  What happens to spin-polarised 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-randomised quasiparticles. Injecting spin-polarised electrons into a
superconductor thus creates both spin and charge imbalances [1, 2, 3, 4, 5, 6,
7] (respectively Q* and S*, cf. Ref. [4]). These must relax when the injection
stops, but not necessarily over the same time (or length) scale as spin
relaxation requires spin-dependent interactions while charge relaxation does
not. These different relaxation times can be probed by creating a dynamic
equilibrium between continuous injection and relaxation, which leads to
constant-in-time spin and charge imbalances. These scale with their respective
relaxation times and with the injection current. While 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] despite intriguing theoretical predictions of spin-charge
separation effects [11, 12]. These could occur e.g. if the spin relaxation time
is longer than the charge relaxation time, i.e. Q* relaxes faster than S*.
Fundamentally, spin-charge decoupling in superconductors is possible because
quasiparticles can have any charge between e and -e, and also because the
condensate acts as a particle reservoir [13, 11, 12]. Here we present evidence
for an almost-chargeless spin imbalance in a mesoscopic superconductor.
