Two-dimensional (2D) topological insulators (TIs), also known as quantum spin
Hall (QSH) insulators, are excellent candidates for coherent spin transport
related applications because the edge states of 2D TIs are robust against
nonmagnetic impurities since the only available backscattering channel is
forbidden. Currently, most known 2D TIs are based on a hexagonal (specifically,
honeycomb) lattice. Here, we propose that there exists the quantum spin Hall
effect (QSHE) in a buckled square lattice. Through performing global structure
optimization, we predict a new three-layer quasi-2D (Q2D) structure which has
the lowest energy among all structures with the thickness less than 6.0 {\AA}
for the BiF system. It is identified to be a Q2D TI with a large band gap (0.69
eV). The electronic states of the Q2D BiF system near the Fermi level are
mainly contributed by the middle Bi square lattice, which are sandwiched by two
inert BiF2 layers. This is beneficial since the interaction between a substrate
and the Q2D material may not change the topological properties of the system,
as we demonstrate in the case of the NaF substrate. Finally, we come up with a
new tight-binding model for a two-orbital system with the buckled square
lattice to explain the low-energy physics of the Q2D BiF material. Our study
not only predicts a QSH insulator for realistic room temperature applications,
but also provides a new lattice system for engineering topological states such
as quantum anomalous Hall effect.Comment: 17pages, 4 figures Accepted by nano letter
