Spin-dependent transport through a chiral molecule in the presence of spin-orbit interaction and non-unitary effects

TL;DR

This paper models spin-dependent electron transport in chiral molecules, showing how non-unitary effects like leakage break symmetry and produce measurable spin polarization, aligning with experimental observations.

Contribution

It introduces a tight-binding model incorporating spin-orbit interaction and non-unitary effects to explain chiral-induced spin selectivity in electron transport.

Findings

Spin polarization increases with helix length.

Leakage causes evanescent waves with spin-dependent decay.

Maximal polarization occurs at a finite angle from the helix axis.

Abstract

Recent experiments have demonstrated the efficacy of chiral helically shaped molecules in polarizing the scattered electron spin, an effect termed as chiral-induced spin selectivity (CISS). Here we solve a simple tight-binding model for electron transport through a single helical molecule, with spin-orbit interactions on the bonds along the helix. Quantum interference is introduced via additional electron hopping between neighboring sites in the direction of the helix axis. When the helix is connected to two one-dimensional single-mode leads, time-reversal symmetry prevents spin polarization of the outgoing electrons. One possible way to retrieve such a polarization is to allow leakage of electrons from the helix to the environment, via additional outgoing leads. Technically, the leakage generates complex site self-energies, which break unitarity. As a result, the electron waves in the helix become evanescent, with different decay lengths for different spin polarizations, yielding a net spin polarization of the outgoing electrons, which increases with the length of the helix (as observed experimentally). A maximal polarization can be measured at a finite angle away from the helix axis.