  Spin-orbit coupling (SOC) can induce spin polarization in nonmagnetic 3D
crystals when the inversion symmetry is broken, as manifested by the bulk
Rashba (R-1) and Dresselhaus (D-1) effects. We determine that these spin
polarization effects originate fundamentally from specific atomic site
asymmetries, rather than from the generally accepted asymmetry of the crystal
space-group. This understanding leads to the recognition that a previously
overlooked hidden form of spin polarization should exist in centrosymmetric
materials. Although all energy bands must be doubly degenerate in
centrosymmetric materials, we find that the two components of such doubly
degenerate bands could have opposite polarizations each spatially localized on
one of the two separate sectors forming the inversion partners. We demonstrate
such hidden spin polarizations in centrosymmetric crystals (denoted as R-2 and
D-2) by first-principles calculations. This new understanding could
considerably broaden the range of currently useful spintronic materials and
enable control of spin polarization via operations on atomic scale.
