Direct k-space mapping of the electronic structure in oxide-oxide interfaces

Novel quantum phases can form at oxide heterointerfaces. Famous is the 2D electron system (2DES) in LaAlO$_3$/SrTiO$_3$ (LAO/STO). Its origin has been related to electronic reconstruction (ER). There electrons are transferred to the interface to compensate the potential gradient due to the polar discontinuity. The novel Al$_2$O$_3$ (AO)/STO also exhibits a 2DES but with much higher mobility [1]. In contrast to LAO, AO is regarded to be non-polar [1]. Hence ER should not be at work. It is assumed that O vacancies (Ovac) at the STO side of the interface induce the 2DES. We have directly mapped the k-resolved electronic structure of the interface states by resonant soft x-ray photoemission [2]. While we find a dichotomy of mobile and trapped charge -- the latter being ascribed to Ovac --, in both systems, they also show remarkable differences regarding the proportion of mobile and trapped carriers, the electron dispersions and Fermi surfaces, shedding light on the different role of Ovac.\\[4pt] [1] Y.Z. Chen et al, Nat. Comm. 4, 1371 (2013)\\[0pt] [2] G. Berner et al, PRL 110, 247601 (2013)