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Abstract : Superconductors out-of-equilibrium have been extensively studied since the advent of thin-film technologies in the 1960s, with a recent revival of interest driven by ‘quasiparticle poisoning’ phenomena in superconducting quantum circuits. Comparatively little attention has, however, been paid to the spin degree of freedom. A few years ago, we demonstrated the existence of a long-lived ( 10ns) almost-chargeless spin imbalance in the quasiparticle (excitation) population of a mesoscopic superconductor with a Zeeman-split density of states. This imbalance exists as a result of the dynamic equilibrium between spin injection (enabled by the Zeeman effect) and relaxation. We have since confirmed the spin imbalance lifetime independently in frequency-domain measurements. Moving beyond the simple existence of spin imbalances, we have begun to look at the spin-dependent dynamics of quasiparticles – this will be the main focus of my talk, after an introduction to out-of-equilibrium physics and ‘spin statics’ in superconductors. Notably, we have demonstrated quasiparticle spin resonance in superconducting aluminium using two novel, on-chip microwave power meters. The quasiparticle spin decoherence time obtained ( 100 ps), and its dependence on the sample thickness are consistent with Elliott-Yafet spin-orbit scattering as the main decoherence mechanism, and suggests that the spin imbalance lifetime is limited by inelastic processes. I’ll also discuss experiments in progress on the spin-dependence of the recombination dynamics of quasiparticles.

References C. H. L. Quay, Y. Chiffaudel, C. Strunk and M. Aprili, ‘Quasiparticle spin resonance and coherence in superconducting aluminium,’ Nature Communications 6, 8660 (2015).

C. H. L. Quay, C. Dutreix, D. Chevallier, C. Bena and M. Aprili, ‘Frequency-domain measurement of the Spin imbalance lifetime in superconductors,’ Physical Review B 93, 220501® (2016).

C. H. L. Quay, D. Chevallier, C. Bena and M. Aprili, ‘Spin-imbalance and spin charge separation in a mesoscopic superconductor,’ Nature Physics, 9, 84–88 (2013).

Contact : Benjamin Sacepe