Current-Induced Nonequilibrium Hidden Spin Polarization in Topological Dirac Semimetals

The effect of hidden spin polarization (HSP) has expanded the material pool for spintronics and provided new physical insights into relevant hidden phenomena such as optical polarization, valley polarization, orbital polarization, and Berry curvature. Here, we uncover an intrinsic HSP in topological Dirac semimetals originating from the three-dimensional quantum Hall effect. This local spin polarization arises through nonequilibrium processes, where the Landau levels tilt due to the PT-symmetry breaking induced by crossed electric and magnetic fields. We find that the nonequilibrium HSP exhibits directional selectivity owing to the spin-momentum-surface locking of the Fermi arcs. As a result of the Weyl-orbit quantization, the local spin polarization exhibits quantum oscillations with sample thickness and displays plateaus as the Fermi energy varies. This nonequilibrium HSP, which can be detected through current-induced nonlocal spin-orbit torques, is unique to the Weyl orbits. Our findings present a novel mechanism for detecting Weyl orbits and for nonlocally controlling magnetization using external electric fields.