Spin and Spin-Wave Dynamics in Josephson Junctions

TL;DR

This paper develops a quantum spin dynamics framework in Josephson junctions, revealing novel spin nutations and unconventional spin wave behaviors influenced by superconducting correlations, which affect tunneling currents.

Contribution

It introduces an exact Keldysh-based approach to analyze spin and spin-wave dynamics in Josephson junctions, uncovering new phenomena like spin nutations and modified spin wave behavior.

Findings

Single S=1/2 spins exhibit nutations triggered by superconducting pairing.

Magnets show unconventional spin wave dynamics in Josephson junctions.

Tunneling current is affected by the modified spin dynamics.

Abstract

We extend the Keldysh formulation to quantum spin systems and derive exact equations of motion. This allows us to explore the dynamics of single spins and of ferromagnets when these are inserted between superconducting leads. Several new effects are reported. Chief amongst these are nutations of single S=1/2 spins in Josephson junctions. These nutations are triggered by the superconducting pairing correlations in the leads. Similarly, we find that on rather universal grounds, magnets display unconventional spin wave dynamics when placed in Josephson junctions. These lead to modifications in the tunneling current.