Electronic Phases and Phase Separation in the Hubbard-Holstein Model of a Polar Interface

TL;DR

This study uses mean-field analysis of the Hubbard-Holstein model to explore diverse electronic phases at polar interfaces, revealing how interactions influence confinement, phase separation, and magnetic phenomena.

Contribution

It introduces a comprehensive mean-field framework to predict various electronic phases and phase separation phenomena at polar interfaces based on competing interactions.

Findings

Identification of 2D metallic and insulating phases at interfaces.

Prediction of 3D spreading of electrons into the bulk.

Discovery of Jahn-Teller polaronic phase and magnetic Kondo centers.

Abstract

From a mean-field solution of the Hubbard-Holstein model, we show that a rich variety of different electronic phases can result at the interface between two polar materials such as LaAlO$_3$/SrTiO$_3$. Depending on the strengths of the various competing interactions, viz., the electronic kinetic energy, electron-phonon interaction, Coulomb energy, and electronic screening strength, the electrons could (i) either be strongly confined to the interface forming a 2D metallic or an insulating phase, (ii) spread deeper into the bulk making a 3D phase, or (iii) become localized at individual sites forming a Jahn-Teller polaronic phase. In the polaronic phase, the Coulomb interaction could lead to unpaired electrons resulting in magnetic Kondo centers. Under appropriate conditions, electronic phase separation may also occur resulting in the coexistence of metallic and insulating regions at the interface.