Discovery of asymmetric NaXBi (X= Sn /Pb) monolayers with non-trivial topological properties

Abstract

Two-dimensional (2D) Bi-based films have attracted intensive attention recently. However, materials with spatial asymmetry are rarely reported, impeding their practical application. In the present work, based on density functional theory (DFT) calculations, we propose a new type of 2D asymmetric NaXBi (X = Sn and Pb) monolayer, which can realize the coexistence of a topological phase and the Rashba effect. The dynamical and thermal stability are confirmed by the phonon spectra and ab initio DFT molecular dynamic simulations. Analysis of the band structures reveals that NaPbBi is an intrinsic 2D topological insulator with a gap as large as 0.35 eV, far beyond room temperature. The non-trivial topology, caused by p_xy − p_z band inversion, is confirmed by the Z₂ topological index and helical edge states. Remarkably, unlike Bi(111) or BiX (X = H, F, Cl) monolayers, the inversion-symmetry breaking in NaPbBi gives rise to a sizable Rashba splitting energy of 64 meV, which is tunable under external strains (−1 to 7%). Also, an effective tight-binding (TB) model is constructed to understand the origin of the non-trivial topology of NaPbBi. Our work opens a new avenue to designing a feasible 2D asymmetric material platform for application in spintronics.