  Two-dimensional (2D) topological insulators (TIs) hold promise for
applications in spintronics based on the fact that the propagation direction of
edge electrons of a 2D TI is robustly linked to their spin origination. Here,
with the use of first-principles calculations, we predict a family of robust 2D
TIs in monolayer square transition metal dichalcogenides (MoS2, MoSe2, MoTe2,
WS2, WSe2, and WTe2). Sizeable intrinsic nontrivial bulk band gaps ranging from
24 to 187 meV are obtained, guarantying the quantum spin Hall (QSH) effect
observable at room temperature in these new 2D TIs. Significantly different
from most known 2D TIs with comparable band gaps, these sizeable energy gaps
originate from the strong spin-orbit interaction related to the pure d
electrons of the Mo/W atoms around the Fermi level. A single pair of
topologically protected helical edge states is established for the edge of
these systems with the Dirac point locating in the middle of the bulk band gap,
and their topologically nontrivial states are also confirmed with nontrivial
topological invariant Z2 = 1. More interestingly, by controlling the applied
strain, a topological quantum phase transition between a QSH phase and a
metallic phase or a trivial insulating phase can be realized in these 2D
materials, and the detailed topological phase diagram is established.
