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Topological insulators (TIs) are promising for achieving dissipationlesstransport devices due to the robust gapless states inside the insulating bulkgap. However, currently realized 2D TIs, quantum spin Hall (QSH) insulators,suffer from ultra-high vacuum and extremely low temperature. Thus, seeking fordesirable QSH insulators with high feasibility of experimental preparation andlarge nontrivial gap is of great importance for wide applications inspintronics. Based on the first-principles calculations, we predict a novelfamily of two-dimensional (2D) QSH insulators in transition-metal halide MX (M= Zr, Hf; X = Cl, Br, and I) monolayers with large nontrivial gaps of0.12$-$0.4 eV, comparable with bismuth (111) bilayer (0.2 eV), stanene (0.3 eV)and larger than ZrTe$_5$ (0.1 eV) monolayers and graphene-based sandwichedheterstructures (30$-$70 meV). Their corresponding 3D bulk materials are weaktopological insulators from stacking QSH layers, and some of bulk compoundshave already been synthesized in experiment. The mechanism for 2D QSH effect inthis system originates from a novel d$-$d band inversion, which is differentfrom conventional band inversion between s$-$s orbitals, or p$-$p orbitals. Therealization of pure layered MX monolayers may be prepared by exfoliation fromtheir 3D bulk phases, thus holding great promise for nanoscale deviceapplications and stimulating further efforts on transition metal-based QSHmaterials.

New Family of Quantum Spin Hall Insulators in Two-dimensional Transition-Metal Halide with Large Nontrivial Band Gaps