Probing Out-of-Equilibrium Spin Modes in a Mesoscopic Superconductor

While energy and charge-mode excitations in a superconductor (associated to out-of-equilibrium Bogoliubov quasiparticles) are well known, spin modes have been addressed only recently. If spins are injected into a mesoscopic superconductor, a long-range out-of-equilibrium magnetization can be observed [1], and both the spin accumulation [2] and spin relaxation [3] times have been measured, being ~10-20 ns and ~50 ps respectively.
However a key question still remains: can the system be described by a truly spin dependent distribution function? In this work we studied the diffusion of quasiparticles injected into a spin-split superconducting Al nanowire, and performed a spectroscopic study of the quasiparticle population using spin sensitive detectors located at different distances from the injection point. We will present evidence for a spin-mode non-equilibrium distribution at time / length scales shorter that the spin-flip one (about 250 nm) as well as an energy-mode spin accumulation above it.
[1] C. H. L. Quay, D. Chevallier, C. Bena, and M. Aprili, Nat. Phys. 9, 84 (2013).
[2] C. H. L. Quay, M. Weideneder, Y. Chiffaudel, C. Strunk, and M. Aprili, Nat. Commun. 6, 9660 (2015).
[3] C. H. L. Quay, C. Dutreix, D. Chevallier, C. Bena, and M. Aprili, Phys. Rev. B 93, 220501(R) (2016)