Artificial oxide heterostructures with non-trivial topology

TL;DR

This paper explores the design of oxide heterostructures with Rashba spin-orbit interactions to realize two- and three-dimensional topological insulators, emphasizing the role of symmetry and layer coupling in protecting topological states.

Contribution

It introduces a design framework for stacking oxide Rashba layers to create topological insulators, including the protection mechanism by reflection symmetry and the realization of quadratic coupling through buckling.

Findings

A 2D topological insulator phase emerges with negative coupling between 2DEGs.

Stacking layers can produce 2D or 3D topological insulators depending on coupling.

3D topological insulators are protected by reflection symmetry, classifying as topological crystalline insulators.

Abstract

In the quest for topological insulators with large band gaps, heterostructures with Rashba spin-orbit interactions come into play. Transition metal oxides with heavy ions are especially interesting in this respect. We discuss the design principles for stacking oxide Rashba layers. Assuming a single layer with a two-dimensional electron gas (2DEG) on both interfaces as a building block, a two-dimensional topological insulating phase is present when negative coupling between the 2DEGs exists. When stacking multiple building blocks, a two-dimensional or three-dimensional topological insulator is artificially created, depending on the intra- and interlayer coupling strengths and the number of building blocks. We show that the three-dimensional topological insulator is protected by reflection symmetry, and can therefore be classified as a topological crystalline insulator. In order to isolate the topological states from bulk states, the intralayer coupling term needs to be quadratic in momentum. It is described how such a quadratic coupling could potentially be realized by taking buckling within the layers into account. The buckling, thereby, brings the idea of stacked Rashba system very close to the alternative approach of realizing the buckled honeycomb lattice in [111]-oriented perovskite oxides.