Electronic structure of exotic states in some correlated-fermion materials : From hidden-order in URu2Si2 to metallic 2D electron gases in transparent insulating oxides

Orateur : Andrés SANTANDER-SYRO

Strong correlationsf-electron systems 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, multiferroicity, or photo-catalytic behavior. To understand such novel states of matter, harness the diverse functionalities of correlated-electron materials, and guide potential applications, it is essential to comprehend their microscopic electronic structure, which is ultimately responsible for their macroscopic behavior. To that end, we use in our group angle-resolved photoemission spectroscopy (ARPES), an experimental technique to directly image the band structure of a solid and how the many-body interactions and phase transitions affect it. In this talk, I will present an overview of our current research on the electronic structure of some correlated-electron materials. I will focus on two systems : (i) The two-dimensional electron gases (2DEGs) at the surface of transition-metal oxides. We recently discovered how to create 2DEGs at the surface of some insulating oxides [1, 2]. I will show that one can also tailor their electronic structure and symmetries by choosing the confining surface [3, 4]. Then, I will discuss our recent observation of a giant spin splitting, of 100 meV, of bands with opposite spin chiralities in the 2DEG at the surface of SrTiO3 [5]. These results show that confined electronic states at oxide surfaces can be endowed with novel, non-trivial properties that are not simple extensions of the bulk bands, and are promising for technological applications. (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. Following our observation of a heavy-electron Fermi-surface instability occurring at the transition [6], we recently studied the changes in electronic structure symmetries, and opening of a momentum-dependent energy gap, across the transition [7, 8]. I will show how these data provide a unified microscopic picture of the large entropy loss, gap opening and Fermi-surface reconstruction inferred from thermodynamic and magneto-transport measurements. [1] Nature 469, 189 (2011). [2] Phys. Rev. B 86, 121107(R) (2012). [3] Sci. Rep. 4, 3586 (2014). [4] Phys. Rev. Applied 1, 051002 (2014). [5] Nature Mater. DOI : 10.1038/NMAT4107 (2014). [6] Nature Physics 5, 637-641 (2009). [7] Phys. Rev. Lett. 110, 156404 (2013). [8] Nature Communications 5, 4326 (2014).

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