Second-order topological insulator in Bilayer borophene

TL;DR

This paper identifies bilayer borophene as a two-dimensional second-order topological insulator, characterized by a bulk quadrupole moment, stabilized by interlayer bonds, and protected by rotational symmetry, expanding the material candidates for such topological states.

Contribution

The study demonstrates that bilayer α5-phase borophene is a second-order topological insulator, providing a realistic material candidate with large bulk gaps and symmetry protection.

Findings

Bilayer borophene is a 2D second-order topological insulator.

The material has large bulk gaps (~0.55-0.62 eV).

Topological properties are protected by C2 rotational symmetry.

Abstract

As the novel topological states, the higher-order topological insulators have attracted great attentions in the past years. However, their realizations in realistic materials, in particular in two dimensional systems, remains the big challenge due to the lack of adequate candidates. Here, based on the first-principle calculation and tight-binding model simulations, we identify the currently \emph{existing} bilayer $\alpha_{5}$-phase borophenes as the two-dimensional second-order topological insulators, protected by the $C_{2}$-rotational symmetry. The formation of interlayer B-B covalent bonds, stabilizing the bilayer borophenes and opening the large direct bulk gaps ($\sim 0.55-0.62$ eV) at Fermi level, plays the key roles. The second-order topology is characterized by the bulk quantized quadrupole momentum. Our results enriches the candidates for the second-order topological insulators, and also provide a way to study topological states in borophenes.