Topological bands in two-dimensional orbital-active bipartite lattices

TL;DR

This paper explores how certain two-dimensional bipartite lattices with degenerate orbitals can host large-gap quantum spin Hall and quantum anomalous Hall states, using band representation theory and tight-binding models.

Contribution

It demonstrates the feasibility of realizing topological insulators in various 2D lattices with specific symmetries and provides concrete models for experimental exploration.

Findings

Degenerate multi-orbitals in bipartite lattices can induce QSH states.

Ferromagnetism extends QSH to QAH with larger gaps.

Topological states depend on lattice symmetry and electron filling.

Abstract

The search for large gap quantum spin Hall (QSH) and quantum anomalous Hall (QAH) insulators is important both for fundamental and practical interests. The degenerate multi-orbitals $p_x,p_y$ in honeycomb lattice provides a paradigm for QSH state with a boosted topological gap of the first order in atomic spin-orbit coupling. By using elementary band representation, we explore the feasibility of this mechanism for QSH in general two-dimensional lattices, and find that the biparticle lattices with $C_{3v}$ or $C_{4v}$ symmetry and degenerate multi-orbitals could work. We further provide concrete tight-binding models on honeycomb, kagome and square lattices to demonstrate the desired topological physics. By introducing ferromagnetism into QSH state, we extend the mechanism to QAH state with a boosted gap. The QSH and QAH states can be achieved when Fermi level is at integer filling only for honeycomb lattice, but at certain fractional filling for other lattices. We conclude with a brief discussion on the possible material venues for such mechanism.