Anisotropic spin-split states with canted persistent spin textures in two-dimensional Janus $1T^{'}$ $MXX'$ ($M$ = Mo, W; $X\neq X'$= S, Se, Te) controlled by surface alloying

Abstract

Two-dimensional tungsten-based transition metal dichalcogenides (TMDCs), $MX_{2}$ ($M$: W, Mo; $X$: S, Se, Te) monolayers (MLs) with a $1T'$ structure, serve as significant-gap quantum spin Hall insulators. However, due to the centrosymmetric nature of these crystals, spin degeneracy persists throughout their electronic band structures, limiting their potential for spintronic applications. By modifying the chalcogen ($X$) atoms in the TMDCs ML surface to create a higly stable Janus $MXX'$ MLs structure, we demonstrate through density-functional theory calculations that substantial spin splitting of the electronic states can be achieved. Taking the Janus $1T'$ WSTe ML as a representative case, we identify pronounced anisotropic spin-splitting bands, with maximum spin splittings of 0.14 eV and 0.10 eV occurring at the highest occupied states and lowest unoccupied states, respectively. These significant band splittings give rise to canted persistent spin textures (PST) in the spin polarization, which differ significantly from those in commonly studied PST materials. We demonstrate that this intricate spin splitting and unique spin textures stem from strong in-plane $p-d$ orbital interactions between tungsten (W) and the chalcogen atoms (Te and Se), driven by the reduced symmetry of the crystal's point group. Further analysis using a $\vec{k}\cdot\vec{p}$ model derived from symmetry considerations corroborates the origins of the observed anisotropic spin splitting and canted PST. the spin-split states are highly sensitive to surface imperfections caused by surface alloying effects, such as variations in the chalcogen composition on the monolayer surface. These findings underscore the potential of Janus $1T'$ $MXX'$ MLs as promising candidates for next-generation spintronic devices.