Synergistic effect of carrier velocity and density on chirality-induced spin selectivity in helical organic devices

The synergistic effect of the carrier velocity and density is studied to understand chirality-induced spin selectivity (CISS) in helical spin-filtering devices. Unlike most previous theories that emphasize the asymmetry between the transmitted spin-related electron densities, we reveal two types of dynamics for the spin polarization: (i) spin conversion dynamics between spin-up and spin-down electrons and (ii) spin-dependent transport dynamics leading to different propagation velocities of spin-up and spin-down electrons. The former dominate in the high-bias-voltage regime by electron tunneling, while the latter dominate in the low-bias-voltage regime by polaron transport. Our theory reproduces well the recently observed extreme spin polarization under a certain bias voltage in a helical spin-filtering device, and it provides a general microscopic perspective for reconsidering CISS.