Spin-orbit induced chirality of Andreev states in Josephson junctions

TL;DR

This paper investigates how Rashba spin-orbit coupling in Josephson junctions induces chirality in Andreev states, leading to unconventional Josephson effects influenced by spin polarization and magnetic fields.

Contribution

It demonstrates the link between spin polarization in the normal state and the chirality of Andreev states, revealing new effects in Josephson junctions with spin-orbit coupling.

Findings

Strong correlation between Andreev state spin and Cooper pair transport direction

Anisotropic Josephson behavior under weak magnetic fields

Unconventional Josephson effects due to spin-momentum locking

Abstract

We study Josephson junctions (JJs) in which the region between the two superconductors is a multichannel system with Rashba spin-orbit coupling (SOC) where a barrier or a quantum point contact (QPC) is present. These systems might present unconventional Josephson effects such as Josephson currents for zero phase difference or critical currents that \textit{depend on} the current direction. Here, we discuss how the spin polarizing properties of the system in the normal state affect the spin characteristic of the Andreev bound states inside the junction. This results in a strong correlation between the spin of the Andreev states and the direction in which they transport Cooper pairs. While the current-phase relation for the JJ at zero magnetic field is qualitatively unchanged by SOC, in the presence of a weak magnetic field a strongly anisotropic behavior and the mentioned anomalous Josephson effects follow. We show that the situation is not restricted to barriers based on constrictions such as QPCs and should generically arise if in the normal system the direction of the carrier's spin is linked to its direction of motion.