Spin imbalance and spin-charge separation in a mesoscopic superconductor

What happens to spin-polarized electrons when they enter a superconductor? Superconductors at equilibrium and at finite temperature contain both paired particles (of opposite spin) in the condensate phase as well as unpaired, spin-randomized quasiparticles. Injecting spin-polarized electrons into a superconductor (and removing pairs) thus creates both spin and charge imbalances(1-7), which must relax when the injection stops, but not necessarily over the same time (or length) scale. These different relaxation times can be probed by creating a dynamic equilibrium between continuous injection and relaxation; this leads to constant-in-time spin and charge imbalances, which scale with their respective relaxation times and with the injection current. Whereas charge imbalances in superconductors have been studied in great detail both theoretically(8) and experimentally(9), spin imbalances have not received much experimental attention(6,10,11) despite intriguing theoretical predictions of spin-charge separation effects(12,13). Here we present evidence for an almost-chargeless spin imbalance in a mesoscopic superconductor.