Rectangular Tantalum Carbide Halides TaCX (X = Cl, Br, I) monolayer: Novel Large-Gap Quantum Spin Hall Insulator

TL;DR

This paper predicts a new family of monolayer tantalum carbide halides with large energy gaps and topologically protected edge states, suitable for room-temperature quantum spin Hall applications.

Contribution

First-principles calculations reveal a novel large-gap QSH insulator family in TaCX monolayers with unique rectangular lattice and tunable electronic properties.

Findings

Large direct energy gaps of 0.23-0.36 eV in TaCX monolayers.

Presence of topologically protected edge states with high Fermi velocity.

Band inversion mechanism differs from conventional QSH systems.

Abstract

Quantum spin Hall (QSH) insulators possess edge states that are topologically protected from backscattering. However, known QSH materials (e.g. HgTe/CdTe and InAs/GaSb quantum wells) exhibit very small energy gap and only work at low temperature, hindering their applications for room temperature devices. Based on the first-principles calculations, we predict a novel family of QSH insulators in monolayer tantalum carbide halide TaCX (X = Cl, Br, and I) with unique rectangular lattice and large direct energy gaps larger than 0.2 eV, accurately, 0.23$-$0.36 eV. The mechanism for 2D QSH effect in this system originates from a intrinsic d$-$d band inversion, different from conventional QSH systems with band inversion between s$-$p or p$-$p orbitals. Further, stain and intrinsic electric field can be used to tune the electronic structure and enhance the energy gap. TaCX nanoribbon, which has single-Dirac-cone edge states crossing the bulk band gap, exhibits a linear dispersion with a high Fermi velocity comparable to that of graphene. These 2D materials with considerable nontrivial gaps promise great application potential in the new generation of dissipationless electronics and spintronics.