Non-equilibrium charge and spin transport in SFS point contacts

TL;DR

This paper explores how a precessing classical spin at a superconductor interface influences charge and spin transport, revealing novel effects like spin-flip scattering, sidebands in Andreev states, and potential for rapid spin reversal.

Contribution

It introduces a model of a Josephson junction with a precessing spin, showing how spin dynamics affect Andreev states, currents, and enable control of spin orientation.

Findings

Precessing spin induces sidebands in Andreev states.

Spin current is supported by spin-triplet correlations.

Resonances can reverse spin direction rapidly.

Abstract

The conventional Josephson effect may be modified by introducing spin-active scattering in the interface-layer of the junction. Here, we discuss a Josephson junction consisting of two s-wave superconducting leads coupled over a classical spin that precesses with the Larmor frequency due to an external magnetic field. This magnetically active interface results in a time-dependent boundary condition with different tunnelling amplitudes for spin-up and -down quasiparticles and where the precession produces spin-flip scattering processes. As a result, the Andreev states develop sidebands and a non-equilibrium population that depend on the details of the spin precession. The Andreev states carry a steady-state Josephson charge current and a time-dependent spin current, whose current-phase relations could be used for characterising the precessing spin. The spin current is supported by spin-triplet correlations induced by the spin precession and creates a feed-back effect on the classical spin in the form of a torque that shifts the precession frequency. By applying a bias voltage, the Josephson frequency adds another complexity to the situation and may create resonances together with the Larmor frequency. These Shapiro resonances are manifested as torques and are, under suitable conditions, able to reverse the direction of the classical spin in sub-nanosecond time. Another characteristic feature is the subharmonic gap structure in the dc charge current displaying an even-odd effect that is attributable to precession-assisted multiple Andreev reflections.