Large-Gap Quantum Spin Hall States in the Bilayer Hexagonal Structure of Rhenium and Technetium Dinitrides: A First-Principles Study

Abstract

Layered transition-metal dinitrides are the recently proposed cousins of transition-metal dichalcogenides, which exhibit extraordinary electronic and magnetic properties. In this work, through first-principles calculations, we have identified the lowest energy geometry of the ReN2 nanosheet, which is a bilayer hexagonal structure (BHS) akin to the MoN2 system. Such a BHS-ReN2 nanosheet possesses robust dynamical, thermal, and mechanical stabilities at the free-standing state. Interestingly, without spin–orbit coupling (soc), this BHS-ReN2 system exhibits a semimetallic feature with a d-character Dirac cone, for which a band gap is opened at the Dirac point when the soc effect is taken into account. As a result, the BHS-ReN2 nanosheet becomes a quantum spin Hall (QSH) insulator in the presence of soc, whose bulk gap reaches up to 384 meV, sufficiently large for the room-temperature QSH effect. The nontrivial topological feature is characterized by a Z2 = 1 invariant and a pair of topologically protected ed...