Two-dimensional honeycomb borophene oxide: Strong anisotropy and nodal loop transformation

TL;DR

This paper predicts a new two-dimensional honeycomb borophene oxide with topological nodal loops, exhibiting strong anisotropy and tunable topological phases, using first-principles calculations on a light-element material.

Contribution

It introduces a novel 2D light-element material, h-B2O, with topological properties and demonstrates how strain can induce topological phase transitions.

Findings

h-B2O is structurally stable and mechanically anisotropic.

The electronic structure features a mirror-symmetry protected nodal loop.

Lattice strain can transform the nodal loop topology.

Abstract

The search for topological semimetals is mainly focused on heavy-element compounds as following the footsteps of previous research on topological insulators, with less attention on light-element materials. However, the negligible spin orbit coupling with light elements may turn out to be beneficial for realizing topological band features.Here, using first-principles calculations, we propose a new two-dimensional light-element material-the honeycomb borophene oxide (h-B2O), which has nontrivial topological properties.The proposed structure is based on the recently synthesized honeycomb borophene on Al (111) substrate [Sci. Bull. 63, 282 (2018)]. The h-B2O monolayer is completely flat, unlike the oxides of graphene or silicene. We systematically investigate the structural properties of h-B2O, and find that it has very good stability and exhibits significant mechanical anisotropy. Interestingly, the electronic band structure of h-B2O hosts a nodal loop centered around the Y point in the Brillouin zone, protected by the mirror symmetry. Furthermore, under moderate lattice strain, the single nodal loop can be transformed into two loops, each penetrating through the Brillouin zone. The loops before and after the transition are characterized by different Z*Z topological indices. Our work not only predicts a new two-dimensional material with interesting physical properties, but also offers an alternative approach to search for new topological phases in 2D light-element systems.