A Group-Theoretic Approach to the Origin of Chirality-Induced Spin-Selectivity in Nonmagnetic Molecular Junctions

Spin–orbit coupling gives rise to a range of spin-charge
interconversion phenomena in nonmagnetic 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 nonmagnetic 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 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.