  It was previously believed that the Bloch electronic states of non-magnetic
materials with inversion symmetry cannot have finite spin polarizations.
However, since the seminal work by Zhang et al. [Nat. Phys. 10, 387-393 (2014)]
on local spin polarizations of Bloch states in non-magnetic, centrosymmetric
materials, the scope of spintronics has been significantly broadened. Here, we
show, using a framework that is universally applicable independent of whether
hidden spin polarizations are small (e.g., diamond, Si, Ge, and GaAs) or large
(e.g., MoS2 and WSe2), that the corresponding quantity arising from orbital -
instead of spin - degrees of freedom, the hidden orbital polarization, is (i)
much more abundant in nature since it exists even without spin-orbit coupling
and (ii) more fundamental since the interband matrix elements of the
site-dependent orbital angular momentum operator determines the hidden spin
polarization. We predict that the hidden spin polarization of transition metal
dichalcogenides is reduced significantly upon compression. We suggest
experimental signatures of hidden orbital polarization from photoemission
spectroscopies and demonstrate that the current-induced hidden orbital
polarization may play a far more important role than its spin counterpart in
antiferromagnetic information technology by calculating the current-driven
antiferromagnetism in compressed silicon.
