Nonlinear Rashba effect and spin Hall conductivity in the 2D Janus MXenes

Rashba spin-orbit coupling (RSOC) facilitates the manipulation of electron spins without requiring external magnetic fields, paving the way for exciting spintronic devices. While the linear Rashba-Bychkov model is widely used to estimate RSOC strength and spin precession lengths, it often falls short in systems with higher symmetries and non-parabolic bands, necessitating the inclusion of cubic momentum terms (k³) in the original Rashba Hamiltonian. In this study, we explore the effects of crystal momentum nonlinearity and anisotropy on RSOC and its impact on spin Hall conductivity (SHC) in a newly predicted 2D Janus monolayer family, W₂COX (X = S, Se, Te). These materials possess significant nonlinear RSOC at the Γ point of Brillouin zone near the Fermi level, with W₂COS exhibiting the highest k³ contribution of -45.9 eVų, despite having the lowest linear Rashba constant. They also exhibit nontrivial topological behavior, with substantial SHC primarily governed by nonlinear RSOC. Notably, their spin Hall angles (SHA), ranging from 0.018 to 2.5, are comparable to those of well-known bulk topological insulators, such as Bi₂Se₃ and Bi₂Te₃. The substantial SHA, SHC, and nontrivial topological behavior make these materials promising for advanced spintronic applications.