Strain modulated quantum spin Hall effect in monolayer NiB

Abstract

Since boron has diverse coordination chemistry, it is capable of bonding with transition metals to form a new family of two-dimensional (2D) transition metal borides (MBenes) that exhibit rich electronic properties. Here, we propose a 2D planar monolayer NiB with topologically nontrivial electronic structures as the new member of the MBenes family. The NiB monolayer exhibits metallic behavior with two Dirac cones residing slightly below the Fermi level in the absence of spin–orbit coupling (SOC). When a tensile strain of 11% is applied, the NiB monolayer transforms into a semimetal. A gap opening of 18 meV is observed at the Dirac points when SOC is considered. Based on the results of Z2 invariant (Z2=1) and the helical edge states, we identify the pristine NiB as a topological metal, while the stretched monolayer under 11% strain as a quantum spin Hall (QSH) insulator. In addition, the NiB monolayer undergoes a phase transition from topological metal to QSH insulator and then to topological metal with the increase in strain from 0% to 14%. Our findings demonstrate that the electronic properties of NiB, the new member of the MBene family, can be modulated by strain.