Enhanced spin pumping into superconductors provides evidence for superconducting pure spin currents

Unlike conventional spin-singlet Cooper pairs, spin-triplet pairs can carry spin. Triplet supercurrents were discovered in Josephson junctions with metallic ferromagnet (FM) spacers, where spin transport can only occur within the FM and in conjunction with a charge current. Ferromagnetic resonance (FMR) injects a pure spin current from a precessing FM into adjacent non-magnetic materials. For spin-singlet pairing, FMR spin pumping efficiency decreases below the critical temperature (Tc) of a coupled superconductor (SC). Here we present FMR experiments in which spin sink layers with strong spin-orbit coupling are added to the SC. Our results show that the induced spin currents, rather than being suppressed, are substantially larger in the superconducting state compared with the normal state; although further work is required to establish the details of the spin transport process we show that this cannot be mediated by quasiparticles and is most likely a triplet pure spin supercurrent.