  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.
