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The established spin splitting in a monolayer (ML) of transition metal dichalcogenides (TMDs) induced by inversion symmetry breaking is dictated by mirror symmetry operations to exhibit the fully out-of-plane direction of spin polarization. Through first-principles density functional theory calculations, we show that polarity inducing mirror symmetry breaking leads to sizable spin splitting having in-plane spin polarization. These splittings are effectively controlled by tuning the polarity using biaxial strain. Furthermore, admixtures between the out-of-plane and in-plane spin-polarized states in the strained polar systems are identified, which are expected to influence the spin relaxation through the Dyakonov–Perel mechanism. Our study clarified that polarity plays an important role in controlling the spin splitting and spin relaxation in the TMD ML, which is useful for designing future spintronic devices.

The persistent spin helix (PSH) that has been widely and exclusively studied in zinc-blende structures is revealed for the first time on the surface of a wurtzite structure. Through first principles calculations of the surface of ZnOð1010Þ, a quasi-one-dimensional orientation of the spin textures is identified. Furthermore, the wavelength of this particular PSH is smaller than that observed in various zinc-blende quantum well structures, thus indicating that wurtzite-structured surfaces are suitable for spintronics applications.