Unusual Spin Polarization in the Chirality Induced Spin Selectivity

TL;DR

This paper redefines the understanding of chirality-induced spin selectivity (CISS), showing that chiral molecules act as spin polarizers with the same spin polarization in transmitted and reflected electrons, rather than as spin filters.

Contribution

It challenges the traditional spin filter model of CISS, proposing that chiral molecules polarize spins in a manner consistent with equilibrium principles and provides a new interpretation of experimental results.

Findings

Both transmitted and reflected electrons exhibit the same spin polarization.

The spin polarization direction depends on molecule chirality and incident electron direction.

The magnitude of spin polarization is influenced by local spin-orbit coupling.

Abstract

Chirality-induced spin selectivity (CISS) refers to the fact that electrons get spin polarized after passing through organic chiral molecules in a nanoscale device. In CISS, chiral molecules are commonly believed to be a spin filter through which one favored spin transmits and the opposite spin gets reflected, i.e., transmitted and reflected electrons exhibit opposite spin polarization. In this work, we point out that such a spin filter scenario contradicts the principle that equilibrium spin current must vanish. Instead, we find that both transmitted and reflected electrons present the same type spin polarization, which is actually ubiquitous for a two-terminal device. More accurately, chiral molecules play the role of a spin polarizer rather than a spin filter. The direction of spin polarization is determined by the molecule chirality and the electron incident direction. And the magnitude of spin polarization replies on local spin-orbit coupling in the device. Our work brings a deeper understanding on CISS and interprets recent experiments, for example, the CISS-driven anomalous Hall effect.