Electron Spin Polarization as a Predictor of Chiroptical Activity in Helical Molecules

TL;DR

This study demonstrates that electron spin polarization can predict chiroptical activity in helical molecules, revealing a fundamental link involving spin-orbit coupling and symmetry breaking.

Contribution

It establishes a novel connection between spin polarization and chiroptical activity in a simple helical model including spin-orbit coupling.

Findings

SP norm predicts COA in helical molecules

TRS breaking is necessary for non-zero SOC contribution to COA

Model captures key physics of chiroptical phenomena

Abstract

Chiral structures, breaking spatial inversion symmetry, exhibit non-zero chiroptical activity (COA) due to the interaction between their electric and magnetic responses under external electromagnetic fields, an effect that is otherwise absent in achiral systems. Non-magnetic chiral structures also exhibit Chiral Induced Spin Selectivity (CISS), where spin-polarization (SP) emerges without external magnetic influence. We have obtained a COA-SP connection for a model system of an electron constrained to a helix including spin-orbit coupling (SOC), and in the presence of an external electromagnetic field. Despite its simplicity, this model captures the relevant physics required to address the problem. In particular, our results reveal that the norm of the SP vector can be used as a predictor of COA. In addition to SOC and the breaking of space inversion, a non-vanishing SP requires the breaking of time-reversal symmetry (TRS), as demanded by Onsager's reciprocity. Beyond the relationship between SP and COA, we obtain the novel result that TRS breaking is also necessary to yield a non-vanishing contribution of the SOC to the COA.