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