Group 14 element-based non-centrosymmetric quantum spin Hall insulators with large bulk gap

To date, a number of two-dimensional （2D） topological insulators （TIs） have been realized in Group 14 elemental honeycomb lattices, but all are inversionsymmetric. Here, based on first-principles calculations, we predict a new family of 2D inversion-asymmetric TIs with sizeable bulk gaps from 105 meV to 284 meV, in X2-GeSn （X = H, F, Cl, Br, I） monolayers, making them in principle suitable for room-temperature applications. The nontrivial topological characteristics of inverted band orders are identified in pristine X2-GeSn with X = （F, Cl, Br, I）, whereas H2-GeSn undergoes a nontrivial band inversion at 8% lattice expansion. Topologically protected edge states are identified in X2-GeSn with X = （F, Cl, Br, I）, as well as in strained H2-GeSn. More importantly, the edges of these systems, which exhibit single-Dirac-cone characteristics located exactly in the middle of their bulk band gaps, are ideal for dissipationless transport. Thus, Group 14 elemental honeycomb lattices provide a fascinating playground for the manipulation of quantum states.