  The generally accepted view that spin polarization is induced by the
asymmetry of the global crystal space group has limited the search for
spintronics [1] materials to non-centrosymmetric materials. Recently it has
been suggested that spin polarization originates fundamentally from local
atomic site asymmetries [2], and therefore centrosymmetric materials may
exhibit previously overlooked spin polarizations. Here by using spin- and
angle-resolved photoemission spectroscopy (spin-ARPES), we report helical spin
texture induced by local Rashba effect (R-2) in centrosymmetric monolayer
PtSe$_2$ film. First-principles calculations and effective analytical model
support the spin-layer locking picture: in contrast to the spin splitting in
conventional Rashba effect (R-1), the opposite spin polarizations induced by
R-2 are degenerate in energy while spatially separated in the top and bottom Se
layers. These results not only enrich our understanding of spin polarization
physics, but also may find applications in electrically tunable spintronics.
