Direct measurement of quasiparticle evanescent waves in a dirty superconductor

We have used a perpendicular-current spin-polarization detector, in the form of a Permalloy-based exchange-biased spin valve, to measure the length scale for penetration of quasiparticles through superconducting-Nb thin films under diffusive transport conditions. In the low-temperature limit, this penetration length is $\sim 16\mathrm{nm},$ comparable to the estimated “dirty-limit” zero-temperature coherence length of our Nb films.