  By the generalized gradient approximation in framewok of density functional
theory, we investigate a 2D topological insulator of new silicon allotrope
(call dumbbell silicene synthesized recently by Cahangirov et al) through
tuning external compression strain, and find a topological quantum phase
transition from normal to topological insulator, i.e., the dumbbell silicene
can turn a two-dimensional topological insulator with an inverted band gap. The
obtained maximum topological nontrivial band gap about 12 meV under isotropic
strain is much larger than that for previous silicene, and can be further
improved to 36 meV by tuning anisotropic strain, which is sufficiently large to
realize quantum spin Hall effect even at room-temperature, and thus is
beneficial to the fabrication of high-speed spintronics devices. Furthermore,
we confirm that the boron nitride sheet is an ideal substrate for the
experimental realization of the dumbbell silicene under external strain,
maintaining its nontrivial topology. These properties make the two-dimensional
dumbbell silicene a good platform to study novel quantum states of matter,
showing great potential for future applications in modern silicon-based
microelectronics industry.
