A group-theoretic approach to the origin of chirality-induced spin selectivity in non-magnetic molecular junctions

TL;DR

This paper uses group theory and symmetry analysis to explain the origins of chirality-induced spin selectivity (CISS) in molecular junctions, highlighting the importance of electrode and molecular symmetries in spin polarization.

Contribution

It introduces a geometrical, symmetry-based framework for understanding CISS, emphasizing the role of electrode and junction symmetries beyond molecular chirality alone.

Findings

Electrode symmetries influence spin polarization in molecular junctions.

Achiral molecules can exhibit CISS if the junction as a whole is chiral.

Predictions of spin-polarization changes upon enantiomer substitution are validated by DFT calculations.

Abstract

Spin-orbit coupling gives rise to a range of spin-charge interconversion phenomena in non-magnetic systems where certain spatial symmetries are reduced or absent. Chirality-induced spin selectivity (CISS), a term that generically refers to a spin-dependent electron transfer in non-magnetic chiral systems, is one such case, appearing in a variety of seemingly unrelated situations ranging from inorganic materials to molecular devices. In particular, the origin of CISS in molecular junctions is a matter of an intense current debate. Here we derive a set of geometrical conditions for this effect to appear, hinting at the fundamental role of symmetries beyond otherwise relevant quantitative issues. Our approach, which draws on the use of point-group symmetries within the scattering formalism for transport, shows that electrode symmetries are as important as those of the molecule when it comes to the emergence of a spin-polarization and, by extension, to the possible appearance of CISS. It turns out that standalone metallic nanocontacts can exhibit spin-polarization when relative rotations which reduce the symmetry are introduced. As a corollary, molecular junctions with $\textbf{achiral}$ molecules can also exhibit spin-polarization along the direction of transport, provided that the whole junction is chiral in a specific way. This formalism also allows the prediction of qualitative changes of the spin-polarization upon substitution of a chiral molecule in the junction with its enantiomeric partner. Quantum transport calculations based on density functional theory corroborate all of our predictions and provide further quantitative insight within the single-particle framework.