Orbital and spin polarizations induced by current through a helical atomic chain

Current-induced spin and orbital polarizations are theoretically studied in a helical atomic chain with ${p}_x, {p}_y$, and ${p}_z$ orbitals in each atom in the linear response to the chemical-potential difference between source and drain electrodes. It is shown that these polarizations are parallel to the helix axis and proportional to the curvature of the cylinder surface defined by the helix in the small curvature region. This proportionality suggests that the curvature generates the coupling between the current and the atomic orbital angular momentum and therefore influences the magnitude of induced spin polarization through the $LS$ coupling. It is also found that current-induced polarizations are nonzero even when the pitch is enforced to vanish. This suggests the possibility that nonchiral structures with curvature can produce current-induced polarizations. With this possibility in mind, it is proposed that the chirality in the current, rather than that in the structure, determines the existence of such current-induced polarizations.