  Large bulk band gap is critical for application of the quantum spin Hall
(QSH) insulator or two dimensional (2D) topological insulator (TI) in
spintronic device operating at room temperature (RT). Based on the
first-principles calculations, here we predict a group of 2D topological
insulators BiX/SbX (X = H, F, Cl, and Br) monolayers with extraordinarily large
bulk gaps from 0.32 to a record value of 1.08 eV. These giant-gaps are entirely
due to the result of strong spin-orbit interaction related to px and py
orbitals of Bi/Sb atoms around the two valley K and K' of honeycomb lattice,
which is different significantly from the one consisted of pz orbital just like
in graphene/silicene. The topological characteristic of BiX/SbX monolayers is
confirmed by the calculated nontrivial Z2 index and an explicit construction of
the low energy effective Hamiltonian in these systems. We show that the
honeycomb structures of BiX monolayers remain stable even at a temperature of
600 K. These features make the giant-gap TIs BiX/SbX monolayers an ideal
platform to realize many exotic phenomena and fabricate new quantum devices
operating at RT. Furthermore, biased BiX/SbX monolayers become a quantum valley
Hall insulator, showing valley-selective circular dichroism.
