Electrons and polarons at oxide interfaces explored by soft-X-ray ARPES

TL;DR

This paper reviews how soft-X-ray ARPES reveals detailed electronic structures, polaronic effects, and phase separation phenomena at oxide interfaces like LaAlO3-SrTiO3, advancing understanding of their complex physics and potential device applications.

Contribution

It demonstrates the application of SX-ARPES to study oxide interfaces, highlighting polaronic charge carriers, electron-phonon interactions, and phase separation, with implications for future spin and device research.

Findings

Identification of dxy-, dyz-, dxz-derived subbands forming Fermi surface.

Observation of Holstein-type large polarons affecting mobility.

Evidence of phase separation and tunable polaronic effects via oxygen vacancies.

Abstract

Soft-X-ray ARPES (SX-ARPES) with its enhanced probing depth and chemical specificity allows access to fundamental electronic structure characteristics - momentum-resolved spectral function, band structure, Fermi surface - of systems difficult and even impossible for the conventional ARPES such as three-dimensional materials, buried interfaces and impurities. After a recap of the spectroscopic abilities of SX-ARPES, we review its applications to oxide interfaces, focusing on the paradigm LaAlO3-SrTiO3 interface. Resonant SX-ARPES at the Ti L-edge accentuates photoemission response of the mobile interface electrons and exposes their dxy-, dyz- and dxz-derived subbands forming the Fermi surface in the interface quantum well. After a recap of the electron-phonon interaction physics, we demonstrate that peak-dip-hump structure of the experimental spectral function manifests the Holstein-type large polaron nature of the interface charge carriers, explaining their fundamentally reduced mobility. Coupling of the charge carriers to polar soft phonon modes defines dramatic drop of mobility with temperature. Oxygen deficiency adds another dimension to the rich physics of LaAlO3-SrTiO3 resulting from co-existence of mobile and localized electrons introduced by oxygen vacancies. Oxygen deficiency allows tuning of the polaronic coupling and thus mobility of the charge carriers, as well as of interfacial ferromagnetism connected with various atomic configurations of the vacancies. Finally, we discuss spectroscopic evidence of phase separation at the LaAlO3-SrTiO3 interface. Concluding, we put prospects of SX-ARPES for complex heterostructures, spin-resolving experiments opening the totally unexplored field of interfacial spin structure, and in-operando field-effect experiments paving the way towards device applications of the reach physics of oxide interfaces.