Large-band-gap non-Dirac quantum spin Hall states and strong Rashba effect in functionalized thallene films

The quantum spin Hall state materials have recently attracted much attention owing to their potential applications in the design of spintronic devices. Based on density functional theory calculations and crystal field theory, we study electronic structures and topological properties of functionalized thallene films. Two different hydrogenation styles (Tl₂H and Tl₂H₂) are considered, which can drastically vary the electronic and topological behaviors of the thallene. Due to the C_3v symmetry of the two systems, the p_x and p_y orbitals at the Γ point have the non-Dirac band degeneracy. With spin-orbit coupling (SOC), topological nontrivial band gaps can be generated, giving rise to non-Dirac quantum spin Hall states in the two thallium hydride films. The nontrivial band gap for the monolayer Tl₂H is very large (855 meV) due to the large on-site SOC of Tl p_x and p_y orbitals. The band gap in Tl₂H₂ is, however, small due to the band inversion between the Tl p_x/y and p_z orbitals. It is worth noting that both the Tl₂H and Tl₂H₂ monolayers exhibit strong Rashba spin splitting effects, especially for the monolayer Tl₂H₂ (α_R = 2.52 eVÅ), rationalized well by the breaking of the structural inversion symmetry. The Rashba effect can be tuned sensitively by applying biaxial strain and external electric fields. Our findings provide an ideal platform for fabricating room-temperature spintronic and topological electronic devices.