strong elec-tron-phonon coupling, and in some cases, superconductivity.
In the 2D case of graphene, it was recently shown that the electron-phonon coupling strength can be engineered through the choice of alkali adatom deposited on
the surface.[1] A recent theoretical work explored the connection between the bulk
and 2D cases, and suggested that lithium adatoms offered a potential pathway to
superconducting monolayer graphene.[2] Despite this, experimental evidence for
superconductivity in monolayer graphene has remained elusive.
Using angle-resolved photoemission spectroscopy (ARPES), we show that lithium
ada-toms form an ordered superstructure on a monolayer graphene surface at low
temperature. This leads to a modification of the phonon density of states, and a
strong enhancement of the electron-phonon coupling. We observe the opening of
a small, temperature-dependent gap in the electronic density of states at the Fermi
level in part of the Fermi surface. This suggests for the first time that Li-decorated
monolayer graphene is superconducting at 3.5K.
[1] Fedorov, A. V. et al. Observation of a universal donor-dependent vibrational mode in
graphene. Nat. Commun. 5:3257 (2014).
[2] Profeta, G. et al. Phonon-mediated superconductivity in graphene by lithium deposition. Nat. Phys. 8, 131-134 (2012)
Corresponding author: B. M. Ludbrook
Department of Physics & Astronomy, University of British Columbia,
Vancouver, British Columbia V6T 1Z1, Canada
bartludbrook@gmail.com
24
Session 2 - Two-dimensional Electron Gas II
Thursday 25/9
Aula Amaldi
Department of Physics
Session 3
Superconductors I
Session 4
Superconductors II
thu 25
8.50
9.10
Hundness and nematicity in Iron Pnictides
L. Fanfarillo, A. Cortijo, B. Valenzuela
Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC),
Sor Juana Inés de la Cruz 3, Campus Cantoblanco, Madrid, E-28049 Spain
There are two exciting research scenarios in iron pnictides, nematicity[1] and Hundness[2] believed to be key to understand superconductivity in these compounds. In
the nematic state the rotational symmetry of the ionic lattice is broken. It is also
found in other strongly correlated electron systems such as cuprates or ruthenates.
In iron pnictides experiments seem to indicate that the nematic state is driven by
electronic degrees of freedom as evidenced by charge probes[3], spin probes[4],
and orbital probes[5]. On the other hand, Hund’s coupling has been proposed as
the origin of columnar magnetic ordering and as the responsible for the bad metallic behavior and the orbital differentiation in the normal state[2] with some of the
orbitals more correlated than others. In this work we derive an effective Landau
model from a microscopic Hamiltonian[6] via a Hubbard-Stratonovich transformation. We extend the derivation done in [7] to address the dependence of the Landau
coefficients on the Hund’s coupling, Hubbard’s coupling and on the orbital content. We find that Hund’s coupling reduces the critical magnetic temperature which
agrees with the small Neel temperature found in iron superconductors. Hund’s coupling also favors columnar ordering magnetism in agreement with other theoretical
techniques. We define an orbital nematic order parameter that depend on the Fermi
surface orbital weight. We discuss the interplay between magnetism and orbital
degrees of freedom in the nematic state in view of this effective model[8].
[1] R. M. Fernandes, et al. Nat. Phys. 10, 97–104 (2014).
[2] A. Georges, et al. Annual Reviews of Condensed Matter Physics 4, 137 (2013)
[3] J.-H. Chu et al. Science 329, 824 (2010); M.A. Tanatar et al., Phys. Rev. B 81,
184508 (2010); A. Dusza et al. EPL 93, 37002 (2011); M. Nakajima et al. PNAS 108,
12238 (2011); Y. Gallais, et al. Phys. Rev. Lett. 111, 267001 (2013); E. P. Rosenthal, et
al., Nat. Phys. (2014).
[4] S. Kasahara, et al. Nature 486, 382–385 (2012); C. Dhital, et al., Phys. Rev. Lett.
108, 087001 (2012).
[5] M. Yi, et al. Proc. Natl Acad. Sci. USA 108, 6878_6883 (2011).
[6] M.J. Calderon, B. Valenzuela, E. Bascones, Phys. Rev. B 80, 094531 (2009).
[7] R.M. Fernandes, et al. Phys. Rev. B 85, 024534 (2012).
[8] Laura Fanfarillo, Alberto Cortijo, Belén Valenzuela, in preparation
Corresponding author: Laura Fanfarillo email: laura.fanfarillo@icmm.csic.es
26
Session 3 - Superconductors I
Seebeck effect of iron-based materials: a sensitive probe of
carrier-spin wave coupling
thu 25
9.10
9.30
F. Caglieris1,2, A. Braggio1,2 , A. Provino3,2, I. Pallecchi2, G. Lamura2, A. Jost4,
U. Zeitler4, E. Galleani D’Agliano1, P. Manfrinetti3,2, M. Putti1,2
1. Department of Physics, University of Genova, Via Dodecaneso 33, 16146 Genova, Italy
2. Institute SPIN-CNR, Corso Perrone 24, 16152 Genova, Italy
3. Department of Chemistry, University of Genova, Via Dodecaneso 31, 16146 Genova, Italy
4. High Field Magnet Laboratory, Radboud University of Nijmegen NL-65000 GL Nijmegen
Thermoelectric properties have been proven to be very sensitive probes of many aspects of interest in solid state physics. Among them, in the last years, unconventional superconductivity in iron pnictides has been a largely debated topic. Considering
the earliest reports1,2,3 of Seebeck (S) effect in iron pnictides, a complicated scenario emerges. For the 1111 family, having general chemical composition REFeAsO
(RE=rare earth), S curves generally exhibit abrupt variations, local maxima and
changes in sign as evident signatures of different mechanisms in competition. A
multiband picture and changes in scattering mechanisms have been proposed to
account for the observed behavior. In particular Matusiak et al.3 have explored the
low temperature regime, where S curves usually exhibit a local minimum. The
significant sensitivity of such feature to the application of an external magnetic
field has suggested to the authors the plausibility of magnon-drag scenario. In this
work, we carried out a careful analysis of the S effect in the 1111 parent compounds
with different RE and different degrees of disorder. We explored the dependences
on temperature, observing a puzzled and articulated phenomenology not explainable in terms of diffusive-multiband transport. In order to find an evident signature
of another mechanism we performed measurements of Seebeck effect as a function of magnetic field (B) up to 30 T at the HFML laboratories of Nijmegen. The
Seebeck coefficient increases in magnitude with increasing field with a tendency
to saturation observed at the low temperature. For this scenario we propose an
interpretation based on magnon-drag by antiferromagnetic spin waves. To support
our thesis we have developed a theoretical model for AFM-magnon drag contribution, identifying a scaling behavior as a function of the ratio B/T, well obeyed by
experimental data. The existence of a dominating magnon-drag contribution in the
1111-parent compounds is an important evidence of the strong interaction between
charge carriers and spin waves. Within this picture the Seebeck effect comes out
to be a privileged property which effectively probes the coupling mechanisms supposed to induce unconventional superconductivity. This work is supported by P7EU project SUPER-IRON (No. 283204) and PRIN2012X3YFZ2.
1
M.A.McGuire et al, New J. of Phys. 11, 025011 (2009) ; 2Q. Tao et al., J. Phys.:Condens.
Matter 22, 072201 (2010); 3M. Matusiak et al., Phys. Rev. B 79,212502 (2009)
Corresponding author: Federico Caglieris University of Genova, Via Dodecaneso 33, 16146
Genova (IT) - CNR-SPIN, Corso Perrone 24, 16152 Genova (IT) Phone: +39 010 353 6323
e-mail: federico.caglieris@spin.cnr.it
Session 3 - Superconductors I 27
27
thu 25
9.30
9.50
Fe-Chalcogenides thin films and monolayers
G. Profeta1, C. Tresca2, F. Ricci,1, A. Continenza2
1. CNR-SPIN and Dipartimento di Scienze Fisiche e Chimiche, Università degli Studi
dell’Aquila, Italy
2. Dipartimento di Scienze Fisiche e Chimiche, Università degli Studi dell’Aquila, Italy
Fe-Chalcogenides (FeCh) is the family of Fe-based superconductors (FeSCs)
having the simplest crystal structure. Despite this, they show very intriguing superconducting behaviour.
In particular, the recent successful fabrication of one monolayer (ML) FeSe grown
on SrTiO3 (STO), represented a real breakthrough in the search for materials with
higher critical temperatures, showing a Tc ~ 65 K. This is still the record Tc among
FeSCs.
The peculiar electronic properties of FeSe monolayer as measured in ARPES experiments, questioned the validity of the existing models on the origin of the superconducitng instabulity in FeSCs.
We performed ab-initio calculations of FeSe thin films and compare the predicted
electronic properties with ARPES measured Fermi Surfaces.
The calculated band structure, properly renormalized to take into account the strong
correlations effects, are found in good agreement with experiments.
Then we calculated the electronic and magnetic properties of FeSe and FeTe ML’s
as a function of tensile strain in comparison with existing experimental results.
We surprisingly found that, although FeTe is not superconducitng under ambient
pressure, a tiny strain field can induce a collinear magnetic ground state, which can
make it an interesting superconducting compound.
Corresponding author: Gianni Profeta, Via Vetoio, 10, 67100 L’Aquila (Italy).
e-mail: gianni.profeta@aquila.infn.it
28
Session 3 - Superconductors I
thu 25
9.50
10.10
Tuning the Magnetic and
Structural Phase Transitions of oxy-pnictides via Spin Dilution
S. Sanna1, P. Carretta1, P. Bonfà2, R. De Renzi2, Y.Yiu3, M. A. McGuire4,
A. Huq5, S. E. Nagler6, G. Lamura7, A. Palenzona7, A. Martinelli7, M. Putti7
1. Dept. of Physics, Pavia University, 27100, Pavia, Italy
2. Dept. of Physics and Earth Sciences, University of Parma
3. Department of Physics and Astronomy, University of Tennessee, Tennessee, USA
4. Materials Science and Technology Division, Oak Ridge National Laboratory, Tennessee
5. Chemical and Engineering Materials Division, Oak Ridge National Laboratory,
Tennessee
6. CIRE, University of Tennessee, Tennessee,USA
7. CNR-SPIN and University of Genova, Genova, Italy
The precise role of magnetism and its coupling to the lattice is a central problem
in the physics of unconventional iron based superconductors and related materials. In general, the undoped parent compounds of the iron-pnictide superconductors are tetragonal paramagnets at high temperatures. Upon cooling they display a
tetragonal-orthorhombic structural transition at TS, followed or accompanied by a
spin density wave transition at TSDW .
To shed light on this intriguing topic, we discuss the evolution of the electronic
phase diagram of LnFe1-XRuXAsO for Ln = La, Pr via diamagnetic Ru for Fe spin
dilution. The substitution of Ru for Fe is isoelectronic [1,2], hence allows for investigations of the underlying physics without the complication of extrinsic modifications of the Fermi surface.
Muon spin relaxation measurements show that the Ru/Fe spin dilution directly
acts on the gradual disruption of magnetism which eventually disappears at the
dilution threshold of x=0.6, as predicted for a J1-J2 frustrated square-lattice model
[1,3]. Neutron diffraction measurements show that the structural tetragonal-to-orthorhombic transition TS, detectable up to x≈0.4, closely precedes the magnetic
transition TSDW, thus indicating a significant magneto-elastic coupling within the
FeAs layers.
The data will be discussed in the framework of the Ising-nematic scenario predicted for localized spins in the J1-J2 model [4].
1. P. Bonfà et al., Phys. Rev. B 85, 054518 (2012).
2. M. Mazzani et al., Phys. Status Solidi B 251, 974 (2014)
3. A. Martinelli et al., J. Phys.: Condens. Matter 25, 395701 (2013).
4. Y. Yiu, S. Sanna et al., in preparation.
29
Session 3 - Superconductors I 29
thu 25
10.10
10.30
Very Slow Spin fluctuations in Ba(Fe1-xRhx)2As2 iron-based
superconductors, revealed by Nuclear Magnetic Resonance
Spectroscopy
L. Bossoni1, P. Carretta1, W. P. Halperin2, S. Oh2, A. Reyes3, P. Kuhns3, P.C. Canfield4
1. Department of Physics, University of Pavia-CNISM, Via Bassi 6, I-27100 Pavia, Italy
2. Department of Physics and Astronomy, Northwestern University, Evanston, IL, USA
3. National High Magnetic Field Laboratory, Tallahasse, FL, USA and
4. Ames Laboratory US DOE and Department of Physics and Astronomy, Iowa State
University, Ames, IA 50011, USA
Within the iron-based superconductors, the close proximity of superconductivity
to spin density wave (SDW) order, together with Fermi surface reconstruction and
the strange-metal behavior have suggested the insurgence of a quantum critical
phase transition, prompted by chemical pressure. The study of spin fluctuations, at
finite temperature, is then fundamental to unveil these non-trivial states. Nuclear
Magnetic Resonance (NMR) spectroscopy happens to be an ideal local probe for
spin dynamic.
In this study we show that in the Ba(Fe1−xRhx)2As2 family of iron-pnictides, very
slow spin fluctuations persist up to the over-doped region. Indeed, the spin-lattice
relaxation rate (1/T1), and the spin-echo decay rate (1/T2) provide insights onto
such spin dynamic. 75As NMR T2 measurements in Ba(Fe1−xRhx)2As2 superconductors are done for a large range of doping [1]. We show that 1/T2 increases upon
cooling, in the normal phase, suggesting the onset of an unconventional very lowfrequency dynamic. The motion is favored at large Rh content, and it is influenced
by the magnetic field intensity. A further dynamic, evidenced by the enhanced spinlattice relaxation rate, and the narrowing of the spectral linewidth, seem to be in
agreement with the T2 phenomenology.
We firstly attempt to derive the correlation times of the fluctuations and their energy barriers. Secondly we study the interaction of the dynamic with static fields
up to 17 T.
These results are discussed in the light of the J1-J2 Heisenberg model, where frustration may give rise to antiferromagnetic nematic fluctuations, involving domain
walls motion [2]. A second interpretation, based on the occurrence of fluctuating
charge stripes is discussed in the light of the analogies with the behavior observed
in the cuprates [3].
Finally we build a new phase diagram, based on our recent observations.
[1] L. Bossoni et al., PRB 88, 100503(R) (2013).
[2] P. Carretta et al., PRL 88, 047601 (2002).
[2] T. Wu et al., Nature 477, 191 (2011).
Corresponding author: Dr. Lucia Bossoni, Department of Physics - University of Pavia
30
Session 3 - Superconductors I
thu 25
11.00
11.20
Synthesis and Characterization of Metal Intercalated Polycyclic
Hydrocarbons
G. A. Artioli1, F. Hammerath2, P. Carretta2, L. Malavasi1
1. Dept. of Chemistry, University of Pavia and INSTM, Viale Taramelli 16, Pavia, Italy
2. Dept. of Physics, University of Pavia, Via Bassi 6, Pavia, Italy
The discovery of 18 K Tc in K-doped picene (K3picene) reported by Mitsuhashi and
colleagues in March 2010 has triggered a huge interest towards superconducting
hydrocabons [1]. Starting from this discovery a new era in the superconducting
research has been opened and the potential to discovered analogous new HTSC
has been extremely exciting. Since the paper of Mitsuhashi some other related
systems have shown to be superconductors such as metal-intercalated phenantrene
and coronene [2, 3]. To date, however, the evidence of bulk superconductivity in
these systems has not confirmed by other researchers. The aim of the present work
is to present our recent research activity devoted to the synthesis of alkali-metal
doped phenacene (family to which picene belongs) compounds. Different synthetic
routes were explored in order to obtain superconducting high purity crystalline
samples and to understand the intercalation process. Systems investigated have
been picene, phenantrene and chrysene doped with alkali metals and rare earths.
The efforts have been directed to the systematic investigation of the alkali metal
doped phenacene as a function of dopant and stoichiometry (and intercalation procedure) followed by a thorough study of magnetic and structural properties of the
samples prepared [4].
[1] R. Mitsuhashi, Y. Suzuki, Y. Yamanari,H.Mitamura, T. Kambe, N. Ikeda, H. Okamoto,
A. Fujiwara, M. Yamaji, N. Kawasaki, Y. Maniwa and Y. Kubozono, Nature, 2010, 464, 76.
[2] Y. Kubozono, H. Mitamura, X. Lee, X. He, Y. Yamanari, Y. Takahashi, Y. Suzuki, Y.
Kaji, R. Eguchi, K. Akaike, T. Kambe, H. Okamoto, A. Fujiwara, T. Kato, T. Kosugi and
H. Aoki, PCCP, 2011, 13, 16476
[3] X. Wang, R. H. Liu, Z. Gui, Y. L. Xie, Y. J. Yan, J. J. Ying, X. G. Luo and X. H. Chen,
Nat. Commun., 2011, 2, 507.
[4] G.A. Artioli, L. Malavasi, J. Mater. Chem. C, 2014, 2, 1577.
Corresponding author: Lorenzo Malavasi, Dept. of Chemistry and INSTM, Viale Taramelli 16,
27100 Pavia, Italy, Tel. +39 382 987291, email: lorenzo.malavasi@unipv.it
31
Session 3 - Superconductors I 31
thu 25
11.20
11.40
Optical anisotropy in the electronic nematic phase
of underdoped detwinned Ba(Fe1-XCoX)2As2.
C. Mirri1, A. Dusza1, S. Bastelberger, J.-H. Chu2, H.-H. Kuo2, I.R. Fisher2, and
L. Degiorgi1
1. Laboratorium für Festkörperphysik, ETH-Zürich, 8093 Zürich, Switzerland
2. Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator
Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA; Geballe
Laboratory for Advanced Materials, Stanford University, Stanford, California
94305, USA; Stanford Department of Applied Physics, Stanford University, Stanford,
California 94305, USA
The iron-pnictide superconductors are excellent materials where one can study the
competition between structural, magnetic and superconducting phases. In their so
called underdoped regime an antiferromagnetic transition occurs at TN, always preceeded by or coincident with a tetragonal-to-orthorhombic structural distortion at
TS ≥ TN. The breaking of the four-fold rotational symmetry of the tetragonal phase
due to the ferro-elastic transition implies the onset of a nematic phase, which can
lead to important implications with respect to superconductivity. The in-plane anisotropy of several measurable quantities in the orthorhombic phase is masked by
the formation of twin domains, which can be aligned (i.e. detwinned) by applying
strong magnetic fields or an uniaxial stress along a specific crystallographic direction.
Here we report on an optical investigation performed with electromagnetic radiation polarized along the orthorhombic a and b axes of Ba(Fe1-xCox)2As2 single crystals (with x=0, 2.5 and 4.5%). We developed a new technique that allows in-situ
variation of uniaxial pressure (i.e., degree of detwinning) in order to probe the
polarization dependence of optical reflectivity at several temperatures across the
tetragonal-to-orthorhombic structural transition. By using Kramers-Kronig transformations, we extract from the reflectivity data the frequency-dependent optical
conductivity at different combinations of temperature T and pressure p.
At T < TS we discover a remarkable optical anisotropy as a function of the applied
p at energy scales far away from the Fermi level consistent with a hysteretic-like
behavior associated with twin boundary motion in the orthorhombic state. At T ≥ TS
the anisotropy turns into a reversible linear pressure dependence, underlying a thermal assisted motion of the domain walls. The optical anisotropy in the underdoped
regime gets progressively depleted with increasing Co-doping level, consistent
with the evolution of the orthorhombicity, but contrary to the non-monotonic
behavior observed for the dc anisotropy upon Co-doping.
Our results bear testimony for the electronic nature of the ferro-elastic transition
and imply the relevance of ferro-orbital ordering as its driving mechanism.
Corresponding author: Chiara Mirri, Laboratorium für Festkörperphysik, ETH-Zürich, 8093
Zürich, Switzerland chiara@phys.ethz.ch
32
Session 3 - Superconductors I
thu 25
11.40
12.00
Nanoscale probe of magnetism, superconductivity, orbital occupation, and structural distortions in iron-based superconductors
C. Cantoni1, A. S. Sefat1, M. A. McGuire1, M. Pan2, and B. C. Sales1
1. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak
Ridge, TN 37831, USA
2. Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak
Ridge, TN 37831, USA
Local probes of atomic and electronic structures with sub-nanometer spatial resolution can provide valuable insights into the physics of iron-based superconductors
(FBS) by resolving inhomogeneities that are typically averaged over by bulk-sensitive techniques. Here we apply aberration-corrected scanning transmission electron
microscopy (STEM) coupled with electron energy loss spectroscopy (EELS), and
scanning tunneling microscopy (STM) to several FBS and parent compounds to
decipher the interplay between crystal distortions, local magnetic moment (LMM),
orbital occupancy, and charge doping.
Our measurements reveal universal trends for hole concentration and LMM across
families of FBS, and provide a more complete understanding of how dopants introduce superconductivity in Pr-doped Ca122 [1] and Co-doped Ba122, including the
dependence of the superconducting gap on the dopants local distribution.
Although the common understanding has been that both long-range and LMM decrease with doping, we find that, in 122 arsenides near the onset of superconductivity, the LMM increases and shows a dome-like maximum near optimal doping,
where no ordered magnetic moment is present. In addition, we discover that magneto-elastic coupling breaks the C4 tetragonal symmetry at room temperature in unstrained single crystals, lowering the symmetry to orthorhombic (I2mm), and that
all of the crystals are twinned with domains the size of a few nanometers. Moreover, the domain size correlates with the magnitude of the dynamic Fe LMM, and
both are enhanced near optimal doping where the ordered moment is suppressed.
This is a remarkable direct observation of the effects of spin-lattice coupling and
magnetic fluctuations, and suggest they likely play a role in the superconducting
mechanism.
Research was supported by the Materials Sciences and Engineering Division Office of Basic Energy Sciences, U.S. Department of Energy
[1] K. Gofryk, M. Pan, C. Cantoni, B. Saparov, J. E. Mitchell, and A. S. Sefat. “Local inhomogeneity and filamentary superconductivity in Pr-doped CaFe2As2” Phys.
Rev. Lett. 112, 047005 (2014).
Corresponding author: Claudia Cantoni, Correlated Electron Materials, Materials Science and
Technology Division, Oak Ridge National Laboratory,P.O. Box 2008, MS-6056, 1 Bethel Valley
Road, Oak Ridge, TN 37831-6056 Email: cantonic@ornl.gov
Phone: (865) 576-2834 Fax: (865) 576-5023.
Session 3 - Superconductors I 33
33
thu 25
12.00
12.40
Magnetism, self-doping and incipient Mott Physics
in the “double” iron superconductor LiFeO2FeSe
C. Heil, L. Boeri, H. Sormann, W. von der Linden and M. Aichhorn
Institute for Theoretical and Computational Physics, Graz University of Technology, Austria
About one year ago, superconductivity with a critical temperature (Tc) of 43 K was
reported in LiFeO2FeSe, i.e. a very anomalous iron chalchogenide in which the
FeSe layers alternate with LiFeO2 layers. [1] Together with Sr2VO3FeAs, [2] this
is to date the only example of iron-based superconductor with a magnetic buffer
layer. What makes this compound even more special is that the magnetic atom in
the buffer layer – Fe(Li) – is also an iron atom, but in a nominal d 5 configuration,
while the iron in the chalchogenide planes – Fe(Se) – is in the usual d 6 configuration of Fe-based superconductors.
In this work, we employ density functional theory (DFT) and dynamical mean field
theory (DMFT) calculations to study the electronic structure of this compound.
We show that at a DFT level both Fe(Li) and Fe(Se) states contribute to the Fermi
surface, which is therefore more complicated than in usual FeSe superconductors.
The ground state is an antiferromagnetic metal, in which Fe(Li) and Fe(Se) have
sizable magnetic moments – 3.6 and 2.6 mB respectively – with a strong mutual
antiferromagnetic coupling. Including dynamical correlations in DMFT results in a
very different behavior for the two Fe atoms: Fe(Se)-derived bands retain a quasiparticle character, with a mass renormalization comparable to that of pure FeSe
[4]; on the other hand, since Fe(Li) is close to an orbital-selective Mott transition
regime,[5] the relative bands have a strongly incoherent character and are almost
completely removed from the Fermi level.
[1] X.F. Lu et al, Phys. Rev. B 89, 020507R, 2014.
[2] X. Zhu et al., Phys. Rev. B 79, 220512R, (2009).
[3] C. Heil et al, in preparation.
[4] M. Aichhorn et al., Phys. Rev. B 82, 064504 (2010).
[5] L. de’ Medici et al., Phys. Rev. Lett. 107, 256401 (2011).
Corresponding author: Lilia Boeri, Institute for Theoretical Computational Physics, TU Graz,
8010 Graz, Austria.
34
Session 3 - Superconductors I
thu 25
14.00
14.40
Coherent Control of High-Temperature Superconductivity
D. Nicoletti1, S. Kaiser1, W. Hu1, C. R. Hunt1, R. Mankowsky1, M. Först1,
M. Le Tacon3, T. Loew3, A. Subedi4, A. Georges4,5,6, B. Keimer3, A. Cavalleri1,2
1. Max Planck Institute for the Structure and Dynamics of Matter, Hamburg, Germany
2. Department of Physics, Oxford University, Clarendon Laboratory, Oxford, UK
3. Max Planck Institute for Solid State Research, Stuttgart, Germany
4. Centre de Physique Théorique, École Polytechnique, CNRS, Palaiseau, France
5. Collège de France, Paris, France
6. DPMC-MaNEP, Université de Genève, Genève, Switzerland
Strong electric field transients at THz frequencies can be used to control the electronic properties of high-temperature superconductors. In particular, resonant excitation of vibrational modes of the crystal lattice [1] and high-energy charge excitation [2] have been used in single-layer La-based cuprates to remove the competing
charge- and spin-order in the so-called stripe phase, thus transiently inducing superconductivity.
In this non-equilibrium superconducting state, the dynamics of different diffraction peaks was then mapped out with femtosecond X-ray diffraction, revealing a
decoupling of charge and lattice order on ultrafast timescales [3].
We recently applied a similar approach also to the well-known bilayer compound
YBa2Cu3O6+x. Here, by inducing large-amplitude modulations of the lattice,
and in particular of the position of the so-called apical oxygen atoms, inter-bilayer
superconducting coherence could be transiently enhanced at the expense of intrabilayer coupling, with this effect being observed even up to room temperature
[4,5].
By combining then ultrafast X-ray diffraction and ab-initio density functional calculations, the lattice distortions that trigger such redistribution of interlayer tunneling could be determined [6].
The coherent control of high-temperature superconductors, achieved by combining
a broad spectrum of time-resolved techniques, may even lead in the next future to
new strategies to enhance superconductivity in steady state.
[1] D. Fausti et al., Science 331, 6014 (2011).
[2] D. Nicoletti et al., arXiv:1404.6796 (2014).
[3] M. Först et al., Phys. Rev. Lett. 112, 157002 (2014).
[4] W. Hu et al., Nature Materials 13, 705 (2014).
[5] S. Kaiser et al., Phys. Rev. B 89, 184516 (2014).
[6] R. Mankowsky et al., arXiv:1405.2266 (2014).
Corresponding author: Daniele Nicoletti
Max Planck Institute for the Structure and Dynamics of Matter
Building 99 (CFEL) - Luruper Chaussee 149, 22761 Hamburg, Germany
email: daniele.nicoletti@mpsd.mpg.de
35
Session 4 - Superconductors II 35
thu 25
14.40
15.00
Study of CaCuO2/SrTiO3 single interface
D. Di Castro1, C. Cantoni2, F. Ridolfi1, C. Aruta1, C. Schlueter3, A. Tebano1,
G. Balestrino1
1. CNR-SPIN and Dipartimento di Ingegneria Civile e Ingegneria Informatica, Università di Roma Tor Vergata, 00133 Roma, Italy
2. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak
Ridge, Tennessee 37831, USA
3. ESRF, 6 rue Jules Horowitz, Boîte Postale 220, 38043 Grenoble, CEDEX 9, France
In the recently discovered superconducting superlattice CaCuO2/SrTiO3, significant experimental evidences suggest the confinement of superconductivity within
few unit cells at the CaCuO2/SrTiO3 interface and the important role of additional
oxygen atoms entering the interfaces during growth in oxygen rich environment.
In particular, the overall experimental data points toward an interfacial reconstruction by oxygen redistribution that, in case of excess oxygen, generates holes in the
CuO2 planes, allowing superconductivity in the CCO layer. Since each CCO block
forms two distinct interfaces with the top and the bottom STO block, the question
arises if both or only one of the two interfaces is responsible for superconductivity. We thus investigated the behavior of a single CCO/STO bilayer forming only
one of the two kinds of interface depending on the deposition sequence on NGO
substrate, that is, NGO/CCO/STO or NGO/STO/CCO. The two systems have been
investigated deeply with several experimental techniques, as STEM/EELS, XAS,
HXPES, starting from the structural and transport properties characterizations.
The main result is that the NGO/CCO/STO interface is the one where superconductivity occurs and hole doping has been revealed there. In this system, the extra
oxygen ions are mainly present in the Ca plane of CCO faced to TiO2 plane of STO,
and disappear moving in the deeper Ca planes. This finding proves that the role of
charge reservoir is really played by one layer at the interface. On the other hand,
the other interface NGO/STO/CCO is not superconducting, since no extra oxygen
ions can be introduced and no hole doping has been revealed.
Corresponding author: 1CNR-SPIN and Dipartimento di Ingegneria Civile e Ingegneria Infomatica,
Università di Roma Tor Vergata, 00133 Roma, Italia;
email: daniele.di.castro@uniroma2.it, phone: +39 06 72597230/7245
36
Session 4 - Superconductors II
thu 25
15.00
15.20
Electrodynamics of hetero-structured high temperature
superconductors
A. Perucchi1, P. Di Pietro1, F. Capitani2, P. Dore2, D. Di Castro3, S. Lupi4
1. INSTM Udr Trieste-ST and Sincrotrone Trieste, Area Science Park, I-34012 Trieste,
Italy
2. CNR-SPIN and Dipartimento di Fisica, Università di Roma Sapienza, P.le Aldo
Moro 2, I-00185 Roma, Italy
3. CNR-SPIN and Dipartimento di Ingegneria Civile ed Ingegneria Informatica, Università di Roma Tor Vergata, Via del Politecnico 1, I-00133 Roma, Italy
4. CNR-IOM and Dipartimento di Fisica, Università di RomaSapienza, P.le Aldo
Moro 2, I-00185 Roma, Italy
Both the iron-based and the cuprate high temperature superconductors, are intrinsically multi-layered materials. Particular efforts have thus been devoted to the deposition of thin superconducting films and to artificially synthesize heterostructures
based onto different superconducting materials. The study of these systems can
provide new clues to the understanding of the general mechanism of high temperature superconductivity while offering the possibility to tailor important superconducting properties. An important example is provided by Co-doped Ba122 superlattices, where it was shown that heterostructuring the pristine superconducting
compound can result in a substantial enhancement of the upper critical field, due to
controlled flux pinning. On the other hand, in cuprates, the fabrication of artificial
interfaces between the insulating CaCuO3 and SrTiO3 compounds, results in superconducting interfaces, analogous to the Copper-Oxide planes of the cuprates. We
address here the electrodynamics of both these classes of heterostructured superconductors. Optics allows to probe the electronic structure of these new superconducting states, thereby addressing important issues as the number and symmetry of
the gaps, the density of the charge carriers and their effective masses.
Corresponding author:
Andrea Perucchi - andrea.perucchi@elettra.eu
Elettra – Sincrotrone Trieste, S.S. 14 km 163.5 in AREA SCIENCE PARK, 34149 Basovizza (TS)
Session 4 - Superconductors II 37
37
thu 25
15.20
15.40
Unified and universal electron/hole doped normal-state phase
diagram ruled by spin-fluctuation phenomena
P. Orgiani1,2,, A. Galdi1,2, C. Sacco1,2, R. Arpaia3, S. Charpentier3, F. Lombardi3,
D.G. Schlom4,5, L. Maritato1,2
1. CNR-SPIN, UOS Salerno, 84084 Fisciano, Italy
2. Department of Information Engineering, Electrical Engineering and Applied
Mathematics, University of Salerno, 84084 Fisciano, (SA) Italy
3. Quantum Device Physics Laboratory, Department of Microtechnology and Nanoscience,
Chalmers University of Technology, SE-41296 Goteborg, Sweden
4. Department of Materials Science and Engineering, Cornell University, 14853 Ithaca, USA
5. Kavli Institute at Cornell for Nanoscale Science, 14853 Ithaca, USA
Even though a considerable effort has been spent in investigating the transport
properties of electron doped cuprates, different theoretical interpretations of the
experimental data have been proposed, some of them in apparent contradiction
with each other, ranging from Fermi liquid behavior to spin-fluctuation scattering
mechanisms. To further complicate the general properties of the normal-state physics of electron doped cuprates, the disappearance of any superconductivity phase
has been found for a very low normal-state sheet resistance values. Indeed, such
a crossover has been measured for a kFl value of about 13, sizable higher than the
critical values of 4 and 1 reported for both high- and low-temperature superconductors, respectively. Such a puzzling scenario has thus prevented, up to now, the full
understanding of the low temperature upturn of resistivity and the crossover from
the metallic/superconducting phase to a fully insulating ones.
With the aim of revealing the true nature of normal-state resistivity in electron-doped
cuprates, the transport properties of a wide series of infinite layer Sr1-xLaxCuO2±d
thin films have been detailed investigated. Infinite-layer cuprate structures, as discussed here, offer unique opportunities to define the proper regime of the transport
properties being removed the ambiguity in the definition of the effective thickness
of the conducting layer. Our starting point has been the underdoped regime, where
localization effects are clearly demonstrated by the upturn of the resistivity, proved
to be determined by quantum interference effects (QIEs). Once established, QIEs
have been traced back also in optimal- and over-doped samples, even though in
these last samples a metal-to-insulator transition does not occur. Evidence arises
that QIEs co-exist with a pure metallic phase and severely affects the normal-state
transport properties in proximity of superconducting transition. Such a phase separation scenario has therefore allowed to reveal the true nature of the metallic phase
of electron doped cuprates, which is found to be unambiguously dominated by
spin-fluctuation phenomena at any temperatures. Our results provide ultimate demonstration of a unified and universal electron/hole doped phase diagram.
Corresponding author: Pasquale Orgiani
Research Scientist at CNR-SPIN *presently at CNR-IOM TASC Laboratory - s.s.14 km 163.5 - I
34149 Trieste - Italy phone: (+39)040.3756467 [office]; (+39)040.3758075 [APE-beamline]
e-mail address: pasquale.orgiani@spin.cnr.it - https://sites.google.com/site/pasqualeorgiani/home
38
Session 4 - Superconductors II
thu 25
15.40
16.00
Boson-mediated effects following ultrafast pumping of strongly
correlated materials: thermal and non-thermal response
Giulio De Filippis
SPIN-CNR and Dipartimento di Fisica, Università di Napoli Federico II, I-80126,
Napoli, Italy
The paradigm of time domain studies of complex materials is that different degrees of freedom interact on different timescales. In this framework, borrowed
from semiconductor physics, the details of the photo-excitation processes are often disregarded and simply treated as an impulsive injection of electronic energy
that subsequently thermalizes with vibrations and other degrees of freedom. Here,
starting from a general theoretical framework (Hubbard-Holstein Hamiltonian), we
characterize the wavelength dependent excitation processes of strongly correlated
charge transfer insulators. We demonstrate that variations of the pump wavelength
across the charge transfer gap results in completely different electronic dynamics.
While pump pulses with photon energy larger than the charge-transfer gap lead to
an effective electronic heating, excitation in the excitation tail pilots an instantaneous increase of the coherent motion followed by an ultrafast reaction of
the bosonic field. Our results force a revision of multi-temperature approaches to
correlated electron systems and disclose an anomalous behaviour of electron-boson interaction if abetted by strong correlation between electrons.
Corresponding author: giuliode@na.infn.it
Session 4 - Superconductors II 39
39
thu 25
16.00
16.20
Snapshots of the retarded interaction of charge carriers with
ultrafast fluctuations in cuprates
S. Dal Conte1, L. Vidmar2,3, D. Golež3, M. Mierzejewski4, G. Soavi1, S. Peli5,6,
F. Banfi5,7, G. Ferrini5,7, R. Comin8, B.M. Ludbrook8, N.D. Zhigadlo9, H. Eisaki10,
M. Greven11, S. Lupi12, A. Damascelli8,13, D. Brida1,14, M. Capone15, J. Bonča3,16,
G. Cerullo1, and C. Giannetti5,7
1. IFN-CNR, Dipartimento di Fisica, Politecnico di Milano, 20133 Milano, Italy
2. Department of Physics and Arnold Sommerfeld Center for Theoretical Physics,
Ludwig- Maximilians-Universität München, D-80333 München, Germany
3. J. Stefan Institute, 1000 Ljubljana, Slovenia
4. Institute of Physics, University of Silesia, 40-007 Katowice, Poland
5. i-LAMP (Interdisciplinary Laboratories for Advanced Materials Physics), Università
Cattolica del Sacro Cuore, Brescia I-25121, Italy
6. Department of Physics, Università degli Studi di Milano, Italy
7. Department of Physics, Università Cattolica del Sacro Cuore, Brescia I-25121, Italy
8. Department of Physics and Astronomy, University of British Columbia, Vancouver,
BC V6T 1Z1, Canada
9. Laboratory for Solid State Physics, ETH, 8093 Zürich, Switzerland
10.Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568, Japan
11. School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota
55455, USA
12.CNR-IOM Dipartimento di Fisica, Università di Roma La Sapienza P.le Aldo Moro
2, 00185 Rome, Italy
13.Quantum Matter Institute, University of British Columbia, Vancouver, BC V6T 1Z4,
Canada
14.Department of Physics and Center for Applied Photonics, University of Konstanz,
78457 Konstanz, Germany
15.CNR-IOM Democritos National Simulation Center and Scuola Internazionale Superiore di Studi Avanzati (SISSA), Via Bonomea 265, 34136 Trieste, Italy
16.Faculty of Mathematics and Physics, University of Ljubljana, 1000 Ljubljana, Slovenia
One of the pivotal questions in the physics of high-temperature superconductors is
whether the low-energy dynamics of the charge carriers is mediated by bosons with
a characteristic timescale. This issue has remained elusive since electronic correlations are expected to dramatically speed up the electron-boson scattering processes, confining them to the very femtosecond timescale that is hard to access even
with state-of-the-art ultrafast techniques. Here we simultaneously push the time
resolution and the frequency range of transient reflectivity measurements up to an
unprecedented level that enables us to directly observe the ~16 fs build-up of the
effective electron-boson interaction in hole-doped copper oxides. This extremely
fast timescale, together with the outcome of calculations for the t-J model and the
repulsive Hubbard model, indicates that short-range antiferromagnetic fluctuations
are the bosons that likely mediate the retarded electron interactions in copper oxides close to optimal doping, where the largest critical temperature is reached.
40
Session 4 - Superconductors II
thu 25
16.20
16.40
Photoinduced antinodal metallicity in the pseudogap state of
high-Tc cuprates
F. Cilento1, S. Dal Conte2, G. Coslovich1, S. Peli2, F. Banfi2, G. Ferrini2 ,
H. Eisaki3, M. Greven4, A. Damascelli5, M. Capone7, D. van der Marel6,
C. Giannetti2 , F. Parmigiani1
1. Sincrotrone Trieste S.C.p.A., Basovizza I-34012, Italy
2. Department of Physics, Università Cattolica del Sacro Cuore, Brescia I-25121, Italy;
i-Lamp (Interdisciplinary Laboratories for Advanced Materials Physics
3. Nanoelectronics Research Institute, National Institute of Advanced Industrial
Science and Technology, Tsukuba, Ibaraki 305-8568, Japan
4. School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota
55455, USA
5. Department of Physics and Astronomy, University of British Columbia, Vancouver,
BC V6T 1Z1, Canada; Quantum Matter Institute, University of British Columbia,
Vancouver, BC V6T 1Z4, Canada
6. Département de Physique de la Matière Condensée, Université de Genève, Switzerland
7. International School for Advanced Studies (SISSA), Via Bonomea 265, I-34136
SISSA, Trieste, Italy
A major challenge in understanding the cuprate superconductors is to clarify the
nature of the fundamental electronic correlations that lead to the pseudogap phenomenon. We used ultrashort light pulses to prepare a non-thermal distribution of
excitations, and we performed time-resolved broadband reflectivity measurements
in order to capture novel properties that are hidden at equilibrium. Our framework
unveils a universal pseudogap-like region in the temperature (T) and hole-doping
(p) phase diagram, delimited by a well-defined T*neq(p) line. In this region the
photoexcitation process leads to a quench of local correlations triggering the evolution of antinodal excitations from gapped (localized) to metallic (delocalized)
quasi-particles characterized by a longer lifetime. This photoinduced antinodal
metallicity finds a natural explanation in terms of the single-band Hubbard model,
in which the short-range Coulomb repulsion leads to a k-space differentiation between “nodal” quasiparticles and antinodal excitations, whose self-energy diverges
as in the insulating state.
41
Session 4 - Superconductors II 41
thu 25
16.40
17.00
High-Energy Magnetic Excitations
in the Cuprate Superconductor Bi2Sr2CuO6+ δ
Y. Y. Peng1, M. Moretti Sala1, M. Hashimoto2, W. S. Lee2, G. Ghiringhelli1,
Z. X. Shen2, L. Braicovich1
1. CNR-SPIN and Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da
Vinci 32, I-20133 Milano, Italy
2. Stanford Institute for Materials and Energy Sciences, SLAC National Accel-
erator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA
Geballe Laboratory for Advanced Materials, Departments of Physics and Applied
Physics, Stanford University, California 94305, USA
We investigate the high-energy magnetic excitation spectrum in optimal doped
high-Tc superconductor Bi2Sr2CuO6+δ (Bi-2201) by momentum resolved resonant
inelastic x-ray scattering (RIXS) at the Cu L3 edge. Broad, dispersive magnetic
excitations are observed, with a zone boundary energy ~350 meV and a weak dependence on temperature. The magnon dispersion in Bi2201 is similar to those
in the previous RIXS studies on cuprates1,2,3. We find a dispersion discontinuity
from spectrum intensity and position around q//~0.25 at both temperature 50K and
200K. This may relate to spin-wave order. Possible implications of the observation
will be discussed.
[1] M. Le Tacon et al., Nat. Phys. 7, 725-730 (2011).
[2] M. P. M. Dean et al., Nat. Mater. 12, 1019-1023 (2013).
[3] M. P. M. Dean et al., Phys. Rev. Lett. 110, 147001 (2013)
Corresponding author: Yingying Peng, Dipartimento di Fisica, Politecnico di Milano, Piazza
Leonardo da Vinci 32, 20133 Milano –ITALY, yingying.peng@polimi.it
42
Session 4 - Superconductors II
Friday 26/9
Aula Amaldi
Department of Physics
Session 5
Strongly
correlated systems
Session 6
Magnetic oxides
and Multiferroics
fri 26
8.50
9.30
Superconductivity in layered bismuth chalcogenides
Yoshikazu Mizuguchi
Tokyo Metropolitan University, 1-1, Minami-osawa, Hachioji, Japan
In the field of exploration of new superconducting materials, the breakthrough
discoveries have been obtained in layered materials in recent years. The most outstanding examples are the Cu-oxide superconductors and Fe-based superconductors [1,2]. Both superconductors have a layered crystal structure composed of an
alternate stacking of a characteristic conductive layer and a blocking layer. Due to
the layered structure, those materials exhibit low-dimensional electronic states and
high-Tc (unconventional) superconductivity.
Recently, we have discovered new layered superconductors with a characteristic
conductive layer composed of Bi and S [3,4]. In the new superconductors, rocksalt-type BiS2 layers act as a conductive (superconductive) layer, and the BiS2
layers are separated by a blocking layer. So far, three types of blocking layer
have been found: RE2O2, Sr2F2 and Bi4O4(SO4) layers. The parent phase of BiS2based superconductor is basically an insulator with a band gap. The typical parent material is LaOBiS2. Electron carriers are generated within the BiS2 layers
upon a partial substitution of O by F at the blocking layers. The electron-doped
compound LaO1-xFxBiS2 shows superconductivity, and the maximum Tc of 3 K
is obtained at x ~ 0.5. The Tc largely increases to above 10 K with applying high
pressure (HP). The increase of Tc is correlated to the change in crystal structure [5].
Recently, it was found that the BiSe2 layer could be superconductive [6,7]. Namely,
we now have a great chance to explore new layered superconductors in bismuthchalcogen compounds. In this presentation, I will show a variety of crystal structure of BiS2-based superconductors and related (BiSe2-based) superconductors.
Notable physical properties such as extremely large anisotropy of superconductivity, low-carrier characteristics, anomalous metallic states and a large pressure
effect will be discussed. Finally, I would like to discuss how to increase Tc in the
layered bismuth-chalcogen superconductors.
[1] J. G. Bednorz, K. A. Müller, Z. Phys. B-Condens. Matter 64 (1986) 189.
[2] Y. Kamihara et al., J. Am. Chem. Soc. 130 (2008) 3296.
[3] Y. Mizuguchi et al., Phys. Rev. B 86 (2012) 220510.
[4] Y. Mizuguchi et al., J. Phys. Soc. Jpn. 81 (2012) 114725.
[5] J. Kajitani et al., Solid State Commun. 181 (2014) 1.
[6] A. K. Maziopa et al., J. Phys.: Condens. Matter 26 (2014) 215702.
[7] M. Tanaka et al., arXiv:1406.0734.
Corresponding author: Yoshikazu MIZUGUCHI
Affiliation and address: Tokyo Metropolitan University, 1-1, Minami-osawa, Hachioji, 192-0397,
Japan - e-mail: mizugu@tmu.ac.jp
44
Session 5 - Strongly correlated systems
fri 26
9.30
9.50
Hole propagation in the Kitaev-Heisenberg model
F. Trousselet1, P. Horsch1, A. M. Oleś1,2, and W. You1, 3
1. Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, D-70569
Stuttgart, Germany
2. Marian Smoluchowski Institute of Physics, Jagellonian University,
3. Reymonta 4, PL-30059 Kraków, Poland
4. College of Physics, Optoelectronics and Energy, Soochow University, Suzhou,
Jiangsu 215006, People’s Republic of China
Entanglement of spin and orbital degrees of freedom in correlated systems is known
to lead to intricate quantum phenomena [1], including exotic quantum topological
states [2], which are one of focuses in the condensed matter theory. In iridates
strong spin-orbit coupling generates on-site entanglement leads to effective S=1/2
spins which interact by frustrated interactions on the honeycomb lattice, a combination of Heisenberg and Kitaev exchange. Motivated by earlier experimental
[3] and theoretical [4] results for Na2IrO3, we explore with exact diagonalization
the propagation of a single hole in four magnetic phases of the t-J-like KitaevHeisenberg model on a honeycomb lattice: Neel antiferromagnetic, stripe, zigzag
and Kitaev spin-liquid phase [5]. We find coherent propagation of spin-polaron
quasiparticles in the antiferromagnetic phase by a similar mechanism as in the t-J
model for high-Tc cuprates. In the stripe and zigzag phases clear quasiparticles features appear in spectral functions of those propagators where holes are created and
annihilated on one sublattice, while they remain largely hidden in those spectral
functions that correspond to photoemission experiments. As the most surprising result, we find a totally incoherent spectral weight distribution for the spectral function of a hole moving in the Kitaev spin-liquid phase in the strong coupling regime
t>>J relevant for iridates. At intermediate coupling the finite systems calculation
reveals a well defined quasiparticle at the Γ point, however, we find that the gapless spin excitations wipe out quasiparticles at finite momenta. Also for this more
subtle case we conclude that in the thermodynamic limit the lightly doped Kitaev
liquid phase does not support quasiparticle states in the neighborhood of Γ point,
and therefore yields a non-Fermi liquid, contrary to earlier suggestions based on
slave-boson studies.
This work was partly supported by the Max Planck--UBC Centre for Quantum Materials, the Polish National Science Center (NCN) Project under No. 2012/04/A/
ST3/00331, and the National Natural Science Foundation of China (NSFC) under
Grant No. 11004144..
[1] A.M. Oles, J. Phys.: Condensed Matter 24, 313201 (2012).
[2] W. Brzezicki, J. Dziarmaga, and A.M. Oles, Phys. Rev. Lett. 112, 117204 (2014).
[3] R. Comin et al., Phys. Rev. Lett. 109, 266406 (2012).
[4] F. Trousselet, M. Berciu, A.M. Oles, P. Horsch, Phys. Rev. Lett. 111, 195143 (2013).
[5] F. Trousselet, P. Horsch, A.M. Oles, W.-L. You, arXiv:1308.3373 (2013).
Session 5 - Strongly correlated systems 45
45
fri 26
9.50
10.10
Strong interplay between electron-phonon interaction and
disorder in low doped systems
D. Di Sante1 and S. Ciuchi2
1. University of L’Aquila, Department of Physical and Chemical Sciences, Via Vetoio,
L’Aquila, Italy - CNR-SPIN, Via Vetoio, L’Aquila, Italy
2. University of L’Aquila, Department of Physical and Chemical Sciences, Via Vetoio,
L’Aquila, Italy - CNR-ISC, Via dei Taurini, Rome, Italy
The effects of doping on the spectral properties of low doped systems are investigated by means of Coherent Potential Approximation (CPA) to describe the distributed disorder carried out by the impurities and Non-Crossing Approximation
(NCA) to characterize a wide class of electron-phonon interactions which dominate the low-energy spectral features[1]. When disorder and electron-phonon interaction work on comparable energy scales, a strong entanglement between them
arises, and the effect of disorder can no more be described as a mere broadening of
the spectral features. As a consequence of this entanglement, the low doping Mott
metal-insulator transition, is strongly affected by a weak or moderate electronphonon coupling which is found to stabilize the insulating phase.
We apply this theory to the low-lying electronic states of La-doped Sr2TiO4 ,a quasi-two-dimensional counterpart of the widely investigated perovskite SrTiO3 [2].
We explain, in term of combined action of electron-phonon interaction and disorder, the experimental Luttinger volume which is substantially smaller than the La
dopant concentration.
The spectral function A(k, ω). a) Electron-phonon interaction only λ = 0.22 b)
Disorder only c) Electron-phonon interaction + disorder.
[1] D. DI Sante and S. Ciuchi arXiv:1404.6435
[2] Y.F. Nie, D. Di Sante, S. Chatterjee,P. D. C. King, M. Uchida, T. Birol, D. G. Schlom,
S. Ciuchi, and K.M. Shen (to be published)
Corresponding author: Sergio Ciuchi Department of Physical and Chemical Sciences, Via Vetoio,
L’Aquila, e-mail: sergio.ciuchi@aquila.infn.it
46
Session 5 - Strongly correlated systems
fri 26
10.10
10.30
Spin fluctuations-corrected DFT for itinerant systems and
Fe-based superconductors
L. Ortenzi1, H. Gretarsson2,3, I. Mazin4, P. Blaha5, S. Kasahara6,
K. D. Finkelstein7, W. Wu3, S. R. Julian3, Y.-J. Kim3, and L. Boeri8
1. Institute for Complex Systems (ISC), CNR, U.O.S. Sapienza, v. dei Taurini 19, 00185
Rome, Italy
2. Max-Planck-Institut für festkörperforschung, Heisenbergstraße 1, D-70569 Stuttgart,
Germany
3. Department of Physics, University of Toronto, 60 St.~George St., Toronto, Ontario,
M5S 1A7, Canada
4. code 6390, Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC
20375, USA
5. Institute for Materials Chemistry, Vienna University of Technology, A-1060 Vienna,
Austria
6. Research Center for Low Temperature and Materials Sciences, Kyoto University,
Kyoto 606-8501, Japan
7. Cornell High Energy Synchrotron Source, Cornell University, Ithaca, New York
14853, USA
8. Institute for Theoretical and Computational Physics, TU Graz, Petersgasse 16, 8010
Graz, Austria.
Albeit density functional theory (DFT) is, at the moment, the most appropriate tool
for treating itinerant magnetism, its mean field implementations -local spin density
approximation (LSDA) with or without gradient corrections- underestimate the
effect of non local spin fluctuations. As a result DFT fails in reproducing, at the
same time, the crystal structure and the amplitude of local moment in near critical
systems. In this talk I will present a simple method for correcting the magnetic
properties of itinerant systems in LSDA.
The method is called reduced Stoner theory (RST). I apply it to describe the ferromagnetic-paramagnetic transition under pressure in Ni3Al itinerant ferromagnet
and for explaining the puzzling temperature behavior of the local moment observed
in Ca0.78La0.22Fe2As2 and CaFe2(As1-xPx)2 systems.
1) L. Ortenzi, I. I. Mazin, P. Blaha, and L. Boeri Phys. Rev. B 86, 064437 (2012)
2) “Microscopic explanation of the anomalous temperature dependence of the local magnetic moments in the CaFe2As2 family of materials”
L. Ortenzi, H. Gretarsson, S. Kasahara, K. D. Finkelstein, W. Wu, S. R. Julian, YoungJune Kim, I. I. Mazin, and L. Boeri in preparation.
47
Session 5 - Strongly correlated systems 47
fri 26
10.30
10.50
Spin-orbital physics in hybrid 3d-4d oxides
W. Brzezicki1,2, C. Noce1,2, A. Romano1,2, A. M. Oleś3,4, M. Cuoco1, 2
1. Dipartimento di Fisica ‘E.R. Caianiello’, Università di Salerno, I-84084 Fisciano
(Salerno), Italy
2. SPIN-CNR, I-84084 Fisciano (Salerno), Italy
3. Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, D-70569
Stuttgart, Germany
4. Marian Smoluchowski Institute of Physics, Jagellonian University, Reymonta 4, PL30059 Kraków, Poland
The entanglement of spin, orbital and lattice degrees of freedom in correlated
systems is known to lead to intricate quantum phenomena [1-3], including exotic quantum topological states [4]. The investigation of correlated electrons in
transition-metal oxides (TMO) traditionally emphasizes 3d materials because the
more extended 4d shells would a priori suggest a weaker ratio between the intraorbital Coulomb interaction U and the electron bandwidth W. Nevertheless, the
extension of the 4d shells points towards a strong coupling between the 4d orbitals
and the neighboring oxygen orbitals, implying that these TMO have the tendency
to form distorted structures instead of the ideal ones. Hence, the change in the MO-M bond angle often leads to a narrowing of the d-bandwidth W, bringing the
system on the verge of a metal-insulator transition or into an insulating state. The
interplay between more localized 3d and more delocalized 4d valence states tunes
the competition between correlated metallic and Mott-insulating states and, in turn,
can significantly influence the strength and the nature of the hierarchy in the coupling between the spin-orbital-lattice degrees of freedom. In this framework, we
study how the magnetic and orbital patterns in a uniform 4d host are modified by
the inclusion of 3d impurities substituting the 4d ions. After discussing the most
suitable microscopic models for different types of 3d-4d hybrids, we determine the
phase diagram assuming different conditions for the orderings, both for metallic
and insulating cases. We demonstrate that the coupling between the impurity and
the host, specific of the 3d-4d bonds, can generate a complex competition between
different phases. Ruthenates belong to the Ruddlesden-Popper series and represent
a fertile 4d platform where different spin-orbital ordered states and metal-to-insulator transitions are realized [5-7]. A specific discussion of the Ru-oxides doped
with Mn will also be presented.
[1] G. Khaliullin, Prog. Theor. Phys. Suppl. 160, 155 (2005).
[2] A.M. Oles’, J. Phys.: Condensed Matter 24, 313201 (2012).
[3] W. Brzezicki, J. Dziarmaga, and A.M. Oles’, Phys. Rev. Lett. 109, 237201 (2013).
[4] W. Brzezicki, J. Dziarmaga, and A.M. Oles’, Phys. Rev. Lett. 112, 117204 (2014).
[5] C. Autieri, M. Cuoco, and C. Noce, Phys. Rev. B 89, 075102 (2014).
[6] M. Malvestuto, V. Capogrosso, E. Carleschi, L. Galli, E. Gorelov, E. Pavarini, R. Fittipaldi, F. Forte, M. Cuoco, A. Vecchione, and F. Parmigiani, Phys. Rev. B 88, 195143 (2013).
[7] F. Forte, M. Cuoco, and C. Noce, Phys. Rev. B 82, 155104 (2010).
48
Session 5 - Strongly correlated systems
fri 26
11.20
11.40
Electric field control of remnant states in ferroelectric tunnel junctions
G. Radaelli1,2,3, D. Gutiérrez1, F. Sánchez1, R. Bertacco2, J. Fontcuberta1
1. Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB,
Bellaterra, Spain.
2. LNESS - Dipartimento di Fisica - Politecnico di Milano, Como, Italy.
3. Unitè Mixte de Physique CNRS/Thales, Palaiseau, France.
In the past few years, ferroelectric tunnel junctions (FTJs) have attracted considerable interest owing to the rich physics involved in controlling their properties and
potential application in nanoelectronics and data storage [1]. The bistable electrical
modulation of conductivity in FTJs in response to a ferroelectric polarization of
the tunneling barrier, phenomenon known as the tunneling electroresistance (TER)
effect, is being considered for binary data storage, in analogy with the operation
of ferroelectric random access memories, but with the key advantage of non-destructive readout and simpler device architecture. Recently, the FTJ concept has
experienced a revival driven by significant advances in thin-film deposition and
characterization techniques, leading to the discovery that ferroelectric films can
maintain a stable and switchable polarization at nanometer thicknesses [2].
Here we report on the fabrication and the room-temperature tunneling characteristics of Pt/BaTiO3/La0.7Sr0.3MnO3//SrTiO3(001) FTJs (junction areas from 10 to
100 µm2), grown by combined use of pulsed laser deposition and sputtering techniques with different thicknesses of the BaTiO3 ferroelectric tunneling barrier. I-V
curves have been recorded after applying a poling voltage, which reverses the barrier polarization. Clear TER effect has been observed at room temperature: resistance vs. poling voltage loops unambiguously show the two resistance states which
correspond to the two ferroelectric polarization states of the ferroelectric BaTiO3
barrier. The demonstration of room temperature TER in micro-devices paves the
way to the consideration of such a kind of ferroelectric devices for storage applications. Moreover we show that integration in FTJs of half-doped manganites
(La0.5A0.5MnO3, A = Ca/Sr), intrinsically highly sensitive to external perturbations,
is very promising to obtain an overall further enhancement of the TER effect.
[1] A. Chanthbouala et al., Nature Nanotech. 7, 101-104 (2012).
[2] E. Y. Tsymbal et al., Science 313, 181-183 (2006).
Corresponding author: Greta Radaelli – email: greta.radaelli@gmail.com – Unitè Mixte de
Physique CNRS/Thales, Thales Research and Technology, Campus de l’Ecole Polytechnique, 1
Avenue A. Fresnel 91767 Palaiseau, France.
Session 5 - Strongly correlated systems 49
49
fri 26
11.40
12.00
Nanoscale atomic correlations in Ir1-xPtxTe2 (x=0.00, 0.03 and
0.04): An x-ray absorption fine structure study
B. Joseph1,2, E. Paris1, A. Iadecola3, L. Simonelli4, M. Bendele1, L. Maugeri1,
S. Pyon5, K. Kudo5, M. Nohara5, T. Mizokawa6, N.L. Saini1
1. Dipartimento di Fisica, Università di Roma Sapienza, Piazzale Aldo Moro 2,
2.
3.
4.
5.
6.
I-00185 Rome, Italy
ELETTRA-Sincrotrone Trieste, Area Science Park, I-34012 Trieste, Italy
European Synchrotron Radiation Facility, BP220, F-38043 Grenoble Cedex, France
CELLS - ALBA, Carretera BP 1413, Km 3.3, 08290 Barcelona, Spain
Dep. of Physics, Okayama University, Kita-ku, Okayama 700-8530, Japan
Department of Complexity Science and Engineering, Uni. of Tokyo, 5-1-5
Kashiwanoha, Chiba 277-8561, Japan
Recently, the layered 5d-transition-metal dichalcogenide IrTe2 has been in the
limelight after the observation of superconductivity upon Pt doping [1]. We have
used x-ray absorption fine structure spectroscopy to investigate the unoccupied
electronic states and local geometry of Ir1−xPtxTe2 (x=0.0, 0.03 and 0.04) as a
function of temperature. The Ir L3-edge absorption white line as well as the nearedge features due to the photoelectron multiple scatterings with near neighbors
reveal clear changes in the unoccupied 5d-electronic states and the local geometry with Pt substitution. We find an anomalous spectral weight transfer across
the known first order structural phase transition, Ts ~ 270 K, from trigonal to
monoclinic phase in IrTe2, characterizing the reduced atomic structure symmetry
below the transition temperature. No such changes with temperature are seen in
the Pt substituted superconducting samples [2]. In the parent phase, extended xray absorption fine structure measurements reveal the appearance of longer Ir-Te
bond length (R ~ 0.05 A) for temperature below Ts [3]. Results indicate the important role of the interaction between the Ir 5d and Te 5p orbitals in determining
the properties of Ir1−xPtxTe2.
[1] S. Pyon, K. Kudo, and M. Nohara, J. Phys. Soc. Jpn. 81, 053701 (2012)
[2]B. Joseph et al., (submitted for publication)
[3] B. Joseph, M. Bendele, L. Simonelli, L. Maugeri, S. Pyon, K. Kudo, M. Nohara, T.
Mizokawa, N.L. Saini, Phys. Rev. B 88, 224109 (2013)
Corresponding author: Boby Joseph, ELETTRA-Sincrotrone Trieste, Area Science Park, I-34012
Trieste, Italy (boby.joseph@elettra.eu)
50
Session 5 - Strongly correlated systems
fri 26
12.00
12.20
Programmable MEMS resonator based on local control of VO2
phase transition
N. Manca1,2, L. Pellegrino1, T. Kanki3, S. Yamasaki3, H. Tanaka3, A. S. Siri1,2,
D. Marré1,2
1. CNR-SPIN, Genova (Italy )
2. Dipartimento di Fisica, Università di Genova, Genova (Italy)
3. ISIR, Osaka University, Osaka (Japan)
Transition Metal Oxides (TMOs) are important multifunctional compounds due
to their numerous physical properties and sensitivity to external stimuli, like temperature, stress, electrical/magnetic field or light. Competing interactions between
charge-spin-lattice degrees of freedom determine strongly correlated electron behaviour and complex phase transitions (PTs). These PTs can be exploited to develop new concepts for smart sensors and memories design in view of a “correlated
oxide nanomechanics”.
We will present VO2-based devices where resistive and mechanical properties can
be tuned by driving on the local scale the PT of this correlated oxide. In particular, we will focus an all-oxide micromechanical oscillator having programmable
mechanical eigenfrequency. Dynamic control of mechanical resonances in Nanoelectromechanical Systems (NEMS) has been obtained using electrostatic fields
or exploiting thermal expansion given by electro-thermal heating. However, these
solutions lack of memory effects. A different approach consists on employing materials exhibiting PT at nanoscale such as VO2, a correlated oxide showing an phase
transition with more than three-orders-of-magnitude hysteretic change of electrical
resistance nearby 68 °C associated with a change of its crystal structure from monoclinic to rutile–type. Here, we use Joule self-heating for setting multiple resistive
and mechanical resonance states of a VO2-based microresonator. Our prototype
device is a thin film microcantilever made of a VO2(200nm)/TiO2(110) (20 nm)
heterostructure. The key issue of this work is controlling in selected micrometric
regions the ratio between the two crystalline phases of VO2, with the consequent
change of the built-in stress of the whole device. Discussion of the mechanisms
of memory retention for implementing a multistate mechanical memory will be
given. Due to the intrinsic nanoscale size of VO2 domains on TiO2(110), downscaling to NEMS devices can be envisaged. The opportunity of controlling the
reciprocal ratio of crystalline phases at nanoscale opens promising perspectives for
developing mechanically reconfigurable devices. The use of nanomechanical sensing methods on complex TMO heterostructures may trigger new opportunities for
studying these fascinating materials.
L. Pellegrino et al. Adv. Mat. 2012 24(21), 2929–34
N. Manca et al. Adv. Mat. 2013 25(44), 6430–5
S. Yamasaki et al. APEX 2014 7(2), 023201
Corresponding author: nicola.manca@spin.cnr.it – C.so Perrone 24 – Genova (GE) 16152 – Italy
Session 5 - Strongly correlated systems 51
51
fri 26
12.20
12.40
Ultrafast light control of electron dynamics in molecular solids
M. Mitrano1, G. Cotugno1,2, S. Clark2, R. Singla1, A. Cantaluppi1, S. Kaiser1,
D. Nicoletti1, J. Staehler3, R. Beyer4, M. Dressel4, L. Baldassarre5, A. Perucchi6,
T. Hasegawa7, H. Okamoto8, D. Jaksch2, M. Riccò9, S. Lupi10, A. Cavalleri1
1. Max Planck Research Dept. for Structural Dynamics, University of Hamburg
2. Department of Physics, Oxford University, Clarendon Laboratory
3. Fritz Haber Institute of the Max Planck Society, Berlin
4. 1. Physikalisches Insitut, Universität Stuttgart
5. Center for Life NanoScience@Sapienza, Istituto Italiano di Tecnologia
6. Elettra-Sincrotrone Trieste S.C.p.A
7. National Institute of advanced Industrial Science and Technology, Tsukuba
8. Department of Advanced material Science, University of Tokyo
9. Dipartimento di Fisica e Scienze della Terra, Università di Parma
10.CNR-IOM and Dipartimento di Fisica, Universita` di Roma Sapienza
We report ultrafast reflectivity measurements under pressure to investigate the
quasiparticle recombination in a photo-excited Mott insulator as function of bandwidth (t) and electronic on- and inter-site interaction energies (U, and V) [1]. Our
measurements are performed on bis-(ethylendithyo)-tetrathiafulvalene-difluorotetracyano-quinodimethane (ET-F2TCNQ), a room-temperature quasi-1D half filled
Mott insulator (U/t~20) with weak electron-phonon coupling [2,3]
We apply external pressure in order to tune the electronic bandwidth t and nearest
neighbour interaction V. A pump-probe experiment with a pump pulse resonant to
the charge transfer band is performed to measure the lifetime of photoinjected holons (H) and doublons (D) [1,3,4]. By connecting the lifetimes to the effective parameters U, V, and t (determined with equilibrium optical measurements) and comparing our findings with the evolution of a time dependent effective multiple site
Hubbard model, we find evidence of a competition between the local recombination
of bound HD pairs and the delocalization of H and D right after photo-excitation.
In addition, we present recent results on the photo-excitation of K3C60, a member of
the A3C60 (A=alkali atom) molecular superconductors family that attracted a strong
interest in the last twenty years. Being a BCS superconductor with electron-phonon
coupling to high-energy molecular vibrations [5,6], this system constitutes an ideal
benchmark to investigate the effects of phonon pumping in a conventional superconductor. We excite the system with a pump pulse resonant to an IR active mode (approx. 170 meV) and we directly measure the gap region with THz-TR spectroscopy to
determine the transient changes in the superconducting state.
We will discuss also in this case the connection between equilibrium and far from equilibrium electron dynamics in presence of different competing interaction energy scales.
[1] M. Mitrano et al., Phys. Rev. Lett. 112, 117801 (2014)
[2] T. Hasegawa et al. , Solid State Comm. 103, 489 (1997)
[3] H. Okamoto et al., Phys. Rev. Lett. 98, (2007)
[4] S. Wall et al., Nat Phys 7, 114 (2011)
[5] O. Gunnarsson, Rev. Mod. Phys. 69, 575 (1997)
[6] L. Degiorgi et al., Phys. Rev. B 49, 7012 (1994).
Corresponding author: Matteo Mitrano - Max Planck Institute Tel: +49 (0) 40 8998 -6202 (Off) -6292 (Lab)
email: matteo.mitrano@mpsd.mpg.de
52
Session 5 - Strongly correlated systems
Localized states in dielectrics for advanced applications:
the case study of spin-polarized tunneling across MgO
fri 26
13.40
14.00
F. Schleicher1, U. Halisdemir1, D. Lacour2, M. Gallart1, S. Boukari1, G. Schmerber1, V. Davesne1, P. Panissod1, D. Halley1, H. Majjad1, Y. Henry1, B. Leconte1, A.
Boulard1, D. Spor1, N. Beyer1, C. Kieber1, E. Sternitzky1, O. Cregut1, M. Ziegler1,
F. Montaigne2, E. Beaurepaire1, P. Gilliot1, M. Hehn2, M. Bowen1
1. IPCMS UMR 7504 CNRS, Université de Strasbourg, 23 Rue du Loess, BP 43, 67034
Strasbourg Cedex 2, France
2. Institut Jean Lamour UMR 7198 CNRS, Université de Lorraine, BP 70239, 54506
Vandoeuvre les Nancy, France
Advanced materials such as ferroelectric or multiferroic perovskites are the focus
of intense research due to promising applications. Yet studies on these materials
rarely address the impact of defects on the nominally expected materials property.
We point out that the four addressable electric states of a magnetic tunnel junction
(MTJ) [1] promoted by a ferroelectric barrier [2] may also be achieved using a
mere paraelectric barrier with altered stoichiometry as was shown nearly a decade
ago [3]. As devices go nanoscale, the nominal properties of the dielectric (e.g. ferroelectricity) shall be dominated by those of defects within the dielectric matrix.
Knowledge of defect states in complex oxides mostly remains quite sparse despite
their strong impact, such as conductive domain walls in ferroelectrics [4].
To obtain broad guidelines on the impact of defects, we revisit the comparatively
simple oxide MgO. MgO is the model material system used to study spin-polarized
solid-state tunneling and present a defect-mediated tunneling potential landscape
of localized states due to explicitly identified defect species, against which we
examine the bias and temperature dependence of magnetotransport. Changes in
the carrier population of these ground and excited states upon thermal excitation
alter the charge carrier’s effective tunnel barrier height. We rationalize spectroscopic signatures of these localized states in the bias dependence of the decrease in
tunneling magnetoresistance with increasing temperature. We thus resolve glaring
contradictions between experiment and theory in this otherwise canonical spintronics system, and propose reporting guidelines for the scientific community. Our
work should stimulate similar investigations on transition metal oxides with intrinsic functionalities (ferroelectricity, multiferroicity, etc…).
[1] Miao, G.-X., Münzenberg, M. & Moodera, J. S. Tunneling path toward spintronics.
Reports Prog. Phys. 74, 036501 (2011).
[2] Garcia, V. et al. Ferroelectric control of spin polarization. Science 327, 1106–10
(2010).
[3] Bowen, M. et al. Bias-crafted magnetic tunnel junctions with bistable spin-dependent
states. Appl. Phys. Lett. 89, 103517 (2006).
[4] Guyonnet et al., Conduction at Domain Walls in Insulating Pb(Zr0.2Ti0.8)O3 Thin
Films, Adv. Mat. 23, 5377 (2011)
Corresponding author: Martin Bowen, bowen@unistra.fr
Session 6 - Magnetic oxides and Multiferroics
53
53
fri 26
14.00
14.20
Analysis of operating temperature and bias current as linked
parameters for the optimization of uncooled suspended
La0.7Sr0.3MnO3 bolometers on silicon substrates
L. Méchin1, B. Guillet1, S. Liu1, A. Aryan1, C. Adamo2, D.G. Schlom2,3
1. GREYC (UMR 6072) CNRS ENSICAEN Université de Caen Basse-Normandie
14050 Caen cedex, France
2. Department of Materials Science and Engineering, Cornell University, Ithaca, NY
14853-1501, USA
3. Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, USA
La0.7Sr0.3MnO3 (LSMO) is known to present a metal-to-insulator transition, which
is associated to a large change of the electrical resistivity with temperature, in a
temperature range close to 300 K. The related Temperature Coefficient of Resistance (TCR) is about 2% K-1, which makes LSMO a material of interest for the
realization of uncooled bolometers.
A bolometer being a thermal detector, which means that the incoming radiation is
measured by the temperature elevation caused by its absorption, thermal insulation
is obviously desired. We previously showed that LSMO thin films of high epitaxial
quality can be deposited on silicon substrates by reactive Molecular Beam Epitaxy
using SrTiO3 or CaTiO3 buffer layers [1]. A fabrication process based on the silicon substrate removal by Reactive Ion Etching has been optimized, which lead to
the successful fabrication of suspended LSMO strips with undegraded electrical
properties [2]. A large number of suspended LSMO strips of various width (2 or
4 µm), length (from 50 to 200 µm), and thickness (from 10 to 100 nm) have been
characterized. The thermal conductance of such suspended strips was measured in
the 3×10-8 – 3×10-5 W.K-1 range at 300 K, which is more than 4 orders of magnitude
below the thermal conductance of non suspended strips. The resulting sensitivity
of suspended bolometers are thus increased by more than 4 orders of magnitude. In
addition, using a proper geometry and adapted measuring conditions, we showed
that 1/f noise in LSMO films is very low [3], which is required to get state-of-theart performance.
The presentation will describe a detailed analysis of the electro-thermal parameters
such as operating temperature (280-320K) and bias current (10-200µA) leading to
optimized bolometric performance, close to the theoretical limit, named BLIP for
Background Limited Infrared Detector. In particular self heating has been studied
and it is shown how it can be used to enhance the bolometric performance.
[1] L. Méchin, C. Adamo, S. Wu, B. Guillet, S. Lebargy, C. Fur, J.-M. Routoure, S.
Mercone, M. Belmeguenai, and D. G. Schlom, Physica Status Solidi A Vol. 209 Issue 6
1090-1095 (2012)
[2] S. Liu, B. Guillet, A. Aryan, C. Adamo, C. Fur, J.-M. Routoure, F. Lemarié, D.G Schlom, L. Méchin, Microelectronic Engineering Vol. 111, 101-104 (2013)
[3] L. Méchin, S. Wu, B. Guillet, P. Perna, C. Fur, S. Lebargy, C. Adamo, D.G. Schlom,
J.M. Routoure, J. Phys. D: Appl. Phys. - Fast Track Communication 46 202001 (2013)
54
Session 6 - Magnetic oxides and Multiferroics
fri 26
14.20
14.40
Dual nature of the ferroelectric and metallic state in LiOsO_3
G. Giovannetti, M. Capone
*CNR-IOM-Democritos National Simulation Centre and International School for
Advanced Studies (SISSA), Via Bonomea 265, I-34136, Trieste, Italy
Metals are expected to not exhibit ferroelectricity because static internal electric
fields are screened by conduction electrons. A class of materials known as “ferroelectric metals” was discussed theoretically by Anderson and Blount in 1965 [1],
but in the paste no examples of this class of materials have been reported. Recently
LiOsO3 has been found to be a “ferroelectric metal” in the sense that it is a metal
but it has a second-order phase transition at Ts =140K with the developing of a
broken-symmetry ionic structure [2]. Indeed neutron and x-ray diffraction studies
show that the space group changes from centrosymmetric R-3c to ferroelectric-like
R3c [2]. Using Density functional theory calculations we address the driving force
behind the ferroelectric instability and we study the electronic structure in metallic
LiOsO3. We find the ferroelectric-like distortions to be related to the Li-O modes
while Os- O displacements allow for an enhancement of the hybridization between
Os d and O p orbitals. Performing Dynamical mean field theory calculations we
show that the metallic state of LiOsO3 is controlled by the amount of the electronic
correlations of the t2g states of Os, which are well below a critical value able to
inhibit the insulating ground state.
[1] Anderson and Blount, Phys. Rev. Lett. 14, 217 (1965)
[2] Y. Shi, Y. Guo, X. Wang, A. J. Princep, D. Khalyavin, P. Manuel, Y. Michiue, A. Sato, K.
Tsuda, S. Yu, M. Arai, Y. Shirako, M. Akaogi, N. Wang, K. Yamaura and A. T. Boothroyd,
Nat. Mat. 12, 1024 (2013).
Session 6 - Magnetic oxides and Multiferroics
55
55
fri 26
14.40
15.00
Coexisting Ferro- and Antiferro-Magnetic orders in multiferroic
BiMnO3 epitaxial films
G. M. De Luca1,2, R. Di Capua1,2, F. Chiarella1, M. Minola3, G. Ghiringhelli3,
N. B. Brookes4, P. Barone5, S. Picozzi5, M. Salluzzo1
1. CNR-SPIN Napoli, Complesso Monte Sant’Angelo via Cinthia, 80126 Napoli, Italy
2. Dipartimento di Fisica Complesso Monte Sant’Angelo via Cinthia, 80126 Napoli, Italy
3. CNR-SPIN, Dipartimento di Fisica, Politecnico di Milano, piazza Leonardo da Vinci
32, 20133 Milano, Italy
4. European Synchrotron Radiation Facility, 6 rue Jules Horowitz, B.P. 220, F-38043
Grenoble Cedex, France
5. CNR-SPIN L’Aquila c/o Dipartimento di Fisica Università degli Studi dell’Aquila
Via Vetoio 67010 Coppito (L’Aquila)
In the last years, the research in multiferroics has received a strong impulse by
potential applications in the area of novel multifunctional materials. Multiferroic
BiMnO3 (BMO) thin films are interesting candidates because of the robust ferromagnetism and possible ferroelectricity observed in polycrystalline samples [1].
BiMnO3 thin films, deposited by RF-magnetron sputtering on SrTiO3 (001) and
DyScO3 (110) substrates, showed extremely large magnetization and room temperature ferroelectricity [2], even if the BMO shows a centro-symmetric C2/c space
group. However, recently it has been theoretically proposed that the multiferroic
behavior of centrosymmetric BiMnO3 might be due to the coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) orders [3]: the AFM order is predicted to
break the inversion symmetry of the system, thus inducing a finite polarization. To
establish the relationship between AFM and FM orders in this compound, we have
performed magnetic circular (XMCD) and linear dichroism (XMLD) spectra at the
Mn L2,3 on BiMnO3 epitaxial thin films, which exhibit properties dependent on the
application of local electric field applied using an atomic Force microscopy tip.
The data show unconventional effects on both magnetic and orbital properties of
the system as function of the electric field, strain, magnetic field and temperature,
suggesting that BiMnO3 thin films are examples of improper multiferroics.
[1]Hill, N. A., Rabe, K. M., Phys. Rev. B 59, 8759 (1999).
[2]G.M. De Luca et al. Appl. Phys. Lett. 103, 062902 (2013)
[3] I.V. Solovyev et al., New J. Phys. 10, 073021 (2008).
Corresponding author: gdeluca@na.infn.it Dipartimento di Fisica Università di Napoli and CNRSPIN U.O.S. Napoli
56
Session 6 - Magnetic oxides and Multiferroics
fri 26
15.00
15.20
Improper origin of polar displacements at CaTiO3 and
CaMnO3 twin walls
P. Barone1, D. Di Sante1,2, S. Picozzi1
1. CNR-SPIN U.O.S. L’Aquila, Italy
2. Dipartimento di Fisica, Università dell’Aquila, Italy
Recent years have seen an increasing interest in novel functionalities arising at
domain walls of ferroic materials. Twin walls represent a particularly interesting
situation, arising as nanosized features within a homogeneous solid. They are characterized by a primary order parameter, which dominates in the bulk and disappears at twin walls. However, competing secondary order parameters often show
the opposite behaviour, appearing inside twin walls but being suppressed in the
bulk [1]. In this context, the ferroelastic nonpolar CaTiO3 has been proposed as
a paradigmatic example where the suppression at the twin boundaries of the primary order parameter, describing tilting and rotations of TiO6 octahedra, would
allow for the emergence of a secondary ferroelectric instability, involving polar
off-centerings of Ti ions [2]. Such a prediction, based on an empirical atomic-scale
description of the wall, was later qualitatively confirmed by Transmission-Electron
Microscopy measurements [3]; however, the wall electric polarization turned out
to be locked to the twin angle and local pattern of tilting distortions, thus making
the ferroelectric nature of the wall questionable. In order to achieve a microscopic
understanding of the origin of the observed wall distortions, we performed firstprinciples calculations which provide solid evidence of polar displacements arising
at the twin walls of CaTiO3, in excellent quantitative agreement with the experimental observations [4]. Furthermore, we show that such polar displacements really arise from rotation and tilting distortions - which at the wall act cooperatively
and cause the appearance of an improper electric polarization - rather than from a
proper secondary ferroelectric instability. A similar mechanism is proposed to be at
play in the antiferromagnet CaMnO3, where polar distortions are predicted to occur despite the presence of magnetic ordering, which has been long thought to be
incompatible with ferroelectricity in oxide perovskites [5]. The role of wall distortions in affecting the local magnetic properties will be also discussed, suggesting a
possible pinning of magnetic domain walls at the twin boundaries.
[1] W. T. Lee et al., J. Appl. Phys. 93, 9890 (2003).
[2] L. Goncalves-Ferreira et al., Phys. Rev. Lett. 101, 097602 (2008).
[3] S. Van Aert et al., Adv.Mater. 24, 523 (2012).
[4] P. Barone, D. Di Sante and S. Picozzi, Phys. Rev. B 89, 144104 (2014).
[5] N. A. Hill, J. Phys. Chem. B 104, 6694 (2000).
Corresponding author: paolo.barone@spin.cnr.it, CNR-SPIN UOS
L’Aquila, Via Vetoio, Coppito (L’Aquila), 67100 Italy
Session 6 - Magnetic oxides and Multiferroics
57
57
Marconi building - first floor plane
58
Session 6 - Magnetic oxides and Multiferroics
SuperFox 2014
POSTERS
(in order of submission)
Orbital selective V2O3 quasiparticle coherence
I. Lo Vecchio1, J. D. Denlinger2, and S. Lupi1
1. Dipartimento di Fisica, Università di Roma “Sapienza”, I-00185, Rome, Italy
2. Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA
P1
94720
Electronic correlations can renormalize the coherence temperature of Fermi liquid
quasiparticle excitations (usually around 10000 K) so that this scale is lowered by
some orders of magnitude. In V2O3, the prototype for Mott metal-insulator transitions, a Tcoh=425 K has been observed in an infrared spectroscopy study [1]. However LDA+DMFT calculations have predicted a very different response of coherent quasiparticles if one separately considers the orbitals involved. The imaginary
part of the self-energies predict that egπ orbitals (directed along the a axis) stay
incoherent down to 390 K, while a1g (oriented along the crystallographic c axis)
quasiparticles disappear at higher temperatures [2].
We measured the temperature and linear polarization dependent optical conductivity and photoemission spectra of a well-oriented ac surface of a V2O3 single crystal.
Both techniques show evidence of an orbital selective quasiparticle coherence. We
analyse the temperature dependence of the Drude spectral weight in the first case
and the integrated intensity around EF for the second technique and we find similar
results in good agreement with theoretical predictions.
[1] L. Baldassarre et al., Phys. Rev. B 77, 1131107 (2008).
[2] A. I. Poteryaev et al., Phys. Rev. B 76, 085127 (2007).
Corresponding author: irene.lovecchio@roma1.infn.it
60
Posters
Charge-dynamics in molecular junctions: Nonequilibrium
Green’s function approach made fast
P2
S. Latini1, E. Perfetto1, A.-M. Uimonen2, R. van Leeuwen2
and G. Stefanucci1
1. Dipartimento di Fisica, Universita di Roma Tor Vergata, Via della Ricerca Scientifica 1, I-00133 Rome, Italy
2. Department of Physics, Nanoscience Center, FIN 40014, University of Jyvaskyla,
Finland
NonEquilibrium Green’s function (NEGF) simulations of molecular junctions
(open systems) are numerically expensive due to the large memory storage needed.
In this talk we propose a simplified Green’s function approach based on the Generalized Kadanoff-Baym Ansatz (GKBA) to significantly speed up the calculations
and yet stay close to the NEGF results. This is achieved through a twofold advance:
first we show how to make the GKBA work in open systems and then construct
a suitable quasi-particle propagator that includes correlation effects in a diagrammatic fashion. We also provide evidence that our GKBA scheme, although already
in good agreement with the NEGF approach, can be further improved without increasing the computational cost. These results hold concrete promise for ab initio
NEGF simulations of nanostructured materials and devices.
Posters
61
61
The polaronic framework fully accounts for transport properties
in metallic ferromagnetic manganites
P3
P. Graziosi , A. Gambardella , M. Prezioso , A. Riminucci , I. Bergenti ,
N. Homonnay3, G. Schmidt3,4, D. Pullini5, D. Busquets-Mataix2,6
1. CNR - ISMN, Consiglio Nazionale delle Ricerche - Istituto per lo Studio dei Materiali
1,2
1
1
1
1
Nanostrutturati, v. Gobetti 101, 40129, Bologna, Italy
2. Instituto de Tecnología de Materiales, Universitat Politécnica de Valencia, Camino
3.
4.
5.
6.
de Vera s/n, 46022, Valencia, Spain
Institut für Physik, Universität Halle, 06120 Halle, Germany
Center for materials science, Universität Halle, 06120 Halle, Germany
Centro Ricerche Fiat, 10043, Orbassano (TO), Italy
Departamento de Ingeniería Mecánica y de Materiales, Universitat Politécnica de
Valencia, Camino de Vera s/n, 46022, Valencia, Spain
We propose a model for the consistent interpretation of the transport behaviour of
manganese perovskites in both the metallic and insulating regimes.1 The concept
of polarons as charge carriers in the metallic ferromagnetic phase of manganites
also solves the conflict between transport models, which usually neglects polaronic
effects in the metallic phase, and, at the other hand, optical conductivity, angle
resolved spectroscopy and neutron scattering measurements which identified polarons in the metallic phase of manganites down to 6 K. Transport characterizations of epitaxial La0.7Sr0.3MnO3 thin films in the thickness range 5 – 40 nm and 25
– 410 K temperature interval have been accurately collected. We show that taking
into account polaronic effects allows to achieve the best ever fitting of the transport curves in the whole temperature range. The Current Carriers Density Collapse
picture accurately accounts for the properties variation across the metal-insulator
transitions. The electron-phonon coupling parameter γ estimations are in a good
agreement with theoretical predictions. The results promote a clear and straightforward quantitative description of the manganite films involved in charge transport device applications and promises to
describe other oxide systems involving
a metal-insulator transition. The model
is successfully applied also to manganite single crystals of other compositions
taking data from literature. 2
Figure 1: transport data and fits for a 9 nm
LSMO/STO thin film; the metallic side is fitted with the proposed model while the insulating branch is described by thermal activated adiabatic hopping, which linearizes the data above
the MIT (inset). The whole R(T) is fitted by a
combination of the twos following the CCDC
model.
1 P. Graziosi, et al., Physical Review B Accepted (2014).
2 Y. Lyanda-Geller, S. H. Chun, M. B. Salamon, et al., Physical Review B 63, 184426 (2001).
62
Posters
Quantum Monte-Carlo phase diagram for a model cuprate
Konev V.V, Moskvin A.S, Matveyeva P.G, Vasinovich E.V
P4
Departament of Theoretical Physics, Ural Federal University, Ekaterinburg , Russia
We introduce a minimal model for CT (charge transfer) unstable 2D cuprates
with the on-site Hilbert space reduced to only three effective valence centers,
nominally Cu1+,2+,3+ ions and make use of the S=1 pseudospin formalism.Focusing on the unconventional bosonic-like physics of the model cuprate we neglect the one-particle transport (on-site terms) we write out the effective S=1
pseudospin Hamiltonian for the CuO2 plane of the model cuprate as follows:
(1)
with a charge density constraint:
1
2N
� S
i
iz
= �n ,
where Δn is the deviation from a half-filling.
The first single-site term describes the effects of a bare pseudo-spin splitting, or the
local energy of centers and relates with the on-site density-density interactions.
The second term may be related to a pseudo-magnetic field h1 || Z, in particular,
a real electric field which acts as a chemical potential (µ is the hole chemical potential, and is h1 a (random) site energy).The third term in Hch describes the
effects of the short- and long-range inter-site density-density interactions including screened Coulomb and covalent couplings. The fourth term describes the twoparticle bosonic transport. It is worth noting that in the limit of large negative Δ
we arrive at the Hamiltonian of the hard-core bosons.The effect of the nonisovalent
substitution in the model cuprate La2CuO4 was modeled via a Sr2+ induced impurity potential region within CuO2 plane with varied parameters Δ and V.
Making use of two different QMC methods, the standard stochastic series expansion (SSE) with loop updates and a continuous time world-line QMC, we studied
the ground-state and finite-temperature properties of the Hamiltonian (1) given different parameters values. Our QMC calculations for the model CT unstable cuprates
shows a step-by-step evolution under doping of the parent insulating state into an
uncoventional inhomogeneous supersolid (mixed charge order - Bose superfluid,
or CO+BS) phase formed by electron and hole CuO4 centers. The simulation does
reproduce main features of the T-x phase diagrams for doped cuprates, in particular,
the suppression of antiferromagnetism, a pseudogap regime due to charge ordering,
formation of a local superconductivity at T>Tc, and global 2D superconductivity.
Posters
63
63
P5
Infrared study of the metallization mechanism in LaxMnO3 thin films
W. S. Mohamed1, P. Maselli2, P. Calvani2, L. Baldassarre3, P. Orgiani4,
L. Maritato5, A. Galdi5, A. Nucara2
1. Dipartimento di Fisica, Università di Roma La Sapienza, Piazzale A. Moro 2,
P5
I-00185 Rome, Italy
2. CNR-SPIN and Dipartimento di Fisica, Università di Roma La Sapienza, Piazzale A.
Moro 2, I-00185 Rome, Italy
3. Center for Life NanoScience@Sapienza, Istituto Italiano di Tecnologia, Viale Regina
Elena 291, 00185 Rome, Italy
4. CNR-SPIN and Dipartimento di Fisica “E. R. Caianiello”, Via Ponte don Melillo,
I-84084 Fisciano, Salerno, Italy
5. CNR-SPIN and Dipartimento Ingegneria dell’Informazione, Università di Salerno,
Via Ponte don Melillo, I-84084 Fisciano, Salerno, Italy
LaMnO3 manganites with a massive concentration of La defects can be stabilized
in form of thin films, by exploiting the structural stress produced by a substrate
like SrTiO3 [1,2]. The optical conductivity of LaxMnO3-y films with x = 0.66, 0.88,
0.98 and 1.10, and with y ≈ 0, has been extracted from reflectivity measurements
between 40 - 40,000 cm-1, and between 20 and 320 K. Increasing metallization for
decreasing temperature is observed in the samples with x < 1, by a Drude term that
continues to grow, much below the insulator-to-metal (IMT) transition temperature
[3], at the expenses of a midinfrared band mir 2 peaked around 5000 cm-1. This
transfer of spectral weight occurs through an intermediate step, the growth of a
softer midinfrared band mir 1 at about 1000 cm-1.
We attribute mir 2 to localized small polarons, and mir 1 to itinerant large polarons,
in a phase separation framework which may explain the broad temperature interval
of the metallization process. We also find that optimal doping occurs around x =
0.88, and that the Drude absorption is much narrower than in chemically doped
manganites like La1-xSrxMnO3-y. In the
visible range, an absorption characteristic of Mn2+ could be identified at
16800 cm-1 in the films with x = 0.88
and 0.66, consistently with previous Xray observations that the La vacancies
are occupied by Mn divalent ions.
Fig. 1 Metallization process in a LaxMnO3 thin
film with x = 0.88, through a transfer of spectral weight from a small-polaron band (mir 2)
to a large polaron (mir 1) and a free-carrier
band (drude).
[1] A. Gupta et al., Appl. Phys. Lett. 67, 3494 (1995).
[2] P. Orgiani et al., Appl. Phys. Lett95, 013510 (2009).
[3] P. Orgiani et al., Phys. Rev. B 82, 205122 (2010).
[4] P. Orgiani et al., Phys. J. Nanopart. Res. 15, 1655 (2013).
64
Posters
Two dimensional Silicon-Germanium Hetero-structures for
Innovative Devices
P6
L. Marchetti1,2 and A. Debernardi1
1. Laboratorio MDM, IMM-CNR, 20864 Agrate Brianza (MB), Italy
2. Università degli studi di Milano, Milano, Italy
Silicene and Germanene, two honeycomb two-dimensional (2D) lattices constituted
by Si and Ge respectively, are predicted to present electronic band-structure having
the Dirac’s cone and to be stable in a buckled structure [1]. The recent experimental
measurement of a 2D silicene sheet [2] and of hydrogenated Germanene, known as
Germanane [3] have triggered further attention on these materials.
On the basis of first principles simulations we envisaged novel heterostructures
based on Silicene/Germanene nanoribbons to be used as building block in the new
generation of ultra-scaled electronic devices. In our simulations we considered
nanoribbons with two different orientations: the so-called zig-zag and the armchair terminations. We determined the equilibrium configuration, and we studied
the structural stability of the heterosturctures predicting the critical thickness of
Silicene and of Germanene ribbons growth starting from 2D Si-Ge sheet with different lattice parameters.
We individuate the microscopic mechanisms responsible of the electronic properties of the hetrostructure, we compute the effect of strain, and we studied how the
Dirac’s cone can be modified by a variation of the lattice parameter of the embedding matrix.
[1] S.Cahangirov et al. “2- and 1-D Honeycomb Structures of Si and Ge” Phys. Rev.
Lett. 102 (2009) 236804
[2] P. Vogt et al. “Silicene: Compelling Experimental Evidence for Graphene-like TwoDimensional Silicon”, Phys. Rev. Lett. 108, 155501 (2012)
[3] E.Bianco et al. “Stability and Exfoliation
Corresponding author: CNR-IMM, uos Agrate Brianza, via C. Olivetti, 2 – 20864 Agrate Brianza
(MB), Italy. E-mail: alberto.debernardi@mdm.imm.cnr.it
Posters
65
65
Poisoning effect of Mn in LaFe1-xMnxAsO0.89F0.11:
Unveiling a quantum critical point in the phase diagram of
iron-based superconductors
P7
F. Hammerath1, P. Bonfà2, S. Sanna1, G. Prando3, R. De Renzi2, Y. Kobayashi4,
M. Sato4, P. Carretta1
1. Department of Physics, University of Pavia-CNISM, I-27100 Pavia, Italy
2. Department of Physics, University of Parma-CNISM I-43124 Parma, Italy
3. Leibniz-Institut fur Festkorper- und Werkstoffforschung(IFW) Dresden,
D-01171 Dresden, Germany
4. Department of Physics, Division of Material Sciences, Nagoya University, Furo-cho,
Chikusa-ku, Nagoya 464-8602, Japan
A superconducting-to-magnetic transition is reported for LaFeAsO0.89F0.11 (La1111)
where a per-thousand amount of Mn impurities is dispersed. By employing local
spectroscopic techniques like muon spin rotation (μSR) and nuclear quadrupole
resonance (NQR) on compounds with Mn contents ranging from x = 0.025% to x
= 0.75 %, we find that the electronic properties are extremely sensitive to the Mn
impurities [1]. In fact, a small amount of Mn as low as 0.2% suppresses superconductivity completely, an order of magnitude lower than the Mn content required to
quench superconductivity in Sm1111 and in Nd1111. Static magnetism, involving
the FeAs planes, is observed to arise for x > 0.1% and becomes further enhanced
upon increasing Mn substitution. Also a progressive increase of low-energy spin
fluctuations, leading to an enhancement of the NQR spin-lattice relaxation rate 1/
T1, is observed upon Mn substitution. The analysis of 1/T1 for the sample closest
to the crossover between superconductivity and magnetism (x = 0.2%) points toward the presence of an antiferromagnetic quantum critical point around that doping level for La1111. Moreover, magnetization measurements under hydrostatic
pressure for x around 0.1% evidence that a sizeable increase of the superconducting transition temperature can be achieved at moderate pressures. These results are
discussed in the framework of recent theoretical models which consider the effect
of Mn substitutions on the wave-vector describing the spin fluctuations [2] .
[1] F. Hammerath et al., Phys. Rev. B 89, 134503 (2014).
[2] R.M. Fernandes and A.J.Millis, Phys. Rev. Lett. 110, 117004 (2013).
Corresponding author: Pietro Carretta, Department of Physics University of PaviaVia Bassi 6,
27100 Pavia (Italy) Email: pietro.carretta@unipv.it
66
Posters
Optical excitation of phase modes in strongly disordered
superconductors
P8
T. Cea1, D. Bucheli1, G. Seibold2, L. Benfatto1, and C. Castellani1
1. ISC-CNR Dep. of Physics, “Sapienza” University of Rome, Italy
2. Institut Fur Physik, BTU Cottbus, Cottbus-Senftenberg, Germany
According to the Goldstone theorem the breaking of a continuous U(1) symmetry
comes along with the existence of low-energy collective modes. In the context
of superconductivity these excitations are related to the phase of the superconducting (SC) order parameter and for clean systems are optically inactive. Here
we show that for strongly disordered superconductors phase modes acquire a dipole moment and appear as a subgap spectral feature in the optical conductivity.
This finding is obtained with both a gauge- invariant random-phase approximation
scheme based on a fermionic Bogoliubov-de Gennes state as well as with a prototypical bosonic model for disordered superconductors. In the strongly disordered
regime, where the system displays an effective granularity of the SC properties,
the optically active dipoles are linked to the isolated SC islands, offering a new
perspective for realizing microwave optical devices.
In the last decades the failure of the BCS paradigm of superconductivity in several
materials led to a profound modification of the description of the superconducting
(SC) phenomenon itself. A case in point is the occurrence of Cooper pairing and
phase coherence at distinct temperatures, associated respectively with the appearance of a single-particle gap ∆ and a non-zero superfluid stiffness Ds. This behavior is observed, e.g., in high-temperature cuprate superconductors [1, 2], stronglydisordered films of conventional superconductors [3–8] and recently also in SC
heterostructures [9]. In all these materials the BCS prediction that Ds is of order of
the Fermi energy, much larger than ∆ Tc, is violated due to the strong suppression
of Ds. The resulting scenario, supported by systematic tunneling measurements,
suggests that pairing survives above Tc, leading to a pseudogap state dominated by
phase fluctuations enhanced by the low Ds value. [10]
In all this, optics represents a preferential playground to address the peculiar role
of disorder. Indeed, as we show in this Communication, disorder renders collective modes - optically inactive in a clean superconductor - visible. By analyzing
a prototype fermionic model, the attractive Hubbard model with on-site disorder[11–16], we reveal that thanks to the breaking of translational invariance the
collective modes couple to light via an intermediate particle-hole excitation process. Most remarkably, this coupling leads to the emergence of additional optical absorption, mainly due to phase modes, below the BCS-like threshold for a photon to
break apart a Cooper pair, in agreement with recent experimental observations[17,
18]. Deeper insight into the nature of this disorder-induced optical response is then
gained through a comparison with the XY model in transverse random field. Within
this effective bosonic description of disordered superconductors[19, 20] we show
explicitly how the local inhomogeneity of the superfluid stiffness leads to a finite
electric dipole for the phase modes. At strong disorder, where the system segregates into SC islands of tens of nanometers[5, 8, 15, 16] and the SC dc current
flows along preferential percolative SC paths[16], the finite-frequency optical absorption occurs in the isolated SC regions, thanks to the presence of a finite phase
difference between the opposite sides of the island. This nano-scale selective optical effect, that we propose to test via microscopic imaging[21], can have a strong
impact on the design of devices based on superconducting microresonators[22].
Posters
67
67
Nanoscale electrochemical investigation on SDC thin films for
μ-SOFCs applications
P9
N. Yang1, A. Kumar2, S. Doria1, S. Jesse2, A. Tebano1, A. Belianinov2,
E. Strelcov2, E. Di Bartolomeo3, T. M. Arruda2, S. Licoccia3, G. Balestrino1,
S. V. Kalinin2 and C. Aruta1
1. CNR-SPIN and Department DICII, University of Roma “Tor Vergata”, Rome, 00133 (Italy).
2. The Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak
Ridge, Tennessee 37831 (United States)
3. Department of Chemical Sciences and Technologies, University of Roma
“Tor Vergata”, Rome, 00133 (Italy).
The growing demand for miniaturized systems for energy conversion and storage
has prompted extensive research aimed at fabricating solid-state ionic devices in
thin-film form. Electrochemical devices, such as solid oxide fuel cells (SOFCs),
can be miniaturized for portable applications (μ-SOFCs).[1] This goal requires
reducing the operating temperature and producing components as thin films.[2]
Samarium doped cerium oxide (SDC) possess larger ionic conductivity than yttria-stabilized zirconia (YSZ), the conventional SOFC electrolyte material, and
are thus being intensively studied with the aim of reducing the SOFC operating
temperature. The potential applications of this material is based on the ceria ability
to store and release oxygen vacancies due to the easily accessible oxidation states
of cerium ions (Ce3+ and Ce4+). Particularly, when Ceria is doped with Sm, oxygen
vacancies are formed due to the Ce3+ and Ce4+ formation.
Recently, a new scanning probe microscopy technique, named electrochemical
strain microscopy (ESM) was developed. This technique can be very useful to
investigate of the electrochemical activity at the nano-scale in ionic or proton conductors. [3]
In the present talk I will show our study on the electrochemical activity of pure
CeO2, 10% Sm doped CeO2 and 20% Sm doped CeO2 epitaxial thin films. The
study was performed to investigate the effect of the different doping concentrations
on transport mechanism in terms of the different type of charge carriers [4]. Water
absorption may affect the surface deformation depending on the Sm-doping. In the
case of pure CeO2 we demonstrated the tip-induced electrochemical reactions as
mediated by electronic currents. 20% Sm doped CeO2 nanocrystalline films were
also investigated in order to understand the role of granularity and porosity on the
oxygen ion or proton conduction [5].
[1] U. P. Muecke, D. Beckel, A. Bernard, A. Bieberle-Hutter, S. Graf, A. Infortuna, P.
Muller, J. L. M. Rupp, J. Schneider, L. J. Gauckler, Adv. Funct. Mater. 2008, 18, 3158.
[2] A. Bieberle-Hu¨tter, J. L. Hertz, H. L. Tuller, Acta Mater. 2008, 56, 177.
[3] A. Kumar, F. Ciucci, A.N. Morozovaska, S.V. Kalinin, S. Jesse, Nature Chemistry, 2011, 3,707.
[4] Nan Yang, S. Doria, A. Kumar, Jae Hyuck Jang, T. M. Arruda, A. Tebano, S. Jesse,
Ilia N.Ivanov, A. P. Baddorf, E. Strelcov, S. Licoccia, A. Y. Borisevich, G. Balestrino and
S. V. Kalinin. IOP Nanotechnology 25 (2014) 075701.
[5] S. Doria, Nan Yang, A. Kumar, S. Jesse, A. Tebano, C. Aruta, E. Di Bartolomeo, S.
V.Kalinin, T. M. Arruda, S. Licoccia, G. Balestrino, Applied Physics Letters 103(2013) 171605.
Nan Yang address: Via Politecnico n.1, Rome, Italy. e-mail: nan.yang@uniroma2.it
68
Posters
Fluctuation diamagnetism in Ba(Fe1-xRhx)2As2 iron-based
superconductors
P10
L. Bossoni1, L. Romanò2, P. C. Canfield3, A. Lascialfari4
1. Department of Physics, University of Pavia-CNISM, Via Bassi 6, I-27100 Pavia, Italy
2. Department of Physics, University of Parma, Parco Area delle Scienze 7A, I-43100
Parma, Italy
3. Ames Laboratory US DOE and Department of Physics and Astronomy, Iowa State
University, Ames, IA 50011, USA
4. Dipartimento di Fisica and INSTM, Università degli studi di Milano, Via Celoria 16,
I-20133 Milano, Italy
Within the study of superconductivity, the possibility that liquid vortex excitations
survive above Tc, leading to local superconductivity remains a key open problem,
despite the variety of experimental and theoretical contributions [1-3]. Among all
the experimental tools, the measure of the magnetization is the most straightforward as it gives a clear diamagnetic response Mfl, as soon as the Cooper pairs are
formed.
Another relevant feature of the occurrence of superconducting fluctuations (SF) is
the presence of an upturn field Hup in the isothermal curves of the fluctuating magnetization Mfl. In fact, while the size of fluctuating pairs ξ grows when the temperature approaches Tc, the fluctuating magnetization shows a progressive increase.
On the contrary, very high magnetic fields must quench the superconducting fluctuations. The combination of the two effects leads the isothermal magnetization
curves to exhibit an upturn, the value of which is, for layered superconductors in
the framework of the GL phenomenology, of the order of Φ0/ξ2. Therefore in highTc superconductors, and iron-pnictides, the upturn could be possibly detected only
at very strong fields, even for temperatures close enough to Tc.
In this study, we measured the static uniform spin susceptibility of Ba(Fe1-xRhx)2As2
iron-based superconductors, over a broad range of doping (0.041≤ x≤ 0.094) and
magnetic fields. At small fields (H < 4000 Oe) we observed, above the transition
temperature Tc, the occurrence of precursor diamagnetism, which is not ascribable
to the Ginzburg-Landau theory, while our data fit to a phase fluctuation model,
which has been successful in interpreting a similar phenomenology occurring in
the high-Tc cuprate superconductors [2]. In this framework, unconventional SF are
supposed to develop into precursor superconducting islands, where the amplitude
of the order parameter is frozen, while the long-range phase coherence associated with the bulk superconducting state is prevented by strong fluctuations of the
phase. Finally, when strong fields are applied, the unconventional fluctuating diamagnetism is suppressed, whereas the usual Ginzburg-Landau scenario is restored,
and the experimental findings agree with the 3D XY universality class.
[1] P. Carretta et al., PRB 61, 12420 (2000).
[2] L. Romanò et al., PRL 94, 247001 (2005).
[3] O. Cyr-Chionière et al., Nature 458, 743 (2009).
Corresponding author: Dr. Lucia Bossoni, Department of Physics University of PaviaVia Bassi 6,
27100 Pavia (Italy) Email: lucia.bossoni@unipv.it
Posters
69
69
Vortex Lattice Melting of a NbSe2 single grain probed by
Ultrasensitive Cantilever Magnetometry
P11
L. Bossoni , P. Carretta , M. Poggio
1. Department of Physics, University of Pavia-CNISM, Via Bassi 6, I-27100 Pavia, Italy
2. Department of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel,
1
1
2
Switzerland.
Using dynamic cantilever magnetometry, we studied the Abrikosov vortex lattice
and its corresponding melting transition, in a micrometer-size crystallite of superconducting NbSe2 [1].
Cantilever magnetometry benefits from the nm-µm size of the sample, and the application of fields ranging from the mT up to the Tesla range, a combination that
is rather difficult to achieve with other experimental techniques [2]. In addition,
the high softness of the cantilever allows for the detection of the weak magnetic
response of micro - and nanometer-scale samples.
While working in the sample-on-cantilever geometry, we explored the mechanical properties of the superconducting micro-particle, by systematically sweeping
either the magnetic field, or the temperature, thus reconstructing the mixed phase
diagram of the compound.
We found that the cantilever dissipation increases as the sample enters the vortex
liquid phase, from the normal phase. Moreover the cantilever resonance frequency
increases as the sample undergoes the transition to the solid vortex phase. This
increase reflects the stiffening of the effective cantilever spring constant due to the
magnetization of the fixed vortex lines. In addition, as the vortices solidify, their
hopping correlation time becomes long with respect to the oscillation frequency of
the cantilever. As a result, the cantilever’s low dissipation state is restored along
with the stiffening of its spring constant.
We interpreted the cantilever dissipation in terms of thermally activated vortex
creep motion, and we found the pinning energy barriers, which result in good
agreement with transport measurements on bulk samples [3].
Moreover the solution of the equation of motion turns out in an analytic expression
for the vortex state magnetization, which depends on the measured parameters.
The temperature and field dependence of the cantilever energy dissipation and oscillation frequency reveal the energy barrier of the pinning mechanism, as a function of the field.
Such results show that the ultrasensitive cantilever magnetometry is an effective
technique for measuring the properties of vortex lattice in micro- and nanometerscale samples, and that its results are directly comparable with macroscopic techniques. Finally, this approach allows describing the phase diagram of the crystallite, and is applicable to other micro- or nanometer-scale superconducting samples.
[1] L. Bossoni et al., Appl. Phys. Lett. 104, 182601 (2014).
[2] P. L. Gammel et al., Phys. Rev. Lett. 61, 1666 (1988).
[3] S. D. Kaushik et al., Pramana, Jour. of Phys. 71, 1335 (2008).
Corresponding author: Dr. Lucia Bossoni, Department of Physics University of PaviaVia Bassi 6,
27100 Pavia (Italy) Email: lucia.bossoni@unipv.it
70
Posters
X-ray absorption spectra at the oxygen K-edge of iron doped
P12
zirconia (Zr1-xFexO2-y): ab-initio simulations and experimental results
D. H. Douma1, A. Lamperti1, R. Ciprian3, P. Torelli3, P. Lupo2, E. Cianci1,
D. Sangalli1, F. Casoli2, F. Albertini2, L. Nasi1, and Alberto Debernardi1
1. Laboratorio MDM, IMM-CNR, 20864 Agrate Brianza (MB), Italy
2. CNR-IMEM, Parco Area delle Scienze 37/A, 43124 Parma (PR), Italy
3. Laboratorio TASC, IOM-CNR, S.S. 14 km 163.5, Basovizza, I-34149 Trieste, Italy
We present an ab-initio simulation and experimental study of X-ray Absorption Near
Edge Structure (XANES) of oxygen K-edge in iron doped zirconia (Zr1-xFexO2-y)
with different Fe dopant concentration (from x~0.06 up to x~0.25). Our target is to
determine the structural and magnetic ordering at low and room temperature of this
dilute magnetic oxide [1] to be used as building block in novel spintronic devices
[2]. We found that substituting Zr by Fe atoms conduces to a radical change in
the oxygen XANES spectrum, especially in the pre-edge region where a pre-edge
peak appears. This pre-edge peak is ascribed to dipole transitions from O 1s to O
2p states that are hybridized with the unoccupied 3d states of iron as demonstrated
previously in reference [3] in the case of perovskite-like alloy LaB1-xGaxO3 (B=Fe,
Mn). Our results reveal the increase of the pre-edge peak when Fe concentration
increased as observed experimentally, suggesting the increase of unoccupied Fe
3d states. By comparing theoretical and experimental data, we demonstrated that
the effect of Fe atoms is localized in the first shells surrounding each Fe atom and
disappears as one moves far from them. According to our first principle spectra,
the increase of the pre-edge peak with increasing Fe concentration is related to an
increase of Fe atoms simultaneously associated to the decrease of the number of
oxygen atoms located in the shells of order higher than two.
[1] J.M.D. Coey, Current Opinion in Solid State and Materials Science,10 (2): 83-92,
2006.
[2] Jairo Sinova and Igor Zutic, Nature materials, 11 (5): 368-371, 2012.
[3] S. Lafuerza, G Subias, J Garcia, S. Di Matteo, J. Blasco, V. Cuartero, and C. R Natoli. Journal of Physics: Condensed Matter, 23(32): 325601, 2011.
Corresponding author: Alberto Debernardi,
Laboratorio MDM, IMM-CNR, 20864 Agrate Brianza (MB), Italy,
e-mail:alberto.debernardi@mdm.imm.cnr.it , Tel:+39 039-6037804
Posters
71
71
Disappearance of nodal gap across the insulator–superconductor
P13
transition in a copper-oxide superconductor
Y. Y. Peng1, J. Q. Meng1, C. T. Chen2, Z. Y. Xu2, T. K. Lee3, X.J. Zhou1
1. National Laboratory for Superconductivity, Beijing National Laboratory for
Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences,
Beijing 100190, China.
2. Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing
100190, China.
3. Institute of Physics, Academia Sinica, Nankang, Taipei 11529, Taiwan.
The parent compound of the copper-oxide high-temperature superconductors is a
Mott insulator. Superconductivity is realized by doping an appropriate amount of
charge carriers. How a Mott insulator transforms into a superconductor is crucial
in understanding the unusual physical properties of high-temperature superconductors and the superconductivity mechanism. Here we report high-resolution angleresolved photoemission measurement on heavily underdoped Bi2Sr2–xLaxCuO6+δ
system. The electronic structure of the lightly doped samples exhibit a number of
characteristics: existence of an energy gap along the nodal direction, d-wave-like
anisotropic energy gap along the underlying Fermi surface, and coexistence of a
coherence peak and a broad hump in the photoemission spectra. Our results reveal a clear insulator–superconductor transition at a critical doping level of ~ 0.10
where the nodal energy gap approaches zero, the three-dimensional antiferromagnetic order disappears, and superconductivity starts to emerge. These observations
clearly signal a close connection between the nodal gap, antiferromagnetism and
superconductivity1.
[1] Y. Y. Peng et al., Nat. Commun. 4, 3459 (2013).
Corresponding author: Yingying Peng, Dipartimento di Fisica, Politecnico di Milano, Piazza
Leonardo da Vinci 32, 20133 Milano –ITALY, yingying.peng@polimi.it
72
Posters
Interface confinement and phase separation instability in the
electron gas of oxide heterostructures
P14
N. Scopigno, D. Bucheli, S. Caprara, and M. Grilli
Physics Dept., Università di Roma `Sapienza’
The 2D electron gas (EG) formed at the interface of two insulating oxides like
LaAlO3/SrTiO3 or LaTiO3/SrTiO3 is a remarkable system displaying many physical effects, like magnetic moment formation, gate-driven superconductivity, and
strong Rashba spin-orbit coupling. There are many experimental evidences that
this 2DEG is strongly inhomogeneous on mesoscopic scales ([1-3] and references
therein).
As for every two-dimensional system, disorder is expected to play a relevant role
in these interfaces with defects and impurities acting as an extrinsic mechanisms
to trigger inhomogeneity.
In our work [4] we propose a different intrinsic mechanism, which can be present
even in a perfectly clean and expectedly homogeneous system rendering the 2DEG
inhomogeneous by electronic phase separation.
We consider a finite width potential well in which the 2DEG is confined and we
take into account the quantization of the electronic motion perpendicular to the
interface and the anisotropy of the effective masses of the bands in the STO side,
where the EG mostly resides. The specific features of this heterostructure possibly
leading to an electronic phase separation instability are the huge field-dependent
dielectric constant of STO and the sizable effective mass (and the related density
of states) in some directions of the planar electronic bands.
We calculate by a standard self-consistent Schroedinger-Poisson approach the
structure of the confining potential well and of the electron chemical potential finding that in rather generic cases the electronic compressibility becomes negative.
Owing to the sizable Rashba spin-orbit coupling in these materials, we also investigate its interplay with the self-consistent potential finding that the phase separation
instability is made even easier.
[1]S. Caprara, M. Grilli, L. Benfatto, and C. Castellani, Phys. Rev. B 84, 014514 (2011);
D. Bucheli, S. Caprara, C. Castellani, and M. Grilli, New J. Phys. 15, 023014 (2013)
[2]J. Biscaras et al., Nature Mater. 12, 542 (2013)
[3] S. Caprara et al., Phys. Rev. B 88, 020504(R) (2013)
[4] N. Scopigno, D. Bucheli, S. Caprara, and M. Grilli, in preparation
Niccolò Scopigno – Università ‘La Sapienza’, Dipartimento di Fisica (stanza 339c),
niccolo.scopigno@roma1.infn.it
Posters
73
73
Pressure dependent critical temperature on KFe2As2
F. Ricci and G. Profeta
P15
CNR-SPIN and Dipartimento di Scienze Fisiche e Chimiche,
Università degli Studi dell’Aquila, Italy
In KFe2As2 it was observed that the superconducting critical temperature has a
minimum as a function of the pressure [1]. Recently, the same Tc behavior was
discovered in CsFe2As2 [2]. This intriguing and unique effect among iron-based
superconductors, was ascribed to a changing of the symmetry of the order parameter with pressure.
As deduced from experiments, the gap symmetry would evolve from a d-wave type
below the critical pressure Pc (~ 1.7 GPa in KFe2As2), to an s± above it, probably
through an intermediate s+id state.
However, the symmetry of the gap was not measured and the influence of the
pressure-dependent gap symmetry on Tc is not yet completely demonstrated and,
up today, different hypotheses are still under debate.
In order to understand the evolution of the structural and electronic properties with
pressure, we performed first principle density functional study of the pressure phase
diagram of KFe2As2 considering paramagnetic and antiferromagnetic phases.
We show that the variation of the density of states at the Fermi level with pressure,
calculated in the paramagnetic phase, could justify the trend of Tc, thus excluding
a possible change of the pairing symmetry [3].
However, a different scenario emerges if we consider a strong coupling origin of
the superconducting phase. Indeed, we discovered a magnetic phase transition between different antiferromagnetic orders (checkerboard and collinear stripe) and
the relative evolution of the Heisenberg exchange constants across Pc. These results
would support a change in the pairing symmetry.
We will conclude the talk with an overview of the state-of-the-art of this particular
problem.
[1] F. F. Tafti et al., Nat. Phys. 9, 349 (2013).
[2] F. F. Tafti et al., Phys. Rev. B 89, 134502 (2014).
[3] F. Ricci et al., in preparation.
Corresponding author: Fabio Ricci, Via Vetoio, 10, 67100 L’Aquila (Italy).
e-mail: fabio.ricci@aquila.infn.it
74
Posters
Bulk charge modulations in optimally doped copper
oxide superconductors
P16
G. Dellea1, A. Amorese1, L. Braicovich1, C. Mazzoli1, Y. Peng1,
M. Hashimoto2, W.-S. Lee2, B. Moritz2, T. P. Devereaux2, Z.-X. Shen2,
K. Kummer3, N. B. Brookes3, Y. Yoshida4, H. Eisaki4, Z. Hussain5, M. Salluzzo6,
M. Minola7, G. Logvenov7, M. Le Tacon7, B. Keimer7, E. Schierle8, E. Weshke8,
G. Ghiringhelli1
1. CNR-SPIN, CNISM and Dipartimento di Fisica, Politecnico di Milano, Italy
2. Stanford Univ., SLAC National Accelerator Laboratory, California, USA
3. European Synchrotron Radiation Facility, Grenoble, France
4. Nanoelectronics Research Institute, AIST, Ibaraki, Japan
5. Advanced light source, Lawrence Berkeley National Lab, Berkeley, USA
6. CNR-SPIN, Napoli, Italy
7. Max Planck Institut für Festkörperforschun, Stuttgart, Germany
8. Helmholtz Zentrum Berlin für Materialen und Energie, Berlin, Germany
In the search of the basic mechanism of high Tc superconductivity, the understanding of possibly coexisting and competing ordering phenomena has recently acquired an increasingly importance. Since the first direct observation [1] of a collective charge mode (CCM) in the “123” copper oxides family, efforts have been
devoted to the search of charge modulations in a wide class of compounds and by
means of different techniques. In this scenario, resonant x-ray scattering (RXS)
has played a fundamental role, leading to the discovery of CCM peaks as possible
general features of underdoped cuprates:
YBa2Cu3O6+x (YBCO) [1], NdBa2Cu3O6+x (NBCO) [1], Bi2Sr2CaCu2O8+x (Bi2212)
[2], Bi1.5Pb0.55Sr1.6La0.4CuO6+x (Bi2201) [3] and HgBa2CuO4+x (Hg1201) [4].
Surface sensitive techniques [2-3-6-7] have also suggested that bulk charge modulation can be present not only in the plateau of the Tc(p) phase diagram, but also
close to the summit of the superconducting dome. Using resonant inelastic x-ray
scattering (RIXS) we have indeed found evidence of such bulk charge modulation
in optimally doped Bi2212 [5]
and, more recently, in a wider class of optimally doped copper oxides: NBCO,
YBCO and La2-SrxCuO4 (LSCO). These results demonstrate the presence of charge
modulation over a large region of the pseudogap regime, not constrained to the underdoped phase as thought initially but extending at least up to the optimal doping.
Whether these charge modes are competing or cooperating with the superconductivity is still an unresolved issue.
[1] G. Ghiringhelli et al., Science 337, 821 (2012).
[2] E. H. d. S. Neto et al., Science 343, 393 (2014).
[3] R. Comin et al., Science 343, 690 (2014).
[4] W. Tabis et al., arXiv:1404.7658.
[5] M. Hashimoto et al., arXiv:1403.0061.
[6] Y. He et al., Science 344, 608 (2014).
[7] K. Fujita et al., Science 344, 612 (2014).
Greta Dellea, Ph.D. student - Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da
Vinci 32, I-20133 Milano, Italy Phone (Milano): (+39) 02 2399 6189, e-mail: greta.dellea@polimi.it
Posters
75
75
Comparison of superconducting properties of MgB2-based
conductors manufactured with different techniques
P17
G. Bovone1,2, D. Nardelli3, D. Matera1,2, M. Vignolo2, C. Bernini2
and A. S. Siri1
1. Università degli studi di Genova, Physics Department (DIFI), Via Dodecaneso, 33 16146 Genova, Italy
2. 2 Istituto SPIN-CNR, Corso Perrone, 24 - 16152 Genova, Italy
3. 3 Columbus Superconductors SpA, Via delle Terre Rosse, 30 - 16133 Genova, Italy
MgB2 has been considered as one of the most utilized superconducting material
for actual applications, thanks to its interesting properties. Superconducting and
physical properties of MgB2 are strongly dependent on boron powders used to
perform synthesis. Different ways of tube filling have been developed to enhance
performances of MgB2 powder and tapes, especially critical current density at high
magnetic fields, and to overcome disadvantages of typical in-situ and ex-situ PIT
processes.
In our effort so far we have been able to develop a new boron synthesis procedure
that has proven to be competitive with commercial one for ex-situ application. As
an extension of previous work here we explore the idea to compare MgB2 synthesize from “lab-made” boron with commercial ones, using different tube filling
technique.
Here we report a systematic study on superconducting properties of different MgB2
wire, manufactured by following different procedures and heat treatments at different temperatures, by using lab-made boron as precursor. Physical and chemical
properties we investigated are magnetic and transport critical current density, residual resistivity, critical temperature and powder morphology. Different sintering
(or synthesis) temperature and time have been performed for a wider knowledge.
76
Posters
Modifications in the doping status of Bi-2212 via X-ray
exposure with nanometric resolution
P18
M. Truccato1, A. Pagliero1, L. Mino2, E. Borfecchia2, A. Agostino2,
L. Pascale2, E. Enrico3, N.De Leo3, C. Lamberti2, G. Martínez-Criado4
1. Dept. of Physics and NIS, University of Torino, Torino, Italy
2. Dept. of Chemistry and NIS, University of Torino, Torino, Italy
3. INRIM, National Institute of Metrological Research, Torino, Italy
4. Experiments Division, ESRF, Grenoble Cedex, France
We report the effects of a 17 keV synchrotron radiation nano-beam with space
resolution of 117 × 116 nm2 on the oxygen doping level of Bi-2212 superconducting single crystals. We have succeeded in obtaining both structural and electrical
information from the same crystals as a function of irradiation up to a maximum
dose of about 3 × 1012 Gy [1]. It turns out that a monotonic behavior exists with a
maximum increase in the critical temperature Tc of 1.3 K and a maximum elongation of about 1 Å in the c-axis length, compared to the as-grown single crystals.
These phenomena unambiguously testify a change in the Bi-2212 doping level,
which shows close similarity to the appearance of a two-dimensional electron gas
(2DEG) under the surface of SrTiO3 because of the desorption of surface oxygen
atoms induced by exposure to intense UV synchrotron light [2].
Modeling our different experimental conditions by means of the finite element
method allowed us to exclude heating induced by the X-ray nano-beam as a possible cause for the change in the doping level. Correspondingly, an important role
is suggested to be played by X-ray-induced photoelectrons as a possible source of
knock-on for the O atoms in the BiO layers due to their low displacement threshold energy, which has already been predicted for the O atoms in the CuO chains of
YBCO [3].
These results support the possible use in the future of hard X-rays for a novel
direct-writing, photoresist-free lithographic process to fabricate superconducting
devices, with potential nanometric resolution and 3D capability.
[1] Alessandro Pagliero et al., Nano Lett., 14, 1583-1589 (2014)
[2] W. Meevasana et al., Nature Mater., 10, 114-118 (2011)
[3] Ibrahin Piñera et al., Phys. Stat. Sol. A, 204, 2279–2286 (2007)
Corresponding author: Marco Truccato -Department of Physics, University of Torino
Via P. Giuria 1, I-10125, Torino, Italy
phone: +39-011-6707374, e-mail: marco.truccato@unito.it
Posters
77
77
Synthesis and physical properties of Ca1-xRExFeAs2
with RE = La~Gd
P19
A. Sala1,2, H. Ogino2, H. Yakita2, T. Okada2, A. Yamamoto2, K. Kishio2,
S. Ishida3, A. Iyo3, H. Eisaki3, M. Fujioka4, Y. Takano4, F. Caglieris1, M. Putti1
and J. Shimoyama2
1. Physics Department, University of Genova, via Dodecaneso 33, I-16146 Genova,
Italy
2. Department of Applied Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo,
Tokyo 113 8656, Japan
3. National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba,
Ibaraki 305-8565, Japan
4. National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki
305-0047, Japan
[Introduction]
Recently new 112 type iron-based superconductors – Ca1-xRExFeAs2 have been reported for RE = La[1] and Pr [2]. This structure has a monoclinic space group
P21/m, and is composed of two Ca(RE) planes, anti-fluorite Fe2As2 layers, and As2
zigzag chain layers. The highest Tc reported for this new phase is 43 K for a Sbdoped Ca0.85La0.15FeAs2 sample [3]. Various studies on these new superconductors
are needed in order to investigate their superconducting properties. In this study we
report the synthesis and characterization of Ca1-xRExFeAs2 compounds with RE =
Ce, Nd, Sm, Eu and Gd.
[Experimental]
The Ca1-xRExFeAs2 samples were synthesized by the high-pressure method. Phase
identification was carried out by XRD. Magnetization properties were examined
by a SQUID magnetometer and electrical resistivity was measured by the AC fourpoint-probe method using Physical Property Measurement System.
[Results]
112 phase was successfully formed in all the samples synthesized under moderately high pressure ~2 GPa, small silvery crystals embedded in a uniform opaque
black matrix in the bulk samples. From the XRD analyses the samples are composed of Ca(RE)–112 phase with FeAs and FeAs2 impurity phases. Nd, Sm, Eu and
Gd doped samples exhibited diamagnetism suggesting superconductivity, while
the Ce doped sample showed a paramagnetic like behavior down to 2 K without
any traces of superconductive transition. The Tc(onset)s evaluated for the Nd, Sm, Eu and
Gd doped samples are 13.5 K, 15 K, 10 K and 14 K, respectively. Any clear trend
of Tc varying the RE doping is not observable probably due to difference in actual
doping levels of RE in the 112 crystals among these samples.
[1] N. Katayama et al., J. Phys. Soc. Jpn. 82 (2013) 123702
[2] H. Yakita et al., J. Phys. Soc. Jpn. 82 (2013) 123702
[3] Journal of the Physical Society of Japan 83, 025001 (2014)
Corresponding author: Alberto Sala Physics Department, University of Genova, via Dodecaneso
33, I-16146 Genova, Italy
78
Posters
Defect states in SrTiO3-δ gap probed by resonant photoemission
P20
spectroscopy
G. Drera1, G. Salvinelli1, A. Verdini2, L. Floreano2, A. Cossaro2, L. Sangaletti1
1. I-LAMP and Dipartimento di Matematica e Fisica, Università Cattolica, Via dei
Musei 41, 25121 Brescia, Italy
2. IOM-CNR, TASC Laboratory, S.S.14 Km 163,5 Basovizza 34149 Trieste, Italy
Strontium titanate (SrTiO3 or STO) is a model perovskite oxide with a large impact
in science and technological application. When doped with Nb it became conductive and even superconductive [1] at low temperature; moreover, it is one of the
most common substrate for epitaxial oxides growth and thus can be used as a substrate for complex heterojunctions systems.
STO transport properties are tightly bound to the presence of defects, oxygen vacancies and to external mechanism, such as elemental doping, light exposure and
interface effects. In conductive STO-based heterostrucures, such as in the 2D-electron gas at LaAlO3-SrTiO3 interface (LAO-STO), it is often difficult to separate the
different doping sources.
In most cases, the STO interesting physics properties are marked by the presence
of a small contribution of electronic states in the gap of bulk STO, which cannot be
directly probed with standard photoemission technique (XPS or UPS). However,
these Ti related 3d states can be successfully probed by exploiting the photoelectron resonant enhancement at the 2p-3d absorption threshold. For instance, this
method has been used to probe in-gap states in LaAlO3-SrTiO3 heterostructures [2],
superconductivity in Nb doped [1] SrTiO3-δ and point defects in TiO2 [3,4].
In this work, we present a characterization of SrTiO3-δ for various oxygen substoichiometry level (δ), carried out with resonant and standard photoemission spectroscopy (ResPES and XPS) and X-ray absorption spectroscopy (XAS). The defect
density has been tuned by in vacuum annealing (at 700 °C) and oxygen exposure
cycles.
The comparison of XPS Ti2p core levels, XAS spectra and resonant valence band
measurements can help to discriminate among oxygen vacancies and external
doping sources in oxide based heterostructures such as in the LAO-STO case.
1 - G. Binnig et al., Phys. Rev. Lett. 45, 1352(1980)
2 - G. Drera et al., App. Phys. Lett. 98, 052907 (2011)
3 - G. Drera et al., J. of Phys. Cond. mat. 25, 075502 (2013)
4 - G. Drera et al, Appl. Phys. Lett. 97 (2010), 012506
email: g.drera@dmf.unicatt.it
Posters
79
79
Shielding of magnetic field by superconducting and
hybrid systems
P21
L. Gozzelino1,2, R. Gerbaldo1,2, G. Ghigo1,2, F. Laviano2,3, A. Pagliero2,3,
M. Truccato2,3, A. Agostino2,4, E. Bonometti 4, R. Rabezzana 4
1. Department of Applied Science and Technology, Politecnico di Torino, Torino, Italy
2. I.N.F.N., Sezione di Torino, Torino, Italy
3. Department of Physics, University of Torino, Torino, Italy
4. Department of Chemistry, Università di Torino, Torino, Italy.
Magnetic field shielding is crucial for several applications requiring an ultralow
magnetic field environment (e.g. biomedical application) or the mitigation of the
magnetic field produced by an electronic device to guarantee electromagnetic compatibility with the surrounding environment or to reduce the device electromagnetic signature (e.g. magneto-resonance imaging equipment or military applications).
Magnetic shielding is also mandatory when the exposure to magnetic fields means
possible health hazards (e.g. cardio-phatic patient and workers safety).
In this framework we investigated both experimentally and numerically the efficiency of passive MgB2 superconducting (SC) shields for dc magnetic field mitigation. We considered two geometries: disks and cups, both subjected to a uniform
external magnetic field parallel to their axis. Samples were grown by microwave
assisted Mg-Reactive Liquid Infiltration technique in boron pre-forms [1], technique allowing the easy production of objects with given shape and size, in order to match the application requirements. Shielding capability was measured as
a function of temperature, external magnetic field, position and time by means of
cryogenic Hall probes mounted on a custom-designed stage, moveable along the
sample axis with micrometric resolution [2].
The tuning of the shielding potential of the SC samples was then investigated by
coupling them to coaxial Armco iron ferromagnetic (FM) shield. These hybrid systems result the most efficient solutions at higher magnetic field, where the SC/FM
coaxial cups allow obtaining a shielding efficiency 3-4 times higher than that measured with a single SC cup, coupled with a reduction of the flux creep phenomena
[2,3]. On the contrary at low field the only superconducting solution is the most
efficient [2].
Starting from these experimental results, numerical simulations were performed on
several SC and hybrid systems in order to check how they better match the shielding requirement of different applications. In particular, possible configurations
suitable to mitigate the magnetic field of the SR2S magnet [4] inside the spacecraft
environment are investigated.
Work partially supported by INFN under SR2S-RD experiment.
[1] A.Agostino et al., Mat. Res. Innovat. 8, 75 (2004)
[2] L.Gozzelino et al., Supercond. Sci. Technol. 25, 115013 (2012)
[3] L.Gozzelino et al., IEEE Trans. Appl. Supercond. 23, 8201305 (2013)
[4] R.Musenich et al., IEEE Trans. Appl. Supercond. 24, 4601504 (2014)
Corresponding author: Laura Gozzelino - Department of Applied Science and Technology,
Politecnico di Torino, corso Duca degli Abruzzi 24, 10129 Torino – e-mail: laura.gozzelino@polito.it
80
Posters
Exploring the LaAlO3-SrTiO3 interface with angle-resolved X-ray
P22
photoemission spectroscopy: experiments and modelling of
cation interdiffusion and substitution effects
G. Salvinelli1, G. Drera1, A. Giampietri1, A. Brinkman2, M. Huijben2, L. Sangaletti1
1. Interdisciplinary Laboratories for Advanced Materials Physics and Dipartimento di
Matematica e Fisica, Università Cattolica, via dei Musei 41, 25121 Brescia, Italy
2. MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE
Enschede, Netherlands
LaAlO3 (LAO) and SrTiO3 (STO) are both non-magnetic and insulating oxides.
Nevertheless the LAO-STO interface exhibits a quasi-2D electron gas (2DEG) [1],
along with superconductivity and magnetic effects for suitable growth conditions.
Interestingly, the 2DEG is observed only for LAO film thicker than four unit cells.
The charge polarity discontinuity that occurs at the interface induces a diverging
potential (the so-called polar catastrophe) that drives the system to an intrinsic
electronic reconstruction yielding the 2DEG [2]. Although this scenario is generally agreed the mechanisms that make it actual are still matter of debate. In fact,
the quality of the LAO-STO interface is crucial for the onset of the conductivity. So
far this interface has been chemically mapped using cross-sectional scanning transmission electron microscopy with electron energy loss spectroscopy and effects of
cation interdiffusion have been evidenced for conducting and insulating samples
[3]. A less direct probe of the interface structure was provided by the analysis of
X-ray scattering data showing La deficiency and atomic vacancies in the film [4]
or cation interdiffusion through the interface [5].
In this study insulating and conducting interfaces have been probed by angle-resolved X-ray photoemission spectroscopy. Data have been collected in the 0°-70°
electron emission angular range for each polar scan and the intensity was averaged
on the azimuthal angle thereby minimizing the sharp modulations due to X-ray photodiffraction effects. A Monte-Carlo code for the photoelectron depth distribution
function (DDF) was properly implemented in a computational software [6] aimed
to model the intensity attenuation with angle and retrieve the correct stoichiometry
profile. The experimental results are compared with the profiles obtained for abrupt
interface models, as well as by considering interdiffusion or cation substitution effects. In this case, the DDF modelling is sensitive enough to address the refinement
towards specific cation substitution effects, disclosing a novel possibility to probe
non-destructively the stoichiometry profile across the interface.
[1] A. Ohtomo, et al., Nature 427 (2004 ) 423
[2] N. Nakagawa, et al., Nat. Mat. 5 (2006) 204
[3] M. P. Warusawithana, et al., Nat. Comm. 4 (2013) 2351
[4] V. Vonk, et al., PRB 85 (2012) 045401
[5] P. R. Willmott, et al., PRL 99 (2007) 155502
[6] G. Drera, G. Salvinelli and L. Sangaletti, http://centridiricerca.unicatt.it/ilamp
Corresponding author: Gabriele Salvinelli - g.salvinelli@gmail.com
Posters
81
81
Topological Order in a One-Dimesional Spin-Orbital Model
W. Brzezicki1,2, J. Dziarmaga3, A. M. Oles3,4
1. Dipartimento di Fisica ‘E.R. Caianiello’, Università di Salerno,
P23
I-84084 Fisciano (Salerno), Italy
2. SPIN-CNR, I-84084 Fisciano (Salerno), Italy
3. Marian Smoluchowski Institute of Physics, Jagellonian University, Reymonta 4,
PL-30059 Kraków, Poland
4. Max-Planck-Institut für Festkörperforschung,Heisenbergstrasse 1, D-70569
Stuttgart, Germany
We present rigorous topological order which emerges in a one-dimensional spinorbital model due to the ring topology [1]. The model is similar to the recently
studied model describing spin-orbital separation in Sr2CuO3 [2]. We show that although this model with SU(2) spin and XY orbital interactions is known to exactly
separate spins from orbitals by means of a unitary transformation U on the open
chain, they are not quite independent when the chain is closed, and the spins form
two half-rings carrying opposite quasimomenta. On changing the topology from an
open to a periodic chain, the degeneracy of the ground state is partly lifted and the
spin-orbital entanglement [3] is enhanced, while the low-energy excitation have
a quadratic dispersion as a function of the total quasimomentum. This novel type
of topological order which emerges from changing the topology from an open to
a periodic chain is reminiscent of the infinite-U Hubbard chain. We prove that the
order exhibits truly topological robustness such that the spin state with opposite
quasimomenta does not change if any of the bonds is rescaled by any positive factor which means that the system can be squeezed or stretched without affecting the
order.
This work is supported by the Polish National Science Center (NCN) under Projects
No. 2012/04/A/ST3/00331 and No. 2013/09/B/ST3/01603.
[1] W. Brzezicki, J. Dziarmaga and A. M. Oles’, Phys. Rev. Lett. 112, 117204 (2014).
[2] J. Schlappa et al., Nature (London) 485, 82 (2012).
[3] A.M. Oles’, J. Phys.: Condensed Matter 24, 313201 (2012).
Corresponding author: wbrzezicki@unisa.it
82
Posters
Anomalous Josephson Effect in S-NW-S junctions
G. Campagnano1,2, P. Lucignano1,2, D. Giuliano2,3, A.Tagliacozzo1,2
1. Dipartimento di Fisica, Università di Napoli Federico II, Via Cintia, 80126
P24
Napoli, Italy
2. CNR-SPIN, Via Cintia, 80126, Napoli, Italy
3. Dipartimento di Fisica, Universita` della Calabria & I.N.F.N., Arcavacata di Rende
I-87036, Cosenza, Italy
Recently semiconducting nanowires realised with InAs or InSb have attracted a lot
of attention due their narrow gap and strong spin-orbit interaction.
We theoretically study a short Josephson junction formed by a nanonwire between
to conventional superconductors.
We solve the scattering problem and calculate the Andreev spectrum from the S
matrix.
We show that by carefully tuning the spin-orbit interaction and an applied magnetic
field one can observe a zero-pi transition and, more interesting, also more unusual
states where the free energy is minimum at a phase difference between the two
superconductors in the interval zero-pi.
Corresponding author G.Campagnano
g.campagnano@gmail.com
Posters
83
83
Doping and critical-temperature dependence of the energy
gaps in Ba(Fe1-xCox)2As2 thin films
P25
P. Pecchio , D. Daghero , F. Laviano , K. Iida , F. Kurth , V. Grinenko ,
R. S. Gonnelli1
1. Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, Torino, Italy
2. Leibniz Institute for Solid State and Materials Research, Dresden, Germany
1
1
1
2
2
2
We present the results of the study of the superconducting gaps in epitaxial
Ba (Fe1-xCox)2As2 thin films with different Co nominal content x (0.04 < x < 0.15),
from the heavily underdoped to the overdoped region of the phase diagram, by
means of point-contact Andreev-reflection spectroscopy (PCARS). The very highquality thin films were grown by pulsed laser deposition on (001) CaF2 substrates
[1] and characterized by means of XRD, RHEED, AFM, FESEM, EDX, transport
and magnetic measurements [2]. The superconducting gaps were determined by
fitting the PCARS spectra (taken in different regions of each film) with a twodimensional, two-band Blonder-Tinkham-Klapwijk model [3]. A successful fit was
obtained by using two nodeless, isotropic gaps—although the possible presence of
gap anisotropies cannot be completely excluded [4].
The two gap amplitudes Δ S and Δ L follow similar monotonic trends of a function
of the local critical temperature TAC from 25 K down to 8 K. The values obtained
here were compared to those given by other techniques and reported in literature
[5-6]. As a function of the doping content, the gaps qualitatively follow the trend
of the critical temperature, i.e. the shape of the superconducting dome in the phase
diagram. We analyzed these results within the three-band Eliashberg model. The
results turn out to be perfectly compatible with a mechanism of superconducting
coupling mediated by spin fluctuations, whose characteristic energy scales with TC
according to the empirical law Ω0 = 4.65kBTC. A reduction of the electron-boson
coupling strength is observed in the overdoped regime, which can be rationalized
as being related to the suppression of spin fluctuations in this region of the phase
diagram.
1.
2.
3.
4.
5.
6.
F. Kurth et al., Appl. Phys. Lett. 102, 142601 (2013).
D.Daghero et al. Appl. Surf. Sci., in press.
D.Daghero, R. S. Gonnelli, Supercond.Sci.Technol. 23, 043001 (2010).
P. Pecchio et al., Phys. Rev. B 88, 174506 (2013).
M. Tortello et al., Phys. Rev. Lett. 105, 237002 (2010).
K.Terashima et al., Proc. Natl.Acad. Sci. U.S.A. 106, 7330 (2009).
We acknowledge the financial support of the European Community through the Collaborative EU Japan Project “IRON SEA” (NMP3-SL-2011-283141).
Paola Pecchio, Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino Corso Duca
degli Abruzzi, 24 - 10129 Torino (Italy)
tel: +39 011 5647350\7381 - email: paola.pecchio@polito.it
84
Posters
Characterization of multiferroic BiFeO3 polycrystalline thin films
P26
grown by RF sputtering
A. Giampietri and G. Drera
I-LAMP and Dipartimento di Matematica e Fisica, Università Cattolica, Via dei Musei
41, 25121 Brescia, Italy
Multiferroics are materials which can display ferroelectricity, ferromagnetism, ferroeleasticity simultaneously. Multiferroics have already attracted much attention
due to the promising multifunctional device applications, such as magneto-electric
sensor devices and memories spintronics [1,2]. BiFeO3 is the only single-phase
multiferroic material at room temperature, with an high ferroelectric Curie temperature (TC = 1103 K) and high Neel temperature (TN = 643 K) [3]. The BiFeO3 thin
films are usually grown by PLD (pulsed laser deposition), MBE (molecular beam
epitaxy) and rarely by sputtering techniques [4,5], which are the cheapest and the
more promising deposition methods for the industrial growth of devices.
In order to be suitable for applications, sputtered polycrystalline film must show
the same properties of high quality epitaxial films, both in term of crystal structure,
secondary phases, magnetic and electric properties. The film properties can also be
tailored by after-growth treatment, such post-annealing, for which only few works
are reported in literature.
In this work, a detailed characterization of BiFeO3 thin films grown on a Si substrate by RF magnetron sputtering is reported, carried out with a set of electron
spectroscopy and microscopic techniques (XPS, Raman spectromicroscopy, AFM
and PFM). The homogeneity of the thin films, both grown in vacuum with different substrate temperature and post-annealed ex situ, was evaluated with the AFM
technique and the Raman microscopy. The temperature-dependent Raman analysis
also allowed us to estimate the ideal growth conditions which are needed to obtain
a BiFeO3 perovskite single phase and to avoid the Fe2O3 and Bi2O3 secondary
phase, while the analysis of core-level photoemission data allowed us to evaluate
the thickness of the secondary phase layers which are eventually present over the
BiFeO3 layer. The PFM technique was finally used in order to verify the ferroelectric response of the thin films.
1 - W. Eerenstein et al., Nature 442:759–65 (2006)
2 - C. Catalan et al., Advanced Materials 21:2463–85 (2009)
3 - H. Yan et al., Materials Letters 111, 123-125 (2013)
4 - R.Y. Zheng et al., Journal of Applied Physics 101, 054104 (2007)
5 - Jian et al. Nanoscale Research Letters, 8:297 (2013)
email: g.drera@dmf.unicatt.it
Posters
85
85
Electronic correlations in multi-orbital materials
L. Fanfarillo and E. Bascones
P27
Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC),
Sor Juana Inés de la Cruz 3, Campus Cantoblanco, Madrid, E-28049 Spain
The Mott transition, at which a material predicted to be metallic by conventional
band theory becomes insulator, is one of the most dramatic effects of interactions.
In the case of single orbital systems with local interactions (Hubbard model) the
Mott transition takes place when the interaction U is larger than a critical value and
the orbital is half-filled. The Mott transition can be tuned with pressure (changing
U) or doping. Away from half-filling a clean system is metallic but correlated, being the band mass larger as closer the system is to half-filling. On the other hand
several orbitals have to be considered in most correlated materials. In multi-orbital
systems the Mott transition happens not only at half-filling but at any commensurate filling. New interactions, like Hund’s coupling, play a role in determining the
strength of correlations in iron pnictides, ruthenates and many oxides. It is already
known that depending on orbital filling Hund’s coupling can promote or suppress
metallicity.
Closer inspection shows that even for a given filling Hund’s coupling can enhance
or decrease correlations. After an introduction, in this presentation I will focus on
our recent results on the different effect which Hund’s coupling has on the induced
correlations for a given orbital filling. I will discuss the energy scales which control
this behavior and the resulting mass enhancement. I will then show the dependence
of these interaction scales on doping. Conmensurate and non-conmensurate fillings
will be discussed. Examples will include cases in which the t2g, the eg, or all the
five-d orbitals are relevant.
If time allows I will talk about some specific consequences of this physics on the
properties of iron pnictides and/or the interplay between Hund’s coupling and spinorbit interactions relevant for 5d oxides, like iridates.
Laura Fanfarillo laura.fanfarillo@icmm.csic.es
86
Posters
Polaron Dynamics in multiferroic GaFeO3 revealed by
Anelastic Spectroscopy and Dielectric spectroscopy
P28
F. M. Vitucci1, F. Cordero2, F. Craciun2, A. Paolone1, O. Palumbo1,
J-C Soret3, G. Gruner3 and F. Trequattrini4
1. CNR-ISC, U.O.S. La Sapienza, Piazzale A. Moro 5, 00185 Roma, Italy.
2. CNR-ISC, Area di Ricerca di Roma-Tor Vergata, Via del Fosso del Cavaliere, 100
00133 Roma, Italy.
3. Université François Rabelais, Parc de Grandmont, FR-37200 Tours, France.
4. Dipartimento di Fisica, Sapienza Un.Roma, P.le A.Moro 5, 00185 Roma, Italy
The original definition of multiferroics describes materials exhibiting simultaneously two or more among ferroelectric, ferromagnetic, ferrotoroidic or ferroelastic
ordering in a common temperature range. This definition has been enlarged also to
antiferroic material following the principle to include all systems which present at
least two order parameters. The search for these materials has been driven by the
perspective of achieving new forms of multifunctional devices controlling charges
by applied magnetic fields and spins by applied voltages [1]. For this reason many
study focus on the interaction between electronic and magnetic degrees of freedom. From the theoretical point of view the interest in these materials arises from
the complex physics due to the interplay of different order parameters [2].
The present study was undertaken in order to investigate the interaction between
elastic and magnetic order in multiferroic GaFeO3 (GFO) by means of anelastic
spectroscopy (AS) and dielectric spectroscopy (DS). GFO exhibits piezoelectricity
and ferrimagnetism [3] and is considered to be a very promising material because it
presents high magnetic transition temperature and relatively high magnetoelectric
coupling. The AS and DS are extremely sensitive tools to investigate the elastic
and polar degrees of freedom, and therefore possible influences from the magnetic
transition mediated by multiferroic couplings.
We investigate two GFO samples, one nominally stoichiometric and one annealed
to 950 °C. In both cases an intense thermally activated relaxation process appears
with an activation energy of 0.29 eV, which masks the expected effects from the
magnetic transition. The relaxation process can be slightly reduced in intensity by
high temperature annealing, suggesting hopping of small polarons at the Fe sites,
possibly associated with off stoichiometric oxygen.
Corresponding author: Ph.D. Francesco M. Vitucci - Istituto Sistemi Complessi ISC / CNR UOS
Sapienza - Dipartimento di Fisica, edificio “G. Marconi”, Università di Roma “La Sapienza” P.le Aldo Moro 5, 00185 Roma, Italy - tel: +39 - 06 – 49914400 Email: francesco.vitucci@roma1.infn.it
Posters
87
87
XAS measurements and linear dichroism at the Cu L2,3 and
O K edge on infinite layer e-doped superconductors
P29
A. Galdi1, P. Orgiani1, L. Maritato1, P. Torelli2, B. N. Gobaut3,
D. G. Schlom4, J. W. Harter5, K. M. Shen5
1. DIEM and CNR SPIN u.o.s. Salerno, Università degli Studi di Salerno, Fisciano
(SA), Italy
2. CNR Istituto Officina dei Materiali (IOM), Laboratorio TASC, Trieste, Italy
3. Elettra Sincrotrone Trieste S.C.p.A., Trieste, Italy
4. Department of Materials Science and Engineering and Kavli Institute at Cornell for
Nanoscale Science, Cornell University, Ithaca NY USA
5. Laboratory of Atomic and Solid State Physics, Department of Physics, Cornell University, Ithaca NY USA
Among the cuprate superconductors, the infinite layer compound SrCuO2 is the
one with the simplest crystal structure that can be electron doped (e-doped) by La
substitution (Tc≈40K for x=10%). Many difficulties arise in comparing hole (hdoped) and e-doped cuprates with layered structure, as the parent compounds of
the two families do not share the same crystal structure. Many differences are observed between the two families, such as different magnetic order in the undoped
compounds, stronger antiferromagnetism in the e-doped, more unconventional
transport properties in h-doped, different band structure. However it is not clear
whether these differences are intrinsic of superconductivity in these compounds.
In this respect, infinite layer compounds are interesting, due to their minimal structure, not involving the (magnetic!) rare earth charge reservoir blocks of the layered
compounds (such as (Nd,Ce)2CuO4).
We present x-ray absorption spectroscopy (XAS) measurements at the Cu L2,3 and
O K edge on (Sr,La)CuO2 thin films grown by MBE. The measurements have been
performed by synchrotron radiation at the APE beamline of ELETTRA lightsource.
Samples with different La and O content have been measured at different temperatures ranging from room temperature down to 30K, using linear polarized photons.
Strong linear dichroism is observed at both edges, according to the square planar coordination of Cu ions. In h-doped compounds the O K pre-edge is strongly
modulated with doping, as the oxygen states are influenced by the density of states
at the Fermi level, thus reflecting the deployment of the upper Hubbard band into
the conduction band. On the contrary no particular doping dependence is usually
observed in e-doped compounds, both layered and infinite layers. However we find
some pre-edge modification with La and O content of the samples. The results are
interpreted in terms of hole and electron doping of the compound by either oxygen
defects or La.
A. Galdi, Dipartimento di Ingegneria Informatica, Ingegneria Elettrica e Matematica Avanzata,
Università degli Studi di Salerno. E-mail: agaldi@unisa.it
88
Posters
Low field magnetoresistance at room temperature in
La0.7Sr0.3MnO3 thin films
P30
L. Méchin1, S. Flament1, P. Perna1,2
1. GREYC (UMR 6072) CNRS ENSICAEN Université de Caen Basse-Normandie
14050 Caen cedex, France
2. IMDEA-Nanoscience, c/ Faraday, 9, Ciudad Universitaria de Cantoblanco 28049
Madrid, Spain
La0.7Sr0.3MnO3 (LSMO) is a ferromagnetic material with a Curie temperature of
360 K. We previously showed that substrates with various orientations can induce
different in- plane anisotropies and different magnetization reversal mechanisms
[1, 2]. In particular, 10° vicinal SrTiO3 (STO) substrates induce an uniaxial magnetic anisotropy with the easy axis aligned with the step edge direction, in contrast
to the biaxial anisotropy observed on STO (001) substrates. The low field magnetoresistance (LFMR) has been measured at room temperature in both LSMO / STO
(001) and LSMO / 10° vicinal STO (001) samples of various dimensions, with
magnetic field applied parallel or perpendicular to the length of the strips. Anisotropic magnetoresistive (AMR) effects are reported in LSMO / STO (001) samples, which can be attributed to AMR inside domain walls (DW) [3]. In addition,
LFMR measurements in LSMO / STO (001) performed in LSMO trips of various
lengths showed interesting positive magnetoresistive peaks, which are attributed
to magnetoresistance of DWs. The measured magnetoresistance showed different
behaviour in vicinal LSMO films. LFMR measurements are analysed and commented thanks to magneto-optical Kerr (MOKE) images of the in-plane magnetic
domain arrangement.
[1] P. Perna, C. Rodrigo, E. Jiménez, F. J. Teran, N. Mikuszeit, L. Méchin, J. Camarero,
and R. Miranda J. Appl. Phys. 110 013919 (2011)
[2] P. Perna, L. Méchin, M. Saïb, J. Camarero, S. Flament, New Journal of Physics 12
103033 (2010)
[3] D. Fadil, S. Wu, P. Perna, B. Renault, M. Saïb, S. Lebargy, J. Gasnier, B. Guillet, J.M. Routoure, S. Flament, L. Méchin, J. Appl. Phys. 112 013906 (2012)
Corresponding author: L. Méchin, GREYC (CNRS UMR6072), ENSICAEN, 6 boulevard
Maréchal Juin, 14050 CAEN cedex, France, laurence.mechin@ensicaen.fr
Posters
89
89
Optimum Wannier orbitals for the Hubbard model and
Gutzwiller functionals
P31
V. Brosco1, Z. Ying1,2 , J. Lorenzana1
1. CNR & Università di Roma ``La Sapienza”, Italy.
2. Beijing Computational Science Research Center, China
Density Functional Theory (DFT) has long been the ubiquitous starting point for
electronic structure computations in solid-state physics and quantum chemistry.
Most numerical methods based on DFT, however, fail in systems at or close to the
Mott insulating regime, when the energy associated to the tunneling of electrons becomes small compared with the typical electron-electron repulsion energies. Even
in the simple case of the H2 molecule, the commonest DFT approaches perform
reasonably well at the equilibrium distance, when electrons have substantial tunneling among the atoms, but they fail in the molecular analog of the Mott regime,
when each electron is localized on one H atom. Starting from simple examples, in
this talk we illustrate the key features a density functional should have to describe
the Mott transition. We then turn our attention to study the mapping between continuum and lattice models and we provide a method to find the “best” Wannier orbitals to write the lattice Hamiltonian whose ground state properties are eventually
addressed within a generalized variational Gutzwiller approach. In certain simple
cases, the whole procedure yields a reduced density matrix functional which can
be readily used. Retaining the simplicity of minimal-basis models but using the
“best” Wannier orbitals, the approach described in this talk may provide a natural
extension of Gutzwiller theory and other lattice methods to the continuum.
Corresponding author: Valentina Brosco, Dipartimento di Fisica, Università di Roma “
La Sapienza”, P.le A. Moro, 5 (00185) Roma, Italy - email: valentina.brosco@roma1.infn.it
90
Posters
Electrodynamics of (LaNiO3)n/(LaMnO3)2 superlattices
P. Di Pietro1, J. Hoffman2, A. Bhattacharya2, S. Lupi3, A. Perucchi1
1. INSTM Udr Trieste-ST and Sincrotrone Trieste, Area Science Park, I-34012
P32
Trieste, Italy
2. Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439
3. CNR-IOM and Dipartimento di Fisica, Università di Roma Sapienza, P.le Aldo Moro
2, I-00185 Roma, Italy
In the last years there has been a great interest in the novel electronic and magnetic
properties observed at the interfaces between different complex oxides materials.
These properties arise from both the character of oxygen ions and the correlation
of electrons. At the interfaces of these structures a charge redistribution and a resulting reconstruction of the orbital and spin degrees of freedom (electronic reconstruction) produces a new two-dimensional state.
(SrMnO3)n/(LaMnO3)2n superlattices (SL) have been recently studied [1]. In their
interfaces the alternation of layers of the insulating antiferromagnets SrMnO3
and LaMnO3 leads to the formation of a two-dimensional ferromagnetic metallic
state.
Here, we have studied the optical properties of four films of (LaNiO3)n/(LaMnO3)2
SL (n=2, 3, 4, 5) on SrTiO3 substrates. We have measured the reflectivity at different
temperatures from 20 K to 400 K, and extracted the optical conductivity through a
model-independent fitting of the reflectivity. The two compounds of the alternated
layers in the SL show a very different conductivity. LaMnO3 is an insulator since
its conductivity is almost zero in the IR range. LaNiO3, on the contrary, is a metal
displaying a sharp Drude peak and a background conductivity in the mid-IR. For
increasing n, the (LaNiO3)n/(LaMnO3)2 SL undergoes an insulator-to-metal transition. The presence of a broad mid-IR band shows that the (LaNiO3)n/(LaMnO3)2 SL
is not a pure combination of the LaMnO3 and LaNiO3 conductivities, and confirms
the mixing of the valence on Mn and Ni sites [2].
[1] A. Perucchi et al., Nanoletters 10, 4819 (2010).
[2] J. Hoffman et al., Phys. Rev. B 88, 144411 (2013).
Corresponding author : INSTM UdrR Trieste-ST and Elettra – Sincrotrone Trieste S.S. 14 km
163.5 in AREA SCIENCE PARK 34149 Basovizza, Trieste, Italy
paola.dipietro@elettra.trieste.it
Posters
91
91
Evidence of high-Tc superconductivity in optimally doped
tetragonal YBCO micro-crystals
P33
A. Agostino1, L. Pascale1, A. Pagliero2, M. Truccato2, L. Operti1
1. Department of Chemistry and NIS, University of Turin, Turin, Italy
2. Department of Physics and NIS, University of Turin, Turin, Italy
In the YBa2Cu3O7-x (Y-123) system, the orthorhombic phase is superconducting
due to the formation of ordered chains that act as dopants for the two-dimensional
CuO2 planes. In the tetragonal ordering, these chains are absent thus inhibiting the
charge transfer from the CuO2 planes and making this phase insulating [1].
Within this context, we have investigated the effect of chemical cationic subtitutions as Al, Sb, Te and Ca [2], of anionic doping (O2) [3] and of X-ray nanobeam
irradiation [4] on both the electrical and the structural properties of HTSC microcrystals. These micro-crystals, characterized by a high structural ordering, low defects concentration and excellent superconducting features, represent ideal candidates for the study and the design of THz devices based on intrinsic Josephson
Junctions.
In this work, we present some structural and superconducting modifications induced on Y-123 by chemical doping with both Ca, Al and Te. In particular, we
propose a fully ordered tetragonal crystal structure, with space group P4/mmm,
which is obtained from the orthorhombic ordering of YBa2Cu3O6.5 but with the
chains running along the a axis as well as along the b axis. In this way, the Cu(1)
atoms in the plane of the chains have twofold, square-planar fourfold, and sixfold
coordinations, in contrast to the purely square-planar coordinations in orthorhombic YBa2Cu3O7.
This structure, previously invoked by Gupta et al. [5], results from single crystal Xray diffraction and turns out to be superconducting from R vs T characterization.
[1] J. D. Jorgensen et al., Phys Rev. B, 41(4), 1863-1877, 1990
[2] F. Bertolotti et al., Acta Cryst., B70, 2014
[3] M. M. Rahman Khan et al., Supercond Sci Technol., 22, 085011, 2009
[4] A. Pagliero et al., Nano Lett., 14, 1583-1589, 2014
[5] R. P. Gupta et al., Phys Rev. B, 47(5), 2795-2800, 1993
Corresponding author: Department of Chemistry, Via Pietro Giuria 7, I-10125 Turin,
email: angelo.agostino@unito.it
92
Posters
Interpretation of the temperature dependence of resistivity in
Fe-based superconductors: single crystals and thin films.
P34
S. Galasso1, G.A. Ummarino1, P. Pecchio1, D. Daghero1, R.S. Gonnelli1
F. Kurth2, K. Iida2, B. Holzapfel2, A. Sanna3
1. Politecnico di Torino, C.so Duca degli Abruzzi, 24 Torino (Italy)
2. Leibniz-Institut für Festkörper-und Werkstoffforschung (IFW) Dresden, P.O.Box
270116, 01171 Dresden, Germany
3. Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, D-06120 Halle, Germany
A study of the temperature dependence of the resistivity of Fe-superconductors
belonging to different families is presented. The experimental data can be reproduced very well within a general phenomenological model containing different
kinds of carriers, distinguished by their transport properties. Good results have
been obtained in particular for LiFeAs [1] and Co-doped Ba-122 [2, 3] confirming
the importance of the multiband nature of these compounds also in the normal state
and supporting the idea that the spin fluctuations play an important role in transport
phenomena.
Furthermore, comparisons are made between the resistivity curves of single crystals and thin films of the same compounds. In the latter case some effects of the
strain of the substrate can be observed: Not only the critical temperature changes
from that of crystals [4], but also the temperature dependence results to be very different. In particular, for Ba-122 with 10% of cobalt content [5], we show that the
shape of the resistivity in thin films can be explained within the same model used
for single crystals, provided that one includes a hardening of the electron-boson
(spin-fluctuation) spectral function. This could be qualitatively explained considering the interaction of spin fluctuations with phonons and their modification under
pressure or strain [6].
Eventually, gathering up all the results, and considering also data available in literature concerning other compounds [7], we observe that the electron-boson coupling
in the superconducting state is much stronger than that in the normal state while
the representative bosonic energy increases a lot in the normal state. Such a disparity between normal and superconducting state (found also in cuprates [8]) can be a
unifying principle at the root of superconductivity in the iron-based materials.
[1] G.A. Ummarino et al., Phys. C 492 , 21-24 (2013).
[2] Lei Fang et al., Phys. Rev. B 80 , 140508(R) (2009).
[3] G.A. Ummarino et al., arXiv:1403.0815 (2014).
[4] K. Iida et al., Appl. Phys. Lett. 95 , 192501 (2009).
[5] F. Kurth et al., Appl. Phys. Lett. 102 , 142601 (2013).
[6] K. W. Kim et al., Nature Materials 11 , 497-501 (2012).
[7] A.A. Golubov et al., JETP Lett. 94 , 333-337 (2011).
[8] E.G. Maksimov et al., Advances in Condensed Matter Physics 2010, 1-65 (2010).
Sara Galasso: sara.galasso@polito.it - Politecnico di Torino, C.so Duca degli Abruzzi 24, Torino (Italy)
Posters
93
93
Chemical pressure in novel doped rare-earth manganites
P35
G. G. Simeoni1, S. Geprägs2, M. Opel2, A. Mancini3, L. Malavasi3
1. MLZ – TUM, Lichtenbergstr. 1, 85747 Garching, Germany
2. Walther-Meissner-Institut, Bayerische Akademie der Wissenschaften, Garching, Germany
3. Dipartimento di Chimica, Sezione di Chimica Fisica, Università di Pavia, Pavia, Italy
Manganese-oxide perovskites RMnO3 (R=rare earth) are flexible frameworks for
the development of novel functional materials. Over the past years the doping with
bivalent ions in the form R1-xAxMnO3 (A= Ca,Sr,Ba) led namely to the discovery
of the colossal magnetoresistant (CMR) effect [1,2]. Aiming at combining it with
the huge ferroelectric polarisation observed in the metastable orthorhombic HoMnO3 (o-HoMnO3)[3], which would make way for important applications in spintronics and microelectronic devices, we synthesized a novel family of hole-doped
orthorhombic rare-earth manganese oxides in the form Ho1-xCaxMnO3 . Many
general features can be understood in the framework of a doped anti-ferromagnet
(AFM) at finite temperature, described by the Hubbard or the t-J model [4].
Here we focus on the role played by the chemical pressure on the electronic, magnetic and nuclear degrees of freedom. Specifically, we report about magnetization
and transport measurements, as well as inelastic neutron scattering (INS) and quasi-elastic neutron scattering (QENS) experiments carried out at the time-of-flight
spectrometer TOFTOF at the FRM II. Pure HoMnO3 exhibits hexagonal crystal
structure. The doping with bigger bivalent ions stabilizes the Jahn-Teller distorted
orthorhombic structure, with lattice parameters close to those of the metastable
o-HoMnO3. Moreover, the presence of Ca2+ introduces both electronic and magnetic holes. The first ones are responsible for an unusual metal-insulator transition
(MIT) [5], whereas the second ones are most likely to be at the origin of the magnetic polarons and the polaron liquid-to-glass transition detected by QENS experiments [6]. Their existence, already predicted for electron-doped CaMnO3 [7], is
compatible with Zhang-Rice S=3/2 quadruplets [8].
Our results, beside confirming many theoretical predictions, provide a new insight
into the universality of the relationship between CMR effect, MIT and polaron
glass transition [9].
[1] S.W.Cheong and M.Mostovoy, Nature Materials 6 (2007) 13-20
[2] A. Moreo et al, Science 283 (1999) 2034
[3] S.M.Feng et al, New Journal of Physics 12 (2010) 073006
[4] E. Dagotto, Nanoscale Phase Separation and Magnetoresistance, Springer 2003
[5] M. Imada et al., Rev. Mod. Phys. 70 (1998) 1039
[6] G.G.Simeoni et al., in preparation
[7] H. Meskine and S. Satpathy, J. of Phys.: Cond. Matter 17 (2005) 1889-1906
[8] F.C. Zhang, T.M.Rice, Phys. Rev. B 37 (1988)
3759; E. Dagotto, Rev. Mod. Phys. 66 (1994) 763
[9] C.P.Adams et al, PRL 85 (2000) 3954; D.N.Argyriou et al, PRL 89 (2002) 036401
Corresponding author: giovanna.simeoni@frm2.tum.de
MLZ Heinz Maier-Lebnitz Zentrum and Department of Physics, TUM
Lichtenbergstr.1, 85747 Garching (Germany) 0049-89-28914975
94
Posters
Surface Nanostructures in Manganite Films studied by
Scanning Tunneling Microscopy
P36
A. Gambardella1, P. Graziosi1, I. Bergenti1, M. Prezioso2, D. Pullini3,
S. Milita4, F. Biscarini5 and V. A. Dediu1
1. Istituto per lo studio dei materiali Nanostrutturati-Consiglio Nazionale delle
Ricerche (ISMN-CNR), Via Gobetti 101 Bologna, 40129 (Italy)
2. Department of Electrical and Computer Engineering, University of California Santa
Barbara, CA 93106, (USA)
3. Centro Ricerche FIAT, Strada Torino 50 Orbassano, 10043 (Italy)
4. Istituto per lo studio dei Materiali Magnetici-Consiglio Nazionale delle Ricerche
(IMM-CNR), Via Gobetti 101 Bologna, 40129 (Italy)
5. Dipartimento di Scienze della Vita, Università degli Studi di Modena e Reggio
Emilia, Via Campi 183 Modena, 41100 (Italy)
Complex magnetic oxides display intriguing bulk and surface properties that make
these materials very promising candidates for spintronics and other magnetic applications1. However, controlling material stoichiometry and homogeneity is extremely difficult, especially at the nanoscale. Scanning tunneling microscopy and
spectroscopy are powerful tools for characterising the nanoscale properties of surface electronic states, acquiring local information without significant lateral averaging; however, the information obtained to date is sparse and does not provide
a general or quantitative understanding of manganite surface features. To try to
overcome these deficits, we investigated ultrathin films of the prototypical manganite La0.7Sr0.3MnO3 (LSMO)2, unambiguously imaging the atomic structures.
Our results provide new insights into the surface properties of manganites as well
as suggesting a new mechanism for the film growth of such complex materials3.
We detected a minority phase dispersed on a stoichiometric crystalline matrix; this
phase consisted of small (less than tens of nm2) non-stoichiometric islands that
represent a kinetic intermediate of single-layer growth. We propose a phenomenological mechanism for the formation of such defects suggesting that they are characteristic features of all manganite films, although the detection of these defects is
mainly obstructed at greater thicknesses.
1. M. Bibes and A. Barthélémy, Oxide spintronics. IEEE T. Electron. Dev. 54, 1003-1023
(2007).
2. P. Graziosi et al. Thin Solid Films 534, 83-89 (2013).
3. A. Gambardella et al Scientific Reports (Accepted 2014)
Corresponding author: Alessandro Gambardella, CNR - ISMN, Consiglio Nazionale delle
Ricerche - Istituto per lo Studio dei Materiali Nanostrutturati, v. Gobetti 101, 40129, Bologna,
Italy, a.gambardella@bo.ismn.cnr.it
Posters
95
95
Spin Hall and Edelstein effects in metallic thin films:
Interfacial spin-orbit effects
P37
J.Borge1, C. Gorini2, G. Vignale3 and R. Raimondi1
1. Dipartimento di Matematica e Fisica, Università Roma tre, Via della Vasca Navale
84, Rome, Italy
2. Service de Physique de l’Etat Condensè, CNRS URA 2464, CEA Saclay, F-91191
Gif-sur-Yvette, France and
3. Department of Physics and Astronomy, University of Missouri, Columbia MO 65211, USA
A normal metallic film sandwiched between two insulators may have strong spinorbit coupling near the metal-insulator interfaces, even if spin-orbit coupling is
negligible in the bulk of the film. Two deeply interconnected effects arise from this
interfacial spin-orbit coupling in metallic films. The first is the spin Hall effect,
and the second is the Edelstein effect. At variance with strictly two-dimensional
Rashba systems, we find that the spin Hall conductivity doesn’t neglects even if
spin- orbit interaction with impurities is neglected and “vertex corrections” are
properly taken into account. Even more remarkably, such finite value becomes
“universal” in a certain configuration. This is a direct consequence of the spatial
dependence of spin-orbit coupling on the third dimension, perpendicular to the
film plane. The non-vanishing spin Hall conductivity has a profound influence on
the Edelstein effect because we find an “anomalous” new term in the Edelstein
conductivity proportional to it. Our results, although derived in a specific model,
should be valid rather generally, whenever a non-strictly two-dimensional Rashba
spin-orbit coupling type is present.
E-mail:borge@fis.uniroma3.it
96
Posters
Terahertz reflectivity study of a superconducting
FeSe0.5Te0.5 thin film
P38
B. Joseph1,2, A. Perucchi3, S. Caramazza1, M. Autore1, E. Bellingeri4,
M. Putti5, S. Kawale4, C. Ferdeghini4, S. Lupi1, P. Dore6
1. Dip. di Fisica, Università di Roma Sapienza, Piazzale Aldo Moro 2, I-00185 Rome, Italy
2. ELETTRA-Sincrotrone Trieste, Area Science Park, I-34012 Trieste, Italy
3. INSTM Udr Trieste-ST and Sincrotrone Trieste, Area Science Park, I-34012 Trieste, Italy
4. CNR-SPIN, Corso Perrone 24, I-16152 Genova, Italy
5. Dip. di Fisica, Università degli Studi di Genova,Via Dodecaneso 33, I-16146
Genova, Italy
6. CNR-SPIN and Dip. di Fisica, Università di Roma Sapienza, Piazzale Aldo Moro 2,
I-00185 Rome, Italy
An infrared (IR) spectroscopy study of a 200 nm thick FeSe0:5Te0:5 film grown on
LaAlO3 with Tc = 13.7 K is reported. In particular, the 20 K normal state absolute
reflectance RN measured up to 8000 cm-1, and, the superconducting state reflectance ratio RS/RN, RS being the 6 K reflectance, is measured in the terahertz range
down to 12 cm-1. A two band model can coherently describe the spectra both in
the normal state and in the superconducting sate. Normal state have two Drude
components, of which one is much broader and intense than the other. In the superconducting state, a gap Δ = 37.3 cm-1 opens in the narrow Drude band, while no
such gap opening can be detected at least in the explored frequency range for the
broad Drude band [1]. We discuss our results in comparison with the available IR
results [2-4] and the local lattice and electronic inhomogeneities [5-7] observed in
the FeSe0:5Te0:5 system.
[1]A. Peruchi, B. Joseph,S. Caramazza, M. Autore, E. Bellingeri, M. Putti, S. Kawale,
C. Ferdeghini, S. Lupi, P. Dore (in preparation)
[2]C. C. Homes, et al, Physical Review B 81, 180508 (2010)
[3]C. Mirri, et al, Superconductor Science and Technology, 25, 045002 (2012)
[4]A. Pimenov, et al, New Journal of Physics, 15, 013032 (2013)
[5]X. He, et al, Physical Review B 83, 220502 (2011)
[6]B. Joseph, et al, Physical Review B 82, 020502 (2010)
[7]U. R. Singh, et al, Physical Review B 88, 155124 (2013)
Corresponding author: Boby Joseph, ELETTRA-Sincrotrone Trieste, Area Science Park, I-34012
Trieste, Italy (boby.joseph@elettra.eu)
Posters
97
97
The Hubbard model beyond the two-pole approximation:
a Composite Operator Method study
P39
Adolfo Avella
Dipartimento di Fisica “E.R. Caianiello” and Unità CNISM di Salerno,
Università degli Studi di Salerno, I-84084 Fisciano (SA), Italy
CNR-SPIN, UoS di Salerno, I-84084 Fisciano (SA), Italy
Within the framework of the Composite Operator Method, a three-pole solution for
the two-dimensional Hubbard model is presented and analyzed in detail [1]. In addition to the two Hubbard operators, the operatorial basis comprises a third operator describing electronic transitions dressed by nearest-neighbor spin fluctuations.
These latter, compared to charge and pair fluctuations, are assumed to be preeminent in the region of model-parameter space - small doping, low temperature and
large on-site Coulomb repulsion - where one expects strong electronic correlations
to dominate the physics of the system. This assumption and the consequent choice
for the basic field, as well as the whole analytical approximation framework, have
been validated through a comprehensive comparison with data for local and singleparticle properties obtained by different numerical methods on varying all model
parameters. The results systematically agree, both quantitatively and qualitatively,
up to coincide in many cases. Many relevant features of the model, reflected by the
numerical data, are exactly caught by the proposed solution and, in particular, the
crossover between weak and intermediate-strong correlations as well as the shape
of the occupied portion of the dispersion. A comprehensive comparison with other
n-pole solutions is also reported in order to explore and possibly understand the
reasons of such good performance.
[1] A. Avella, Eur. Phys. J. B 87, 45 (2014).
Dr Adolfo Avella, Dipartimento di Fisica “E.R. Caianiello”, Università degli Studi di Salerno, Via
Giovanni Paolo II, 132, I-84084 Fisciano (SA), Italy, Tel. +39 089 96 9131, Fax: +39 089 96 9658,
E-mail: avella@physics.unisa.it, Web page: www.physics.unisa.it/Homepage.asp?avella
98
Posters
Stability of the critical state, local vortex pinning and nonlocal
interactions in high-temperature superconducting films
P40
F. Laviano1,2,3, R. Gerbaldo1,2, L. Gozzelino1,2, G. Ghigo1,2
1. Department of Applied Science and Technology, Politecnico di Torino, Corso Duca
degli Abruzzi 24, 10129 Torino, Italy
2. I.N.F.N. Sez-To, Via P. Giuria 1, 10125 Torino, Italy
3. Department of Energy, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129
Torino, Italy
The vortex diffusion in type II superconductors is the dominant dissipative mechanism both in transport and magnetization properties. The strong nonlinearity of the
vortex system along with nonlocal interactions causes the variety of phenomena
which are typical of the complex systems. Therefore the detailed knowledge of the
electromagnetic properties on both local and global scales is necessary in order to
understand and then to control the vortex system. Here we present a thorough study
of the vortex critical state, also beyond its stability, in high temperature superconducting YBCO films. In order to measure the local properties of the vortex system,
we use the magneto-optical imaging (MOI) technique, which gives the magnetic
field and supercurrent density distribution, on the whole sample area with micrometric resolution [1]. The time-dependent observation of vortex diffusion under
external magnetic field (up to 0.2T), electrical current, and temperature was carried
out (down to the ms scale), in order to gather deep information about the diffusion dynamics. The direct visualization of the transition to the normal state due to
instability of the critical state will be also presented. The microscopic interaction
between vortices and crystal structure was locally altered in the superconducting
films in a controlled way by means of high energy heavy ion irradiation [2]. In
fact the local critical current density can be finely tuned by changing the density
of defects, either for enhancing or for depleting the as-grown vortex pinning force.
Moreover, the correlated defects created by the ion irradiation allow us to study
also anisotropic pinning properties due to the geometrical effects [3]. Comparison
between experimental observations and models for describing the behaviour of
inhomogeneous pinning in superconducting films will be addressed along with a
quantitative analysis in order to account for dc and rf dissipation mechanisms in
these systems. Recipes for predicting and controlling local pinning properties and
long-range interactions due to proper sample shaping are finally presented.
Acknowledgements
Staff of INFN-LNS and INFN-LNL laboratories is gratefully acknowledged for
supporting irradiation experiments.
[1] F. Laviano et al., Supercond. Sci. Technol 16 71 (2003)
[2] R. Gerbaldo et al., Nucl. Instrum.Meth. B, 272 291 (2012)
[3] F. Laviano et al., Supercond. Sci. Technol. 27 044014 (2014)
Posters
99
99
FeGa/MgO/Fe/GaAs nanostructure:
using a magnetostrictive material in magnetic tunnel junctions
P41
B. Gobaut1, R. Ciprian2, B. R. Salles5, R. Hussain2, S. Roddaro3,
M. Eddrief 4, M. Marangolo4, G. Panaccione2, G. Rossi2 and P. Torelli2
1. Sincrotrone Trieste S.C.p.A., S.S. 14 Km 163.5, Area Science Park, 34012 Trieste, Italy
2. Laboratorio TASC, IOM-CNR, S.S. 14km 163.5, Basovizza, 34149 Trieste, Italy
3. NEST, Sc. Norm. Sup. and Ist. Nanoscienze-CNR, Piazza S. Silvestro 12, 56127 Pisa, Italy
4. Institut des Nanosciences de Paris, UPMC - CNRS, UMR7588, 4 pl. Jussieu Paris,
France
5. Univ. Fed. Rio de Janeiro, Inst. Fis., BR-21941972 Rio De Janeiro, RJ, Brazil
Research is more and more interested in controlling the magnetic order of materials
through application of an electric field. This would allow for the development of
new devices for information and communication technology built smaller, working faster or being energetically more efficient. Research is focusing on materials
intrinsically combining magnetic and electric orders such as BiFeO3 or nanostructures allowing for magneto-electric coupling through different thin layers [1]. Recently, arises the idea of indirectly coupling these properties through strain using
magnetostrictive materials. In this framework, Fe1-xGax has demonstrated its value
because of its relatively high magnetostrictive coefficient [2].
In the present communication, we describe the growth and magnetic properties of a
magnetic tunnel junction built with FeGa as top layer. A structure of Fe1-xGax/MgO/
Fe has been grown by molecular beam epitaxy on GaAs(001) substrate and studied by magneto-optic kerr effect (MOKE) and x-ray magnetic circular dichroism
(XMCD) on APE beamline at Elettra synchrotron. Results revealed a good structural quality of the deposited layers and the hysteresis cycles showed a magnetic
decoupling between FeGa and Fe layers. Moreover a structural patterning of the
sample has been performed by reactive ion etching to build pillars of the nanostructure. Transport measurements have been performed on this patterned sample in
a two point configuration.
All results show the feasibility of using FeGa
magnetostrictive material in magnetic tunnel
junctions and are a step forward in the development of new complex devices.
Fig:Normalised hystersis curves of Fe0.8Ga0.2/MgO/
Fe/GaAs along the axis Fe[100]//GaAs[110] obtained
by MOKE (black) and XMCD (red).
[1] Y.-H. Chu et al. Nature Materials 7 478 (2008).
[2] D. E. Parkes, et al. Scientific Report 3 2220 (2013).
Corresponding author: B. Gobaut (benoit.gobaut@elettra.eu)
100
Posters
Nanoscle atomic disorder in layered superconducting
Ca10Ir4As8(Fe2As2)5 with metallic Ir-As spacer layer
P42
E. Paris1, B. Joseph1,2, A. Iadecola3, C. Marini4, K. Kudo5, D. Mitsuoka5,
M. Nohara5, T. Mizokawa6, N. L. Saini1
1. Dip. di Fisica, Università di Roma Sapienza, Piazzale Aldo Moro 2, I-00185 Roma
2. ELETTRA-Sincrotrone Trieste, Area Science Park, I-34012 Trieste, Italy
3. European Synchrotron Radiation Facility, BP220, F-38043 Grenoble Cedex, France
4. CELLS - ALBA, Carretera BP 1413, Km 3.3, 08290 Barcelona, Spain
5. Dep. of Physics, Okayama University, Kita-ku, Okayama 700-8530, Japan
6. Department of Complexity Science and Engineering, University of Tokyo,5-1-5
Kashiwanoha, Chiba 277-8561, Japan
The newly discovered layered Ca10Ir4As8(Fe2As2)5 is peculiar among the Febased superconductors because of its complex metallic spacer layer, with the Ir
5d states having non negligible weight at the Fermi level. This feature makes the
Ca10Ir4As8(Fe2As2)5 compound a new kind of Fe-based superconductor. In this system, there are several issues to be addressed: (i) interaction between the layers (ii)
nanoscale atomic pair correlations in the Fe2As2 and Ir4As8 layers, (iii) possible
role of spin-orbit (SO) interaction and, (iv) Ir valence state.
Here, we have used temperature dependent x-ray absorption measurements at Fe
K-edge and Ir L3 and L2-edge in order to address these issues in part. In the active
layer, we found the Fe-plane undergoing a large relaxation with significant distortions of the FeAs4 tetrahedra. From Ir L3 XAS we conclude that SO coupling may
not be significant for the physics of the system. Ir L3 EXAFS reveals large nanoscale disorder in the IrAs layer. It is likely that such a disorder could propagate into
the active FeAs-layer making the Fe-Fe bond to be flexible. The latter indicates importance of interlayer interactions in describing the physics of Ca10Ir4As8(Fe2As2)5
system. The results will be discussed in comparison with other Fe-based and Irbased superconductors.
Corresponding author: E. Paris, Dip. di Fisica, Università di Roma Sapienza, Piazzale Aldo Moro
2, I-00185 Rome, eugenio.paris@gmail.com
101
Posters 101
Ultrafast Broadband Optical Reflectivity of
Lanthanum Cuprate (La2CuO4)
P43
A. Mann1, E. Baldini1,2, J. Lorenzana3, C. Arrell2, F.Carbone1
1. Laboratory for Ultrafast Microscopy and Electron Scattering, ICMP,
EPFL, CH-1015 Lausanne
2. 2 Laboratory of Ultrafast Spectroscopy, ISIC, EPFL CH-1015 Lausanne
3. 3Institute for Complex Systems - Consiglio Nazionale delle Ricerche, and Physics
Department, University of Rome “La Sapienza”, I-00185 Rome
We investigate the transient reflectivity of lanthanum cuprate on the femtosecond
(fs) and picosecond (ps) timescale over a wide probing range of 400 to 700 nm.
Pump-probe experiments have been performed for two samples cut along the abplane (corresponding to the Cu-O layers) and the ac-plane for various temperatures and pump fluences. Careful tuning of the polarization of the pump and probe
beam with respect to the crystallographic axes of the samples allows for the study
of emerging coherent oscillatory components and their symmetries. The origin of
these oscillations can be ascribed to elementary excitations by exploiting the analogy between ultrafast optical pump-probe experiments and Impulsive Stimulated
Raman Scattering (ISRS) [1].
This work was motivated by our recent results on optimally doped lanthanum
strontium cuprate (La2-xSrxCuO4, x=0.15) [2]. There, we have been able to selectively excite the superconducting condensate by means of a technique called Coherent Charge Fluctuation Spectroscopy (CCFS). We found a strong coupling of
the Cooper pair excitations to the charge transfer energy around 2.5 eV. The aim
of our present study is to push further the understanding of the emergence of superconductivity by studying the undoped parent compound La2CuO4, an antiferromagnetic charge transfer insulator, and identifying the elementary excitations that
resonate with the charge transfer energy.
1. R. Merlin, Solid State Comm. 102, 207 (1997)
2. B. Mansart et al., Proc. Natl. Acad. Sci. USA 110, 4539-4544 (2013)
Corresponding author:
Andreas Mann, Ecole Polytechnique Federale de Lausanne, EPFL campus, CH J2 499, Station 6,
CH-1015 Lausanne, Switzerland
102
Posters
Ultrafast Spectroscopy of
Coherent Excitations in La1.84Sr0.16CuO4
P44
E. Baldini1,2, A. Mann1, S. Borroni1, C. Arrell2, C. G. Fatuzzo3,
A. Tramontana4, J. Lorenzana4, F. Carbone4
1. Laboratory for Ultrafast Microscopy and Electron Scattering, ICMP, EPFL, CH1015 Lausanne, Switzerland
2. Laboratory of Ultrafast Spectroscopy, ISIC, EPFL, CH-1015 Lausanne, Switzerland
3. Laboratory for Quantum Magnetism, ICMP, EPFL, CH-1015 Lausanne, Switzerland
4. Institute for Complex Systems-CNR and Physics Department, Sapienza, University of
Rome, Piazzale Aldo Moro 5, I-00185 Rome, Italy
Ultrafast spectroscopy has become a powerful tool in the investigation of strongly
correlated electron systems, providing a direct insight into their many-body dynamics far from equilibrium. This is of pivotal importance in the case of cuprates,
since the origin of their unconventional superconductivity is still one of the most
elusive problems in modern condensed matter physics.
In our experiments we exploit the Impulsive Stimulated Raman Scattering (ISRS)
process [1] to coherently perturb the Cooper paired electrons in a slightly overdoped sample of La2-xSrxCuO4 (x = 0.16) and we monitor the effect of nonequilibrium superconductivity on the other degrees of freedom [2]. The photoexcitation is
performed at the pump energies of 1.5 eV and 3.1 eV, in order to explore different
resonance conditions between the ground state and higher energy electronic states,
respectively ascribed to the excitation of charge stripes and the Cu-O charge transfer bands. The resulting transient reflectivity spectrum is subsequently monitored
over a wide spectral range (1.8 - 3.1 eV) through a weak continuum probe.
Here, we investigate the influence of the superconducting gap excitation on the
charge transfer and its coupling to other optical phonons coherently generated in
the system.
[1] T. E. Stevens et al., Phys. Rev. B 65, 144304 (2002)
[2] B. Mansart et al., Proc. Natl. Acad. Sci. USA 110, 4539 (2013)
Corresponding author:
Edoardo Baldini, edoardo.baldini@epfl.ch, ICMP, Ecole Polytechnique Fédérale de Lausanne,
CH-1015 Lausanne, Switzerland
103
Posters 103
Inhomogeneous Quantum Hall States in oxide interfaces with
strong Rashba spin-orbit interaction
P45
N. Bovenzi1, D. Bucheli1, S. Caprara1, M. Grilli1, G. Seibold2
1. Dipartimento di Fisica, Sapienza Università di Roma, Piazzale Aldo Moro 2,
I-00185 Rome, Italy
2. Institut fur Physik, BTU Cottbus-Senftenberg, PBox 101344, 03013 Cottbus, Germany
In the last years, the 2DEGs at the interfaces of perovskite oxide heterojunctions
attracted great attention because, besides magnetism and ferroelectricity, they exhibit, gate-tunable superconductivity. There are both experimental and theoretical
evidences that these systems are intrinsically inhomogeneous due to the occurrence of electronic phase separation (EPS)[1-5]. It has been proposed [1,3] that an
intrinsic mechanism can arise from the Rashba Spin-Orbit Coupling (RSOC). The
Rashba coupling depends on the interfacial charge density, via the electric field
confining the electrons at the interface.
In this work [6], we study the influence of a high magnetic field perpendicular to
the interface, on an electron gas with RSOC, with analytical and numerical approaches. We find a new physical scenario: the discreteness of the energy spectrum
(chiral Landau levels) always drives the system unstable towards EPS with negative compressibility. We
also investigate the topological properties of the
states at the interface between regions characterized by different values
of density (i. e. with different RSOC strenghts),
in the Quantum Hall regime [6].
Fig. 1: (a) Chemical potential as a function of density at B=25T for two different RSOC strenghts.
(b) Real space calculation of the charge configuration, revealing the occurrence of EPS.
[1] S. Caprara, F. Peronaci, and M. Grilli. Phys. Rev. Lett., 109 (2012).
[2] D. Bucheli, S. Caprara, C. Castellani, and M. Grilli, New J. Phys. 15, 023014 (2013).
[3] D. Bucheli, M. Grilli, F. Peronaci, G. Seibold, and S. Caprara. arXiv:1307.5427, to
appear in Phys. Rev. B (2014).
[4] S. Caprara, J. Biscaras, N. Bergeal, D. Bucheli, S. Hurand, C. Feuillet-Palma, A.
Rastogi, R. C. Budhani, J. Lesueur, and M. Grilli, Phys. Rev. B 88, 020504(R) (2013).
[5] S. Caprara, D. Bucheli, M. Grilli, J. Biscaras, N. Bergeal, S. Hurand, C. FeuilletPalma, J. Lesueur, A. Rastogi, and R. C. Budhani, SPIN 4, 1440004 (2014).
[6] N. Bovenzi, D. Bucheli, S. Caprara, M. Grilli and G. Seibold (in preparation).
Nicandro Bovenzi - Dipartimento di Fisica, Sapienza Università di Roma, Piazzale Aldo Moro 2,
I-00185 Rome, Italy. Mobile: 3208130813 E-mail: nicandrobovenzi.89@gmail.com
104
Posters
Magnetic order and low-temperature spin freezing
in La2-xSr xCoO4
P46
G. Allodi1, S. Bordignon1, R. De Renzi1, M. Mazzani1, T. Lancaster2,
A. Boothroyd3, S.J. Blundell3 and D. Prabhakaran3
1. Dipartimento di Fisica e Scienze della Terra, Università di Parma, I-43100 Parma, Italy
2. Centre for Materials Physics, Durham University, Durham DH1 3LE, United Kingdom
3. Department of Physics, Oxford University, Oxford OX1 3PU, United Kingdom
Hole doped lanthanum cobaltates La2−xSrxCoO4 are layered, magnetic compounds
isostructural with the ‘214’ copper oxide family La2−xSrxCuO4 which however, unlike cuprates, do not exhibit any superconductive phase. La-Sr substitution dopes
cobalt with holes which, upon localizing, induce a spinless Co3+ state. Thus, Sr doping depresses the AF order, as signaled by the drop of the Néel temperature from
a reported value of 270K in the undoped compound, down to TN≈30K at relatively
low doping levels. On the other hand, localized holes tend to self-organize into arrays of charged stripes behaving as antiphase domain walls in the AF spin structure,
and giving rise to a magnetic super-structure similar to the ones observed in certain
cuprates. Inelastic neutron scattering also revealed a peculiar “hour-glass”-shaped
dispersion branch in the magnetic excitations of La5/3Sr1/3CoO4 [1], similar to those
observed in several cuprates, either with or without static stripe order. The presence of similar dispersion features in both classes of compounds suggests that the
underlying excitations arise from stripes (either static or fluctuating) in all cases.
We present an investigation on a series of La2−xSrxCoO4 crystals (x=0−0.5) by muon
spin rotation (μSR) and 139La, 59Co NMR, focused on elucidating the magnetic order in the low doping regime (x < 0.1) and the nature of the stripe dynamics close
to the “magic” compositions x=1/3, x=0.5. Preliminary μSR experiments revealed
magnetic ordering temperatures TN ≈90K in the undoped compound, surprisingly
increased to TN ≈180K in the slightly doped x=0.05 member but, in all cases, much
lower than the values determined by neutron scattering at x=0. We argue that the
discrepancy between TN determined by the two techniques is related to the different time windows of the two probes, with neutrons probing the onset of magnetic
order on a very short time scale, which develops as static 3D order at the lower TN
revealed by muons. Further μSR experiments on better characterized samples (with
respect. e.g. to possible oxygen stoichiometry deviations), scheduled by the date of
the conference, will hopefully confirm this scenario.
Evidence for very slow dynamical excitations in the x=1/3 compound, ascribed to
the stripes, was provided by muon spin relaxation rates and by the partial or total
loss of the NMR signal (wipeout), due to short and inhomogeneous T2 relaxation
times. Two wipeout regimes, corresponding to distinct excitations, were detected:
a charge freezing above 100K, and a spin freezing in the 10-30K range, seemingly
due to the magnetic frustrations induced by the frozen-in charge disorder [2].
[1] A.T. Boothroyd et al., Nature 471, 341 (2011).
[2] T. Lancaster et al., Physical Review B 89, 020405(R) (2014)
Corresponding author: Giuseppe Allodi, Dipartimento di Fisica e Scienze della Terra, plesso
fisico, viale delle Scienze 7A, I-43100 Parma, Italy. E-mail: giuseppe.allodi@fis.unipr.it
105
Posters 105
Spin Seebeck effect in Lanthanum Strontium Manganites
F. Telesio1,2, N. Manca1,2, I. Pallecchi2, L. Pellegrino2, D. Marré1,2
1. Dipartimento di Fisica, Università di Genova, Genova (Italy)
2. CNR-SPIN, Genova (Italy )
P47
In ferromagnetic materials it has been observed that a temperature gradient induces
not only a voltage drop due to the Seebeck effect, but also an unbalance in spin
density. This is the Spin-Seebeck effect, one of the most exotic phenomena in the
physics of thermoelectrics, detected in insulators, semiconductor and metals. SpinSeebeck effect shows a quasi-linear spatial dependence along the sample length
and its magnitude proportional to sample magnetization and temperature gradient.
Typical length scale is of the order of millimeters, much larger than spin flip diffusion length and it seems to be a collective effect where magnons and phonons play
a role. Transduction mechanism between spin-accumulation and electrical voltage
is given by the Inverse Spin-Hall Effect (ISHE), obtained by using a non-magnetic
metals with high spin-orbit coupling. By depositing a transverse strip of, for example, platinum, the local spin unbalance is probed, by ISHE effect, as a transverse
voltage drop.
A promising material for Spin Seebeck experiments is (La,Sr)MnO3 (LSMO)
for several reasons. LSMO present a high spin polarization at Fermi level near
to 100%, thus increasing dramatically the expected ISHE signal. Furthermore
LSMO present a complex phase diagram where almost all magnetic and conducting behavior can be found by varying the x between 0 and 0.4 in the stoichiometric formula (La1-xSrx)MnO3. LSMO present also a phase transition in temperature
associated with Jahn-Teller distortion mechanism which suppress magnetism at
high temperature.
Spin-Seebeck effect measurements of LSMO with different Sr substitutions allow to investigate magnons and phonons contribution, because in this material
electric and magnetic properties can be tuned, and that is one of the reasons why
manganites can be useful to better understand this phenomenon. We believe that
LSMO, even more than being an ideal laboratory for Spin-Seebeck fundamental
experiments, shows the potentiality of being a candidate for spin voltage generator in spintronics applications.
We’ll introduce our measurement setup and discuss it in the framework of the different experimental configuration reported in literature. We’ll also analyze sample
fabrication method, based on a simple PLD deposition and thin film lithography
procedure and at last preliminary results of our experiments will be also presented.
Corresponding author: francesca.telesio@spin.cnr.it – Via Dodecaneso 33 – 16146 Genova – Italy
106
Posters
Voltage controlled magnetic anisotropy of ultrathin
CoFeB films on BaTiO3
P48
L. Baldrati, C. Rinaldi, M. Asa, M. Cantoni, R. Bertacco
LNESS – Dipartimento di Fisica – Politecnico di Milano, Via Anzani 42, 22100 Como, Italy.
Magnetoelectric coupling (MEC) in heterostructures made of ferroelectric (FE)
and ferromagnetic (FM) materials, is a promising route towards the electrical control of the magnetization, which would permit the realization of high density and
low power memory and logic devices. [1-2]
In previous work we investigated MEC in the Fe/BaTiO3 (BTO) system, demonstrating the on-off switching of the ferromagnetism in the interfacial Fe layers via
an electric field applied across the BTO. [3-4] In order to use MEC to modulate the
magnetic properties of thicker FM films, materials displaying perpendicular magnetic anisotropy (PMA), such as CoFeB, are highly promising. [5] In this context,
we have studied ferromagnetic ~1 nm thick CoFeB films grown on top of ferroelectric BTO thin films.
A La0.7Sr0.3MnO3(50 nm)/BTO(150 nm)/CoFeB(t)/Ta(2.0 nm) stack was grown on
SrTiO3 substrates by Pulsed Laser Deposition and Magnetron Sputtering, Then
microcapacitors (100x70 μm) were patterned by means of optical lithography and
ion beam etching. A detailed characterization of samples with different thicknesses
of Ta and CoFeB was carried out. It turns out that annealing (275°C for 15’) in a
magnetic field (0.4 T) is necessary to obtain PMA even at small thicknesses. The
effective anisotropy constant was measured as a function of the CoFeB thickness,
using vibrating sample magnetometry (VSM).
The volume and interface anisotropy constants turn out to be Kv=−1067±77 kJ/
m3 and Ki=1.4±0.1 mJ/m2 respectively. PMA is obtained below a CoFeB thickness
of 1.3 nm. In samples with PMA, the out-ofplane coercive field presents a hysteresis loop
versus the voltage applied across the BTO (i.e.
between the CoFeB and LSMO layers), which
can be put in correlation to the loop of BTO polarization versus voltage. Noteworthy the electrically induced switching of the magnetization
has been achieved, as shown in Fig. 1.
Figure 1 - A 80% modulation of the coercive field is
obtained by reversing the polarity of the voltage applied across BTO. The magnetization can be switched
from upwards to downwards by reversing the polarity
of the voltage across BTO in a bias field of -50 Oe.
[1] M. Bibes, Nature Mater. 11, 354 (2012).
[2] J. Ma, J. Hu, Z. Li, and C.-W. Nan, Adv. Mater. 23, 1062 (2011).
[3] G. Radaelli et al, Nat. Comm. 5, 3404 (2014).
[4] G. Redaelli et al, J. App. Phys. 115, 172604 (2014).
[5] P. V. Lukashev et al., J. Phys.: Condens. Matter 24 226003 (2012).
107
Posters 107
Anharmonicity and polarons in perovskites ingredients
of their non-common behaviors
P49
G. Ruani and T. Ivanovska
Consiglio Nazionale delle Ricerche - Istituto per lo Studio dei Materiali Nanostrutturati
CNR-ISMN, via Gobetti, 101 - 40129 Bologna - Italy.
At least in the last 30 years, perovkites were able to attract the attention of an
always growing number of scientists. These materials are showing amazing properties, HTc superconductivity, colossal magneto resistance in oxides and, more recently high efficient components in photovoltaic cell (with conversion efficiency
up 20%) in hybridorganic-inorganic compounds. At the same time there is still not
complete agreement in the scientific community about the main ingredients that are
able to determine/tune their exceptional properties.
In this communication we will report and discuss about some of the results we
have obtained investigating by Raman and photoinduced infrared absorption spectroscopy these materials (cuprates, manganites, etc.) pointing our attention on the
experimental evidences of polaronic
states and anharmonic potential and
the possible role they can play in determining their electronic properties.
Fig. 1 Photoinduced IR absorption in semiconducting YBCO with different oxygen
isotope contents (16O and 18O) showing both
polaronic deformation and anharmonicity of
the associated phonons
Giampiero Ruani: CNR-ISMN, via Gobetti, 101 - 40129 Bologna - Italy - g.ruani@bo.ismn.cnr.it
108
Posters
Room-temperature reversible electrical switching of the
magnetic order at the Fe/BaTiO3 interface
P50
G. Radaelli1,3, D. Petti1, P. Torelli2, B. R. Salles2, I. Fina3, D. Gutiérrez3,
E. Plekhanov4, C. Rinaldi1, M. Cantoni1, M. Varela5, S. Picozzi4, G. Panaccione2,
J. Fontcuberta3, R. Bertacco1
1. LNESS - Dipartimento di Fisica - Politecnico di Milano, Como, Italy.
2. Consiglio Nazionale delle Ricerche, CNR – IOM, Laboratorio TASC, Trieste, Italy.
3. Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, Bellaterra, Spain.
4. Consiglio Nazionale delle Ricerche, CNR-SPIN, L’Aquila, Italy.
5. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak
Ridge, Tennessee, USA; Departamento Fisica Aplicada III, Universidad Complutense
de Madrid, Madrid, Spain.
Interfacial magnetoelectric coupling (MEC) is a viable path to achieve electrical
writing of magnetic information in spintronic devices. For the paradigmatic Fe/
BaTiO3 (BTO) system, sizable changes of the interfacial Fe magnetic moment
upon reversal of the dielectric polarization of BTO have been predicted [1] and
huge variations of magnetoresistance have been observed by reversing the BTO
polarization in Fe/BTO/La0.7Sr0.3MnO3 (LSMO) tunneling junctions [2]. Here, by
using X-ray Magnetic Circular Dichroism and high resolution electron microscopy
tools on fully epitaxial Co/Fe/BTO/LSMO//SrTiO3(001) stacks, we report on an
undisclosed physical mechanism for interfacial MEC in the Fe/BTO system. At the
Fe/BTO interface, an ultrathin iron oxide layer exists, whose magnetization can
be electrically and reversibly switched on/off at room-temperature by reversing
the BTO polarization (see Fig.1). [3] First principles calculations give insight into
the mechanisms for the observed giant magnetoelectric response and provide a rationale for understanding and optimizing interfacial MEC for efficient, low power
spintronic devices.
Fig. 1: XAS spectra for up and down polarization of BTO n microcapacitors (see middle panel).
The XMCD signal at the FeOx shoulder disappears for down polarization.
[1] C-G. Duan et al., Phys. Rev. Lett. 97, 047201 (2006).
[2] S. Valencia et al., Nature Mater. 10, 753-759 (2011).
[3] G. Radaelli et al., Nature Comm. 5, 3404 (2014).
Corresponding author: Riccardo Bertacco
109
Posters 109
Electrochromic Properties of WO3/V2O5/TiO2 Composite
by RF Rotating Plasma
P51
M. Kiristi1, F. Bozduman2, A. U. Oksuz1, L. Oksuz2,3
1. Suleyman Demirel University, Faculty of Arts and Science, Department of Chemistry,32000 Isparta, Turkey
2. Suleyman Demirel University, Faculty of Arts and Science, Department of Physics,32000 Isparta, Turkey
3. Center for Plasma-Aided Manufacturing and Department of Engineering Physics,
University of Wisconsin-Madison, Madison, 53706, United States
Electrochromic (EC) devices can change reversible and persistent their optical
properties in the visible region (400–800 nm) upon charge insertion/extraction according to the applied voltage. Composite of tungsten oxide, vanadium oxide and
titanium oxide was obtained by rotating capacitively coupled radio-frequency (RF:
13.56 MHz) plasma reactor (Fig. 1). Steady state plasma coating parameters during
48 h as follows of treatment were base pressure of 20-30 mTorr; RF power of 50
W (CW). The excited discharge species were measured by an in-situ optical emission spectroscopy (OES) during RF plasma modification process. Thin films have
been deposited on In2O3:SnO2 (ITO) glass using the electron beam evaporation
technique in rich oxygen pressure.
Elemental composition of the composite was determined by using energy-dispersive X-ray Spectroscopy (EDS), the surface microstructure of the films was investigated using a scanning electron microscopy (SEM) and the surface topography was
investigated using an atomic force microscopy (AFM). The characterizations have
indicated that plasma modification was well adsorbed onto the surface of the tungsten oxide particles. In addition, redox potentials and durability during 2000 cycles
were investigated, comparatively. Solid-state EC device
of composite was prepared by
sandwiching the appropriate
gel electrolyte between ITO
electrodes previously coated
one side with the composite
oxides. The in-situ transmittance spectrum of the EC device was measured by optical
spectrophotometer. The electrochromic contrast, switching speed, and durability of
the solid-state devices were
investigated.
Keywords: Electrochromic; RF Plasma, metal oxide composite
110
Posters
Optical spectra of LaMn0.5Ga0.5O3: A contribution to
the assignment of the electronic transitions in manganites
P52
W. S. Mohamed1, A. Nucara1, F. Miletto Granozio2, A. Vecchione3,
R. Fittipaldi3, P.P. Perna2, M. Radovic4, P. Calvani1
1. CNR-SPIN and Dipartimento di Fisica, Università di Roma La Sapienza, Piazzale A.
Moro 2 I-00185 Roma, Italy
2. CNR-SPIN and Dipartimento di Scienze Fisiche, Universitá degli Studi Federico II,
Complesso Universitario di Monte Sant’Angelo, I-80126 Napoli, Italy
3. CNR-SPIN and Dipartimento di Fisica E. R. Caianiello, Università di Salerno, via
Ponte don Melillo, I-84084 Fisciano, Italy
4. Paul Scherrer Inst, Swiss Light Source, CH-5232 Villigen, Switzerland and Paul
Scherrer Inst, SwissFEL, CH-5232 Villigen, Switzerland
Despite the large amount of experimental work on the manganites of lanthanides,
the electronic ground state of LaMnO3 (LMO) and the charge transfer mechanisms
which occur with doping and temperature variations are not fully understood. We
present an optical study of the electronic bands of La0.5Ga0.5MnO3, in comparison
with those of pure LMO, as a contribution to a better understanding of the above
problems.
Ga substitution is a clean tool to study the effect of a replacement of Mn+3 by
anon-magnetic (diamagnetic) ion without introducing lattice distortion and any
additional magnetic exchange interaction due to the replacement. Indeed, Ga+3 has
no magnetic moment and its ionic size (0.62 Å) is very similar to that of Mn+3 (0.65
Å). Moreover, Ga+3 has a 3d10 filled-shell configuration so that eg orbitals are neither involved in exchange interactions nor in charge transport.
We have prepared a LaMn0.5Ga0.5O3 thin film deposited by Pulsed Laser Deposition
on a LSAT substrate and we have measured its transmittance in the visible and near
UV range, from 300 to 10 K. The aim was to shed light on the orgin of the electronic bands in the LaMnO3 family through a comparison with the corresponding
spectra collected previously on the parent compound. Two out of the four bands
detected have been assigned to the intersite d-d transitions between Mn+3 ions. The
other ones were ascribed to the p-d Mn-O charge-transfer transitions. We are thus
led to a”mixed” interpretation where both a Mott-Hubbard and a charge-transfer
approach concur to interpret the electronic spectrum of LMO.
Physics Department, Fermi Building IV floor P.le Aldo Moro 2, 00185 Roma
111
Posters 111
Characterization of multiferroic BiFeO3 polycrystalline thin
films grown by RF sputtering
P53
A. Giampietri and G. Drera
I-LAMP and Dipartimento di Matematica e Fisica,
Università Cattolica, Via dei Musei 41, 25121 Brescia, Italy
Multiferroics are materials which can display ferroelectricity, ferromagnetism, ferroeleasticity simultaneously. Multiferroics have already attracted much attention
due to the promising multifunctional device applications, such as magneto-electric
sensor devices and memories spintronics [1,2]. BiFeO3 is the only single-phase
multiferroic material at room temperature, with an high ferroelectric Curie temperature (TC = 1103 K) and high Neel temperature (TN = 643 K) [3]. The BiFeO3 thin
films are usually grown by PLD (pulsed laser deposition), MBE (molecular beam
epitaxy) and rarely by sputtering techniques [4,5], which are the cheapest and the
more promising deposition methods for the industrial growth of devices.
In order to be suitable for applications, sputtered polycrystalline film must show
the same properties of high quality epitaxial films, both in term of crystal structure,
secondary phases, magnetic and electric properties. The film properties can also be
tailored by after-growth treatment, such post-annealing, for which only few works
are reported in literature.
In this work, a detailed characterization of BiFeO3 thin films grown on a Si substrate by RF magnetron sputtering is reported, carried out with a set of electron
spectroscopy and microscopic techniques (XPS, Raman spectromicroscopy, AFM
and PFM). The homogeneity of the thin films, both grown in vacuum with different substrate temperature and post-annealed ex situ, was evaluated with the AFM
technique and the Raman microscopy. The temperature-dependent Raman analysis
also allowed us to estimate the ideal growth conditions which are needed to obtain a BiFeO3 perovskite single phase and to avoid the Fe2O3 and Bi2O3 secondary
phase, while the analysis of core-level photoemission data allowed us to evaluate
the thickness of the secondary phase layers which are eventually present over the
BiFeO3 layer. The PFM technique was finally used in order to verify the ferroelectric response of the thin films.
1 - W. Eerenstein et al., Nature 442:759–65 (2006)
2 - C. Catalan et al., Advanced Materials 21:2463–85 (2009)
3 - H. Yan et al., Materials Letters 111, 123-125 (2013)
4 - R.Y. Zheng et al., Journal of Applied Physics 101, 054104 (2007)
5 - Jian et al. Nanoscale Research Letters, 8:297 (2013)
email: g.drera@dmf.unicatt.it
112
Posters
‘Stressing’ the limits in Fe-Chalcogenide superconductors
by thin film deposition
P54
S. Kawale1, V. Braccini1, R. Buzio1, A. Gerbi1, G. Lamura1, F. Caglieries1,2,
A. Sala1,2, M. Putti1,2, E. Bellingeri1, C. Ferdeghini1
1. CNR – SPIN Genova, Corso Perrone 24, 16152 Genova, Italy
2. DIFI, University of Genova, Via Dodecaneso 33, 16145 Genova, Italy
One of the most interesting family of Fe-based Superconductors (FBS) is 11 system i.e. Fe-Chalcogenides (Fe-CH). These materials are characterized by simple
layered crystal structure with presence of only conducting layers of Fe-Ch and
presents intrinsically high values of critical fields. In our efforts, here we show that
superconducting properties like critical temperature (TC), critical current densities
(JC) and critical fields (HC2 and Hirr) of this material can be remarkably enhanced
by thin film deposition. High crystalline quality and epitaxial Fe(Se0.5,Te0.5) thin
films were grown by pulsed laser ablation on single crystal [001] CaF2 substrates.
Thanks to the compressive strain induced by typical Volmer-Weber growth mode
TC was enhance up to 20.6 K, over 30% enhancement to the bulk value. Transport
measurements show JC values higher than 1 MA/cm2 with very weak and isotropic
dependence on the magnetic field. Analysis through Transmission Electron Microscopy evidences the presence of defects looking like lattice disorder at a very small
scale, between 5 and 20 nm, which are thought to be responsible for such isotropic
behavior.
Homogeneity of JC distribution was checked by low temperature-scanning hall
probe microscopy. Nernst Effect, resistivity and critical current measurements at
very high field – up to 35 T – were employed in order to realize the (H,T) vortex
phase diagram. We observed the presence of a large region at low temperature but
extremely high magnetic field where the vortices are firmly pinned.
temperature but extremely high magnetic field where the vortices are firmly
pinned.
The fact that very high JC values in self field and at liquid helium temperature, are
reached together with a very weak dependence on the magnetic field and a complete isotropy, joined with the very high rigidity of the vortex lattice at very high
field underlines the potential of the Fe(Se,Te) phase for low temperature (≤ 4.2 K)
and high field (≥ 25 T) applications.
Corresponding author : Shrikant Kawale, CNR-SPIN Institute, Corso Perrone 24, Genova-16152.
Email-shrikant.kawale@spin.cnr.it , Phone no. +39 3209016605
113
Posters 113
Interfaces of SrTiO3 / LaAlO3 studied by Low Energy
Electron Microscopy
P55
A. v.d. Torren1, Z. Liao2, J. Aarts1
1. Kamerlingh Onnes Laboratory, University of Leiden PO Box 9504, 2300 RA Leiden,
The Netherlands
2. Faculty of Science and technology, University of Twente P.O. Box 217, 7500 AE
Enschede, The Netherlands
In growing and characterizing oxide interfaces, knowledge of morphology ot other
characteristics of a substrate or film surface at various temperatures and pressures,
is of obvious importance. We bring a new experimental method to bear on such
questions, by using Low Energy Electron Microscopy (LEEM). The method allows dynamic observation of surfaces in a large regime of temperatures up to
1200 °C, both in ultrahigh vacuum and in low oxygen pressures, and with high
lateral resolution. We demonstrate this by studying the LaO-terminated surface of
LaAlO3, where we find that heating in vacuum leads to a partially reconstructed
surface, indicating a mixed ordered termination.
Next we discuss samples of SrTiO3 (STO) / LaAlO3 (LAO) with the LAO thickness
around the critical thickness dcr of 4 unit cells where the interface becomes conducting. We find that the current-versus-energy characteristics of these samples are
strongly changing around dcr, in such a way that it yields a good contrast mechanism for microscopy. We also find differences between samples with LaO grown
on SrO-terminated STO and TiO2- terminated STO. In this way we observe clear
and confined area’s of non-conducting signature in nominally conducting samples.
Such type of microscopy can be quickly performed over large area’s, as will be
shown.
Corresponding author : Prof. Dr. J. Aarts Kamerlingh Onnes Laboratory, University Leiden
PO Box 9504, 2300 RA Leiden, The Netherlands - aarts@physics.leidenuniv.nl
114
Posters
Wednesday 24/9
8h30
Registration
and Posting
Thursday 25/9
Friday 26/9
Session 3
Superconductors I
Session 5
Strongly correlated systems
Session 1
Two-dimensional Electron Gas I
8h50
Fanfarillo
8h50
Mizuguchi
9h30
Welcome
9h10
Caglieris
9h30
Oles
9h40
Triscone
9h30
Profeta
9h50
Ciuchi
10h20
Radovic
9h50
Sanna
10h10 Ortenzi
10h40
Dreda
10h10 Bossoni
10h30 Brzezicki
11h00
Coffè break
10h30 Coffè break
10h50 Coffè break
11h30
Ortolani
11h00 Perucchi
11h20 Bertacco
11h50
Di Gennaro
11h20 Mirri
11h40 Joseph
12h10
Rubano
11h40 Cantoni
12h00 Manca
12h30
Bucheli
12h00 Boeri
12h20 Mitrano
12h50
End of Session 1
12h40 End of Session 3
12h40 End of Session 5
Lunch and Poster Session
Lunch and Poster Session II
Lunch
Session 2
Session 4
Two-dimensional Electron Gas II Superconductors II
Session 6 Magnetic
oxides and Multiferroics
14h30
Golden
14h00 Nicoletti
13h40 Bowen
15h10
Di Sante
14h40 Di Castro
14h00 Méchin
15h30
Perfetto
15h00 Orgiani
14h20 Giovannetti
15h50
Autore
15h20 De Filippis
14h40 De Luca
16h10
Coffè break
15h40 Giannetti
15h00 Barone
16h40
Citro
16h00 Cilento
15h20 Malavasi
17h00
Piatti
16h20 Peng
15h40 End of Session 6
17h20
Leveratto
16h40 End of Session 4
17h40
Cappelluti
18h00
Ludbrook
17h10 Guided Visit
18h20
End of Session 2
20h00 Conference dinner
16h00 Conference closure
Party and Poster Session
19h45
End of Poster Session
3
Sponsors and exhibitors
We thank all the sponsor for their contribute and support.
Consiglio Nazionale delle Ricerche
116
117
notes
118
notes
119
notes
120

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