Relativistic Effects and Spin Selectivity in Chiral Molecules

The Chirality-Induced Spin Selectivity (CISS) effect refers to the ability of chiral molecules and crystals to transmit spin-polarized currents, first observed in 1999 (\textit{Science} \textbf{283}, 814, 1999). Despite its promise for spintronics and electron transfer, its microscopic origin remains unclear (\textit{Adv. Mater.} \textbf{34}, 2106629, 2022; \textit{Nano Lett.} \textbf{19}, 5253, 2019). The effect is often attributed to enhanced spin-orbit coupling (SOC) in chiral structures (\textit{Phys. Rev. Lett.} \textbf{133}, 036201, 2024), and may involve non-trivial topological states (\textit{Nat. Commun.} \textbf{14}, 5163, 2023). In this work, we explore CISS related mechanisms in chiral carbon based molecular systems, where local curvature, such as near defects induces significant SOC. Using a fully relativistic density functional theory framework, we show that spin-momentum locking and spin polarization naturally arise from relativistic electron dynamics. Our findings suggest that a complete description of CISS requires moving beyond spherically averaged SOC models and call for energy functionals that explicitly depend on the electrons' chirality (\textit{arXiv:2412.18413}, 2024).