Strain-controlled spin-splitting in the persistent spin helix state of two-dimensional SnSe monolayer

Recently, ferroelectric materials have attracted much attention in the field of spintronics since they enable to manipulation of spin splitting and spin textures via electric control of electric polarization. Here, twodimensional ferroelectric SnSe monolayer (ML) comprises a promising candidate for spintronics since it has large and switchable in-plane electric polarization. However, the application of strain strongly modifies the electronic properties of the SnSe ML, which is expected to significantly affect the properties of the spin-splitting bands. By using fully-relativistic density-functional theory calculations, we predict that a strongly anisotropic spin splitting is observed in the energy bands near both the conduction band minimum (CBM) and valence band maximum (VBM), which can be effectively tuned by applying a small uniaxial in-plane strain. We demonstrate that these spin-splitting bands exhibit a unidirectional out-of-plane spin configuration in the momentum space, giving rise to a long-lived persistent spin helix (PSH) state. Importantly, a giant spin splitting (up to 245 meV) is achieved under the uniaxial in-plane tensile strain without losing the nature of the PSH state, thus offering a possibility for realizing efficient spintronic devices enabling operation at room temperature.