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

TL;DR

This study experimentally demonstrates spin-charge separation in a superconductor by observing a spin-bottleneck effect caused by spin-polarized carrier injection, revealing new insights into quasiparticle dynamics.

Contribution

The paper provides the first experimental evidence of spin-charge separation and spin-bottleneck effects in superconductors using tunneling measurements.

Findings

No significant decrease in superconducting gap with spin-polarized injection

Noticeable heating of quasiparticles observed with ferromagnetic injection

No heating observed with paramagnetic injection

Abstract

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.