Employing ab initio electronic calculations, we propose a new type of two-dimensional (2D) topological insulator (TI), monolayer (ML) low buckled (LB) mercury telluride (HgTe) and mercury selenide (HgSe), with tunable band gaps. Monolayer LB HgTe undergoes a transition to a topological nontrivial phase under the appropriate in-plane tensile strain ({\epsilon} > 2.6%) due to the combination effects of strain and spin orbital coupling (SOC). Under the 2.6%< {\epsilon} <4.2% tensile strain, the band inversion and topological nontrivial gap are induced by the SOC. For {\epsilon} >4.2%, the band inversion is already realized by strain but the topological gap is induced by SOC. The band gap of monolayer LB HgTe TI phase can be tuned over a wide range from 0 eV to 0.20 eV as the tensile strain increases from 2.6% to 7.4%. Similarly, the topological phase transition of monolayer LB HgSe is induced by strain and SOC as the strain {\epsilon} >3.1%. The topological band gap can be 0.05 eV as the strain increases to about 4.6%. The large band gap of 2D LB HgTe and HgSe monolayers make this type of material suitable for practical applications at room-temperature.
@article{arxiv.1412.2528,
title = {Two dimensional topological insulators with tunable band gaps: HgTe and HgSe monolayers},
author = {Jin Li and Chaoyu He and Lijun Meng and Huaping Xiao and Chao Tang and Xiaolin Wei and Jinwoong Kim and Nicholas Kioussis and G. Malcolm Stocks and Jianxin Zhong},
journal= {arXiv preprint arXiv:1412.2528},
year = {2015}
}