In solids with strongly-interacting electrons, the competition between the different degrees of freedom leads to competing quantum ground states, from which a rich variety of macroscopic phenomena emerge. In many cases, these phenomena arise from phase transitions described by exotic (or even unknown) order parameters and underlying novel states of matter. Examples of such physical richness are the high-temperature superconductivity in cuprates, the colossal magneto-resistance in manganites, the multiferroic behaviour in bismuth ferrites, the numerous quantum-critical transitions in heavy-fermion materials, or the recently discovered 2D metallic electron gases (2DEGs) at the interfaces of insulating transition-metal oxides.
A direct approach to understand the physics of strongly-correlated materials is to study their band structure and how the many-body interactions and phase transitions affect it. The technique of angle-resolved photoemission spectroscopy (ARPES) does precisely that. ARPES gives access to the band structure, the effective masses and the scattering-rate of electrons (hence the effects of many-body interactions) in the occupied states of the solid.
In this talk, I will first introduce the basic aspects of the ARPES technique. Then, I will discuss its application to the study of the nature and electronic structure of the 2DEGs in SrTiO3-based interfaces. Such 2DEGs are not only a novel playground to study fundamental aspects of the physics of doped correlated-electron systems, but also offer interesting perspectives in applications aiming at functionalizing the numerous properties of transition-metal oxides. Here, very recent ARPES experiments have unveiled the existence of a metallic 2DEG at the surface of insulating SrTiO3 [1, 2], providing detailed information about its electronic structure, and hinting to a novel route to generate 2DEGs at surfaces of functional oxides.
 A. F. Santander-Syro et al. “Two-dimentsional electron gas with universal subbands at the surface of SrTiO3”. Nature 469, 189 (2011).
 W. Meevasana et al. “Creation and control of a two-dimensional electron liquid at the bare SrTiO3 surface”. Nature Materials 10, 114 (2011).