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Accueil > Séminaires > Séminaires passés > Seminaires de 2013 > Andres Santander (CSNSM - UPSud)

Soutenance HDR : Electronic structure of exotic states in correlated-fermion materials

Andres Santander (CSNSM - UPSud)

Jeudi 25 avril 2013 à 14h00 Auditorium Pierre Lehmann, LAL (Bât 200)

The physics of strongly-interacting fermions is the common thread in several challenging open problems at all scales. For instance, such physics is involved in the description of compact nuclear and sub-nuclear matter, in the study of the primitive Universe and the symmetry breakings leading to today’s observable cosmos, in ultra-cold atomic gases in optical lattices, or in electrons in a large class of solids in which low-dimensional or correlated behavior is present. These last systems are the subject of this work.

In this talk, I will present my research on the electronic structure of some correlated-electron materials. I have used angle-resolved photoemission spectroscopy (ARPES), an experimental technique that allows measuring the band structure of a solid and how the many-body interactions and phase transitions affect it. I will focus on two systems :

(i) The two-dimensional electron gases (2DEGs) at the surface of transition-metal oxides. These materials present remarkable properties, such as superconductivity or colossal magneto-resistance. We recently discovered how to create 2DEGs at the surface of some insulating oxides [1, 2]. As 2DEGs are at the basis of modern electronics and of exotic states of matter, such as the quantum Hall effect or the topological insulators, our discovery opens a wealth of perspectives in the fields of correlated electrons and oxide electronics [3].

(ii) The puzzling ‘hidden-order’ phase transition at THO = 17.5 K in URu2Si2. This transition, discovered in the 80’s, is characterized by a large entropy loss and an energy gap of about 10 meV in the density of states at the Fermi level. However, the identification of the associated broken symmetry and order parameter are still a riddle, earning it the epithet of “the Higgs problem of condensed-matter physics”. We demonstrated that there is a heavy-electron Fermi-surface instability occurring at the transition [4], thus highlighting the importance of heavy fermions in the physics behind the hidden order. More recently, we studied how the lattice of heavy-fermions evolves from the paramagnetic state above THO to the hidden-order state [5], and how the symmetry of the electronic structure changes, and opens an energy gap, across the transition [6].

[1] A. F. Santander-Syro et al., Nature 469, 189-193 (2011).
[2] A. F. Santander-Syro et al., Phys. Rev. B 86, 121107(R) (2012).
[3] C. Bareille et al., submitted (2013).
[4] A. F. Santander-Syro et al., Nature Physics 5, 637-641 (2009).
[5] F. L. Boariu et al, Phys. Rev. Lett. 110, 156404 (2013).
[6] C. Bareille et al., submitted (2013).