Tuning Rashba effect, band inversion, and spin-charge conversion of Janus XSn2Y monolayers via an external field

We predict, through first-principles calculations, a different class of Janus two-dimensional (2D) materials $X{\mathrm{Sn}}_{2}Y$ ($X, Y=\mathrm{P}$, As, Sb, and Bi; $X\ensuremath{\ne}Y$). It has found that these 2D monolayers have intrinsic polarization effect owing to presence of mirror-symmetry breaking. $\mathrm{Sb}{\mathrm{Sn}}_{2}\mathrm{Bi}, \mathrm{As}{\mathrm{Sn}}_{2}\mathrm{Sb}$, and $\mathrm{P}{\mathrm{Sn}}_{2}\mathrm{As}$ have a semiconducting feature, however $\mathrm{As}{\mathrm{Sn}}_{2}\mathrm{Bi}, \mathrm{P}{\mathrm{Sn}}_{2}\mathrm{Bi}$, and $\mathrm{P}{\mathrm{Sn}}_{2}\mathrm{Sb}$ have a metallic feature with inverted band structure. When the spin-orbit coupling (SOC) is considered, the double Rashba effects are found in $\mathrm{Sb}{\mathrm{Sn}}_{2}\mathrm{Bi}$. Band inversion coupling with SOC leads to unexpected spin-valley splitting characteristics in $\mathrm{As}{\mathrm{Sn}}_{2}\mathrm{Bi}, \mathrm{P}{\mathrm{Sn}}_{2}\mathrm{Bi}$, and $\mathrm{P}{\mathrm{Sn}}_{2}\mathrm{Sb}$, showing a circle-type Berry curvature. The electronic and spin properties of Janus $X{\mathrm{Sn}}_{2}Y$ monolayers are tunable via applying external strain and electric field, resulting in Rashba-type spin-splitting, ${p}_{z}/{p}_{xy}$ band inversion, and a Dirac cone. In particular, the sombrero band dispersion and Van Hove singularity are revealed in $\mathrm{P}{\mathrm{Sn}}_{2}\mathrm{As}$ by exerting tensile strain. We therefore investigate the Lifshitz transition and inverse Edelstein effect (also referred as spin-charge conversion) of the strained $\mathrm{P}{\mathrm{Sn}}_{2}\mathrm{As}$. On the basis of the k\ifmmode\cdot\else\textperiodcentered\fi{}p model in the clean limit, the Drude coefficient for spin-charge conversion is estimated to be about 0.086 $e$/\AA{} as tensile strain of +3%. Finally, the synergic effect of tensile strain and electric field is considered to manipulate the spin-charge conversion. Our results provide a class of 2D materials to investigate the Rashba effect, Lifshitz transition, and spin-charge conversion.