Coexistence of a spin–valley-coupled Dirac semimetal and robust quantum spin Hall state with significant Rashba spin-splitting in a halogenated BiAs film

Abstract

Two-dimensional (2D) honeycomb layered structures (HLSs) have emerged as a significant pathway for exploring the intercorrelation between two distinct degrees of freedom in Dirac semimetals. The Dirac spin–valley-based characteristics of electrons in HLSs offer an unparalleled opportunity to investigate various physical properties. However, the availability of suitable spin–valley-coupled Dirac semimetals (svc-DSMs) exhibiting robust Dirac spin–valley coupling is quite restricted. Here, we utilize first-principles calculations to explore multiple topological phases in a functionalized bismuth arsenic (BiAs) monolayer. More intriguingly, we demonstrate that it harbors an exceptionally uncommon svc-DSM state under modest tensile strain, which can be attributed to the orbital filtering phenomenon and giant spin splitting (helpful for cutting down spin-flip scattering in spintronics) induced by strong spin–orbit coupling (SOC). The Dirac fermions residing in K/−K valleys exhibit opposite spin moments and Berry curvatures, indicating that the two valleys are inequivalent. We found that strong SOC and broken space inversion symmetry lead to Rashba spin-splitting in the band structure with a noteworthy Rashba parameter of 1.03 eV Å around the M-point of the Brillouin zone. On further increments in strain, the topological phase undergoes a transition from a svc-DSM to a quantum-spin Hall insulator. Through alterations in stacking order with strain, a novel bilayered halogenated BiAs showcases a quantum spin Hall state with a large gap (E_g(k) = 0.26 eV) appearing at the K-point with the inclusion of SOC. Moreover, our finding with the hBN substrate exemplifies that the interaction between the substrate and monolayer does not alter the svc-DSM state of the system. Our findings greatly enhance our understanding of svc-DSMs and offer valuable insights for experimental detection and possible applications at high temperatures.