  Electronic phase separation is crucial for the fascinating macroscopic
properties of the LaAlO3/SrTiO3 (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 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 t2g-electrons co-exist
with localized strongly correlated Ti eg-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 eg-electrons, and repelling of the mobile
t2g-electrons from these clusters. Our results on the irradiation-tuned EPS
elucidate the intrinsic one taking place at the stoichiometric LAO/STO
interfaces.
