Electronic Structure of Oxide Interfaces: A Comparative Analysis of GdTiO$_3$/SrTiO$_3$ and LaAlO$_3$/SrTiO$_3$ Interfaces

TL;DR

This study compares the electronic structures of GdTiO3/SrTiO3 and LaAlO3/SrTiO3 oxide interfaces using ab-initio calculations, revealing distinct behaviors in carrier density and surface charge effects, with implications for understanding 2DEG phenomena.

Contribution

It provides a comparative ab-initio analysis of GdTiO3/SrTiO3 and LaAlO3/SrTiO3 interfaces, highlighting differences in carrier density and surface charge behavior.

Findings

GTO/STO has a carrier density of 0.5 e^-/Ti, independent of GTO thickness.

LAO/STO shows carrier density buildup only beyond a threshold thickness.

Top GTO layer is insulating due to correlation effects, unlike LAO.

Abstract

Emergent phases in the two-dimensional electron gas (2DEG) formed at the interface between two insulating oxides have attracted great attention in the past decade. We present ab-initio electronic structure calculations for the interface between a Mott insulator GdTiO$_3$ (GTO) and a band insulator SrTiO$_3$ (STO) and compare our results with those for the widely studied LaAlO$_3$/SrTiO$_3$ (LAO/STO) interface between two band insulators. Our GTO/STO results are in excellent agreement with experiments, but qualitatively different from LAO/STO. We find an interface carrier density of 0.5$e^{-}$/Ti, independent of GTO thickness in both superlattice and thin film geometries, in contrast to LAO/STO. The superlattice geometry in LAO/STO offers qualitatively the same result as in GTO/STO. On the other hand, for a thin film geometry, the interface carrier density builds up only beyond a threshold thickness of LAO. The positive charge at the vacuum surface that compensates the 2DEG at the interface also exhibits distinct behaviors in the two systems. The top GTO layer is found to be insulating due to correlation-driven charge disproportionation, while the top LAO layer is metallic within band theory and may become insulating due to surface disorder or surface reconstruction.