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

Abstract

Electronic phase separation is crucial for the fascinating macroscopic properties of the LaAlO 3 /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 Ti t 2g electrons coexist with localized strongly correlated Ti e 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 Ti e g electrons, and repelling of the mobile t 2g electrons from these clusters. Our results on the irradiation-tuned EPS elucidate the intrinsic one taking place at the stoichiometric LAO/STO interfaces.

Band-Order Anomaly at the γ-Al2O3/SrTiO3 Interface Drives the Electron-Mobility Boost

“…18,19 However, the observed band dispersions and population stayed constant, following the electronic phase separation scenario typical of the STO-based systems. 18,20,42,43 The irradiation-induced increase of the 2DES signal from the high-n s GAO/STO sample was smaller compared to the low-n s one. All reported ARPES data were acquired after an irradiation time of more than 1 hour to insure saturation.…”

“…Firstly, a typical feature of the STO-based systems is the electronic phase separation where conducting regions co-exist with insulating ones. 18,42,43 While experimental FS reflects the local electron density in the conducting areas, the integral n s can be smaller. Secondly, if the interfacial V(r) is sufficiently long-range, it hosts a ladder of quantum-well (QW) states, 44 and the Luttinger count should include the FS sheets through all of them.…”

Two-dimensional electron systems in perovskite oxide heterostructures: Role of the polarity-induced substitutional defects

“…Another future direction of SW spectroscopic studies on the LAO/STO system is to explore the role of oxygen vacancies. Recently, it is found that the LAO/STO interface electronic structure, which includes the QP (associated with 2DES) and IG states (associated with localized electrons), is tunable through the control of the oxygen vacancies [24,[44][45][46]. SW spectroscopies can show the depth profiles of both the IG and QP states in the oxygen deficient LAO/STO heterostructure, revealing the relationship between oxygen vacancies and 2DES.…”

Identifying Ionic and Electronic Charge Transfer at Oxide Heterointerfaces

“…[11,73] The complex nature of the heterointerfaces, however, may implicate other defect-scenarios with contributions from both STO and LAO defect sites. Given the reported importance of oxygen vacancies for the properties of LAO/STO interfaces, [35,74,75] it needs to be considered if annihilation and incorporation of oxygen vacancies alone can explain all observed data consistently. For one, however, the reversibility of the appearance of the Ti 3+ signature in the Ti 3d core-level rules out a mere effect of growth-induced excess oxygen vacancies on the STO side of the interface.…”