Institut des Sciences Moléculaires d'Orsay



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Accueil > Équipes scientifiques > Surfaces, Interfaces, Molecules & 2D Materials (SIM2D) > Systèmes Fortement Corrélés et Matériaux Quantiques > Strongly Correlated Electron Systems and Quantum Materials

Strongly Correlated Electron Systems and Quantum Materials

par Mayne Andrew - 30 juin 2020 (modifié le 23 septembre 2020)

Strongly Correlated Electron Systems

Leader : Andrés Santander-Syro (MCF, HDR 2013, Junior Chair IUF 2011-2016)

Participants : Emmanouil Frantzeskakis (MCF), Franck Fortuna (IRHC)

Students : Maximillian Thees (PhD), Pedro Rezende-Goncalves (PhD - Belo Horizonte, Brazil)

Previous members :
Ji Dai (PhD-2019, postdoc at EPFL-Laussanne),
Tobias C. Rödel (PhD-2016, Editor of Nature),
Cédric Bareille (PhD-2013, post-doc at ISSP-Tokyo University)

The physics of strongly interacting fermions is the common thread in several challenging open problems at all scales, from the description of compact nuclear and sub-nuclear matter to the behavior of electrons in a large class of systems, also known as "quantum materials", in which low-dimensional confinement or correlated behavior are present. For instance, in transition-metal oxides and f-electron materials, strong correlations lead to a wide realm of phase transitions and exotic, often poorly understood, states of matter showing remarkable macroscopic properties, such as high-temperature superconductivity, large magneto-resistance, or metal-to-insulator transitions.
To understand such novel states of matter, and guide potential applications, we study these systems using angle-resolved photoemission spectroscopy (ARPES), a technique that directly images the electronic structure of solids. We use both our state-of-the-art ARPES and MBE setups, and synchrotron-based experiments all over the world : France (SOLEIL), Germany (BESSY), Japan (KEK-PF and HiSOR). Our main fields of research are :

1) Novel 2D electron systems at the surface of functional oxides
2) Exotic electronic states and phase transitions in correlated-electron materials

1) Novel 2D electron gases at the surface of functional oxides :
The realization of 2DEGs in transition-metal oxides (TMOs), a field of current intense research, is crucial for harnessing the functionalities of these materials for future applications, and go beyond current technologies based on 2DEGs in semiconductor heterostructures. Our team discovered how to create 2DEGs in several transparent functional oxides, such as SrTiO3, the “silicon” of TMOs, or BaTiO3, a versatile ferroelectric, by simply inducing oxygen vacancies at their bare surface. These 2DEGs offer the possibility to explore new physics emerging from the combined effects of electron correlations and low-dimensional confinement, including the emergence and control of superconductivity by voltage gating.

2) Exotic electronic states and phase transitions in correlated-electron materials :
In systems with strong interactions between elementary excitations, the competition between the different degrees of freedom (charge, spin, lattice vibrations, etc.) leads to competing ground states resulting in classical or quantum phase transitions described by exotic (or even unknown) order parameters and underlying novel states of matter. The heavy-fermion material URu2Si2 is a paradigmatic example of such physics. This material undergoes a phase transition to an ordered state below 17.5K, macroscopically characterized by a large entropy loss and a massive reduction of the density of states at the Fermi level. However, the identification of the broken symmetry and microscopic mechanism behind this phase transition have been among the most challenging riddles of modern physics for over 30 years –earning it the nickname of “hidden-order” transition. Recently, pushing ARPES to its limits, our team measured the complete momentum-resolved reconstruction of the electronic structure of URu2Si2, identifying the symmetry changes and gaps opening in its Fermi surface across the hidden order transition. We are now investigating how other external parameters, such as doping of chemical pressure, transform, through quantum phase transitions, the hidden order into other collective states of matter.

Techniques :
State-of-the-art high-resolution ARPES setup with low-temperature manipulator
XPS setup with monochromatic Al source
Molecular Beam Epitaxy chamber with LEED, RHEED, AES
Focussed-Ion Beam (FIB) (Plateform)

External Funding :
ANR projects LACUNES (PI) and Fermi-NESt (partner)
LabEx PALM projects ELECTROX (PI), 2DEG2USE (PI), and 2DTROX (PI)

Collaborations :
ISSP, University of Tokyo, Japan
IMRAM, Tohoku University, Japan
Hiroshima Synchrotron, Japan
University of California, San Diego
MagLab – University of Florida – Tallahassee
CNEA, Buenos Aires, Argentina
University of Würzburg, Germany
DIPC – University of the Basque Country, San Sebastián, Spain


Tunable two-dimensional electron system at the (110) surface of SnO2. We report the observation of a two-dimensional electron system (2DES) at the (110) surface of the transparent bulk insulator SnO2 and the tunability of its carrier density by means of temperature or Eu deposition. The 2DES is insensitive to surface reconstructions and, surprisingly, it survives even after exposure to ambient conditions—an extraordinary fact recalling the well known catalytic properties SnO2. Our data show that surface oxygen vacancies are at the origin of such 2DES, providing key information about the long-debated origin of n-type conductivity in SnO2, at the basis of a wide range of applications. Furthermore, our study shows that the emergence of a 2DES in a given oxide depends on a delicate interplay between its crystal structure and the orbital character of its conduction band.

article : J. Dai, E. Frantzeskakis, F. Fortuna, P. Lömker, R. Yukawa, M. Thees, S. Sengupta, P. Le Fèvre, F. Bertran, J. E. Rault, K. Horiba, M. Müller, H. Kumigashira, and A. F. Santander-Syro. Tunable two-dimensional electron system at the (110) surface of SnO2. Phys. Rev. B. 101, 085121 (2020).

Gate-tunable superconductivity at SrTiO3 surface realized by Al layer evaporation. Electronic properties of low dimensional superconductors are determined by many-body-effects. This physics has been studied traditionally with superconducting thin films and in recent times with two-dimensional electron gases (2DEGs) at oxide interfaces. In this work, we show that a superconducting 2DEG can be generated by simply evaporating a thin layer of metallic Al under ultrahigh vacuum on a SrTiO3 crystal, whereby Al oxidizes into amorphous insulating alumina, doping the SrTiO3 surface with oxygen vacancies. The superconducting critical temperature of the resulting 2DEG is found to be tunable with a gate voltage with a maximum value of 360 mK. A gate-induced switching between superconducting and resistive states is demonstrated. Compared to conventionally-used pulsed-laser deposition, our work simplifies to a large extent the process of fabricating oxide-based superconducting 2DEGs. It will make such systems accessible to a broad range of experimental techniques useful to understand low-dimensional phase transitions and complex many-body-phenomena in electronic systems.

article : Shamashis Sengupta, Emilie Tisserond, Florence Linez, Miguel Monteverde, Anil Murani, Tobias Rödel, Philippe Lecoeur, Thomas Maroutian, Claire Marrache-Kikuchi, Andrés F. Santander-Syro, and Franck Fortuna. Gate-tunable superconductivity at SrTiO3 surface realized by Al layer evaporation. J. Applied Physics 124, 213902 (2018).

High-density two-dimensional electron system induced by oxygen vacancies in ZnO. We realize a two-dimensional electron system (2DES) in ZnO by simply depositing pure aluminum on its surface in ultrahigh vacuum and characterize its electronic structure by using angle-resolved photoemission spectroscopy. The aluminum oxidizes into alumina by creating oxygen vacancies that dope the bulk conduction band of ZnO and confine the electrons near its surface. The electron density of the 2DES is up to two orders of magnitude higher than those obtained in ZnO heterostructures. The 2DES shows two s-type subbands, that we compare with the d-like 2DESs in titanates, with clear signatures of many-body interactions that we analyze through a self-consistent extraction of the system self-energy and a modeling as a coupling of a two-dimensional Fermi liquid with a Debye distribution of phonons.

article : T. C. Rödel, J. Dai, F. Fortuna, E. Frantzeskakis, P. Le Fèvre, F. Bertran, M. Kobayashi, R. Yukawa, T. Mitsuhashi, M. Kitamura, K. Horiba, H. Kumigashira, and A. F. Santander-Syro. High-density two-dimensional electron system induced by oxygen vacancies in ZnO. Phys. Rev. Materials 2, 051601 (2018).