We have used complementary sets of normal and superconducting probes to study the nonequilibrium region near phase-slip centers in narrow superconducting strips. An exponentially decaying branch imbalance potential, ${\mu }_Q$, is observed and the decay length, the quasiparticle diffusion length, is measured directly and compared to the phase-slip resistance.

• Received 14 October 1977

DOI:https://doi.org/10.1103/PhysRevLett.39.1628