Spin polarization in nonmagnetic materials is a textbook effect associated with spin-orbit coupling and the global absence of inversion symmetry. However, we find and validate via first-principles calculations that in centrosymmetric materials with non-centrosymmetric site symmetries (such as MoS2, LaOBiS2, or Bi2Se3) there is a nonzero local spin polarization even though the total spin is zero. Such real-space spin-polarizations are normally hidden by compensation effect, rather than being intrinsically absent, and are already verified by upcoming experiments. This understanding leads to the recognition that a previously overlooked hidden form of spin polarization should exist in a much broader class of 3D bulk solids that own global inversion symmetry, and thus open the possibility to provide new routines for manipulating electron spins.
The concept of hidden spin polarization also explains many previously embarrassing observations of apparent polarization noted in centrosymmetric systems that by common understanding should not have existed. One such example is the recent prediction and subsequent observation of strong circular polarization (CP) in bilayers of transition-metal dichalcogenides that are naturally centrosymmetric and thus should have no “valley polarization” effects. We show that for bilayers where inversion symmetry is present and valley polarization physics is strictly absent, such intrinsic selectivity in CP is to be expected on the basis of fundamental spin-orbit physics.