  Topological insulators (TIs) are promising for achieving dissipationless
transport devices due to the robust gapless states inside the insulating bulk
gap. However, currently realized 2D TIs, quantum spin Hall (QSH) insulators,
suffer from ultra-high vacuum and extremely low temperature. Thus, seeking for
desirable QSH insulators with high feasibility of experimental preparation and
large nontrivial gap is of great importance for wide applications in
spintronics. Based on the first-principles calculations, we predict a novel
family of two-dimensional (2D) QSH insulators in transition-metal halide MX (M
= Zr, Hf; X = Cl, Br, and I) monolayers with large nontrivial gaps of
0.12$-$0.4 eV, comparable with bismuth (111) bilayer (0.2 eV), stanene (0.3 eV)
and larger than ZrTe$_5$ (0.1 eV) monolayers and graphene-based sandwiched
heterstructures (30$-$70 meV). Their corresponding 3D bulk materials are weak
topological insulators from stacking QSH layers, and some of bulk compounds
have already been synthesized in experiment. The mechanism for 2D QSH effect in
this system originates from a novel d$-$d band inversion, which is different
from conventional band inversion between s$-$s orbitals, or p$-$p orbitals. The
realization of pure layered MX monolayers may be prepared by exfoliation from
their 3D bulk phases, thus holding great promise for nanoscale device
applications and stimulating further efforts on transition metal-based QSH
materials.
