Strain-Driven Control of Electric Polarization & Spin Splitting in The Persistent Spin Helix State of XO₂Y₂ (X=W, Mo; Y=Cl, Br, I) Monolayer

The realization of spin field effect transistors requires robust spin splitting and coherent spin precession in the presence of ferroelectric polarization. Recent findings indicate that integrating ferroelectricity with spin-orbit coupling in two-dimensional (2D) non-centrosymmetric systems enables non-volatile control of spin textures in momentum space. However, 2D ferroelectric materials with significant spin splitting are scarce. To navigate the challenge, strain engineering offers an effective approach to tuning electronic properties, including ferroelectric polarization and spin splitting.

Within the framework of density functional theory, we investigated the electronic structure, tunability of spontaneous polarization, and spin texture of XO₂Y₂ (X=W, Mo; Y=Cl, Br, I) monolayer under uniaxial strain. We obtained tunable spin splitting in the conduction band minimum of the WO₂Cl₂ monolayer, with considerable spin-orbit strength reaching up to 2.16 eVÅ, allowing the device to function at room temperature. Additionally, we highlight the emergence of a reversible persistent spin helix across compounds, maintained across strained variants. This spatially periodic mode is caused by the unidirectional spin-orbit field, underscoring their potential for spin field effect transistors.