First-principles study of electric-dipole-induced Rashba splitting in Janus WSSe

Spin-splitting in electronic band structures is a key mechanism in spintronic functionality. Rashba spin-splitting enables electric control of spin states by both external and built-in electric fields induced by structural asymmetry. We analyzed the Born effective charge Z * in monolayer Janus WSSe using first-principles calculations to understand the relation between dipole moment and spin splitting. Our results revealed an out-of-plane dipole moment of -0.194 Debye, with anomalous Z * behavior explaining the dipole direction. Comparison of spin textures between external field-induced spin-splitting in pristine systems and intrinsic spin-splitting in Janus WSSe shows identical Rashba parameter signs, confirming that the built-in electric field is oriented in the positive out-of-plane direction, Se to S. The Rashba parameters in Janus WSSe, 0.12 eV Å, are comparable to other well-known Rashba systems. These findings highlight that Janus asymmetry enables intrinsic Rashba spin-splitting without external gating, providing design principles for next-generation nanoscale spintronic devices.