Electronic phase separation at LaAlO3/SrTiO3 interfaces tunable by oxygen deficiency

Abstract

Electronic phase separation is crucial for the fascinating macroscopic properties of the ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ (LAO/STO) paradigm oxide interface, including the coexistence of superconductivity and ferromagnetism. We investigate this phenomenon using angle-resolved photoelectron spectroscopy (ARPES) in the soft-x-ray energy range, where the enhanced probing depth combined with resonant photoexcitation allow us access to fundamental electronic structure characteristics -- momentum-resolved spectral function, dispersions and ordering of energy bands, Fermi surface -- of buried interfaces. Our experiment uses x-ray irradiation of the LAO/STO interface to tune its oxygen deficiency, building up a dichotomic system where mobile weakly correlated $\mathrm{Ti}\phantom{\rule{0.28em}{0ex}}{t}_{2\mathrm{g}}$ electrons coexist with localized strongly correlated $\mathrm{Ti}\phantom{\rule{0.28em}{0ex}}{e}_{\mathrm{g}}$ ones. The ARPES spectra dynamics under x-ray irradiation shows a gradual intensity increase under constant Luttinger count of the Fermi surface. This fact identifies electronic phase separation (EPS) where the mobile electrons accumulate in conducting puddles with fixed electronic structure embedded in an insulating host phase, and allows us to estimate the lateral fraction of these puddles. We discuss the physics of EPS invoking a theoretical picture of oxygen-vacancy clustering, promoted by the magnetism of the localized $\mathrm{Ti}\phantom{\rule{0.28em}{0ex}}{e}_{\mathrm{g}}$ electrons, and repelling of the mobile ${t}_{2\mathrm{g}}$ electrons from these clusters. Our results on the irradiation-tuned EPS elucidate the intrinsic one taking place at the stoichiometric LAO/STO interfaces.