Observation of spin-bottleneck due to spin-charge separation in a superconductor

An experimental device was designed to measure the effect of the injection of spin-polarized carriers on the superconductive gap and density-of-states (DOS). Quasiparticles were injected from a ferromagnet ($Ni_{0.8}Fe_{0.2}$) through a tunnel junction into a conventional superconductor (Nb), while charge neutrality was maintained by a supercurrent. The DOS of the superconductor was measured through a second tunnel junction with a normal paramagnetic metal. No significant decrease of the superconductive gap was observed while a noticeable heating of the quasiparticles of the superconductor was measured. A similar experiment performed with current injected from a paramagnet (Al or Ag) showed no heating of quasiparticles. These observations are consistent with spin-charge separation of Bogoliubov quasi-particles and spin-bottleneck due to the enhanced recombination time of pure spin-excitations.