Transport and magnetization dynamics in a superconductor/single-molecule magnet/superconductor junction

TL;DR

This paper investigates charge and spin transport in a superconductor/single-molecule magnet/superconductor junction, revealing how spin dynamics influence superconducting currents and identifying regimes with suppressed or enhanced transport properties.

Contribution

It provides a comprehensive analysis of superconducting and spin currents in a junction with a precessing spin, including the effects of phase bias, transparency, and precession frequency, extending understanding of spin-dependent transport in such systems.

Findings

Steady-state charge current flows in phase-biased junctions.

Out-of-equilibrium spin current of frequency Ω is emitted into leads.

Strong suppression of current occurs near phase difference ≈ 0 at high transparency.

Abstract

We study dc-transport and magnetization dynamics in a junction of arbitrary transparency consisting of two spin-singlet superconducting leads connected via a single classical spin precessing at the frequency $\Omega$. The presence of the spin in the junction provides different transmission amplitudes for spin-up and spin-down quasiparticles as well as a time-dependent spin-flip transmission term. For a phase biased junction, we show that a steady-state superconducting charge current flows through the junction and that an out-of-equilibrium circularly polarized spin current, of frequency $\Omega$, is emitted in the leads. Detailed understanding of the charge and spin currents is obtained in the entire parameter range. In the adiabatic regime, $\hbar \Omega \ll 2\Delta$ where $\Delta$ is the superconducting gap, and for high transparencies of the junction, a strong suppression of the current takes place around $\vp \approx 0$ due to an abrupt change in the occupation of the Andreev bound-states. At higher values of the phase and/or precession frequency, extended (quasi-particle like) states compete with the bound-states in order to carry the current. Well below the superconducting transition, these results are shown to be weakly affected by the back-action of the spin current on the dynamics of the precessing spin. Indeed, we show that the Gilbert damping due to the quasi-particle spin current is strongly suppressed at low-temperatures, which goes along with a shift of the precession frequency due to the condensate. The results obtained may be of interest for on-going experiments in the field of molecular spintronics.