  Spin polarization effects in nonmagnetic materials are generally believed as
an outcome of spin-orbit coupling provided that the global inversion symmetry
is lacking, also known as 'spin-momentum locking'. The recently discovered
hidden spin polarization indicates that specific atomic site asymmetry could
also induce measurable spin polarization, leading to a paradigm shift to
centrosymmetric crystals for potential spintronic applications. Here, combining
spin- and angle-resolved photoemission spectroscopy and theoretical
calculations, we report distinct spin-layer locking phenomena surrounding
different high-symmetry momenta in a centrosymmetric, layered material BiOI.
The measured spin is highly polarized along the Brillouin zone boundary, while
is almost vanishing around the zone center due to its nonsymmorphic crystal
structure. Our work not only demonstrates the existence of hidden spin
polarization, but also uncovers the microscopic mechanism of the way spin,
momentum and layer locking to each other, shedding lights on the design metrics
for future spintronic devices.
