Phase Coexistence and Strain-Induced Topological Insulator in Two-Dimensional BiAs

Two-dimensional (2D) binary compounds have been recently reported as promising materials for achieving topological insulator and dissipationless transport devices. On the basis of first-principle calculations combined with a tight binding (TB) model, we investigate a new class of 2D bismuth arsenic (BiAs) polymorphs that are energetically and dynamically stable. The monolayers of alpha-, beta-, and gamma-BiAs allotropes show significant direct band-gap, while the delta- and epsilon-BiAs phases display indirect band-gaps. Using strain-engineering along with spin-orbital coupling (SOC), beta-BiAs transforms from a normal insulator to nontrivial topological phase. Under tensile strain and SOC, the bands are inverted resulting in a topological phase transition with a sizable band-gap of 0.28 eV. The nontrivial topological state is explicitly confirmed by calculating topological invariant, Z(2) = 1, and the characteristic of edge states which are topologically protected in a Dirac cone at the Gamma-point. Hexagonal boron nitride is confirmed as an excellent substrate for supporting the beta-BiAs film without perturbing the topological insulator state. The present results indicate promise for 2D TIs at room temperature.