  Symmetry forbidden effects in crystals may emerge in a local environment that
breaks the symmetries. Yet these hidden physical effects were only discussed in
centrosymmetric crystals. Here we propose that hidden physical effects can be
generalized to almost all crystallographic symmetric groups and hence reveal
their universality and diversity. We systematically discuss certain symmetries
in crystals that may induce hidden spin polarization (HSP), hidden berry
curvature and hidden valley polarization, with a focus on a specific pattern of
HSP whose winding directions of the in-plane spin vectors are the same for
adjacent bands, dubbed anomalous HSP. Such an unprecedented spin pattern arises
from relatively weak spin-dependent inter-sector interaction and is
demonstrated in mirror-symmetric InSe by first-principles calculations. Unique
electric field-dependent splitting into specific spatial polarization pattern
may serve as an experimental signature of anomalous HSP. Our results reveal
abundant hidden physical effects beyond centrosymmetric crystals and provide
new platforms to discuss them for emergent physical effects and future
applications.
