Prediction of polarization vortices, charge modulation, flat bands, and moir\'e magnetism in twisted oxide bilayers

TL;DR

This paper predicts novel electronic and magnetic phenomena in twisted SrTiO3 bilayers, including polarization vortices, charge modulation, flat bands, and moiré-induced magnetism, expanding the potential of moiré oxide heterostructures.

Contribution

It introduces theoretical predictions of complex emergent phenomena in twisted oxide bilayers, a relatively unexplored area compared to van der Waals materials.

Findings

Vortex-antivortex polarization patterns at interfaces

Charge modulation following moiré patterns

Emergence of flat bands and moiré-periodic localized states

Abstract

The recent surge of interest in moir\'e superlattices of twisted van der Waals compounds has spotlighted the emergence of unconventional superconductivity and novel electronic phases. However, the range of moir\'e phenomena can be dramatically expanded by incorporating complex oxide materials into twisted heterostructures. In this study, motivated by the recent breakthroughs in synthesis of free-standing oxide membranes, we explore the emergent structural and electronic properties of twisted oxide bilayers. We focus on the classic perovskite oxide, SrTiO3, and design SrTiO3 bilayers with a relative twist between the individual layers. Using density functional theory calculations, we predict the appearance of vortex-antivortex polarization patterns at the interface of the SrTiO3 bilayers driven by twist. We also predict charge modulation of the interfacial Ti ions induced by varying local coordination which follow the moir\'e pattern. Furthermore, we forecast the emergence of flat bands at large twist angles and the associated localized electronic states with moir\'e-periodic charge density, originating from the interlayer bonding effects resulting in the formation of dangling bonds. Finally, we predict that hole doping induces unconventional d0 magnetism in otherwise nonmagnetic SrTiO3, driven by the exchange splitting of the high-density O-p bands and producing the spin density with moir\'e periodicity. These results demonstrate a broad landscape of emergent phenomena which may occur in moir\'e-engineered oxide heterostructures showing far-reaching perspectives of these material systems for further fundamental studies and potential applications.