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

Journal article, 2010

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 Ω. 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 Ω, is emitted in the leads. Detailed understanding of the charge and spin currents is obtained in the entire parameter range. In the adiabatic regime, ℏΩ⪡2Δ, where Δ is the superconducting gap, and for high transparencies of the junction, a strong suppression of the current takes place around φ≈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 (quasiparticlelike) 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 backaction of the spin current on the dynamics of the precessing spin. Indeed, we show that the Gilbert damping due to the quasiparticle 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 ongoing experiments in the field of molecular spintronics.