Nonequilibrium quasiparticle current at superconducting boundaries

Potential measurements have been made in the nonequilibrium depairing regions of current-carrying superconducting boundaries. Experiments performed on two different systems are reviewed. The first is the boundary between a superconductor and a normal metal. The second is the boundary between two superconductors with one of them driven normal by a current. In both cases, the quasiparticle chemical potential was found to be different from the potential of the pairs with current dependence depending on individual boundary conditions. The spatial extent of the nonequilibrium region was also found to be longer than the coherence length of the superconductor. A new interpretation for the experimental results is proposed. The theory is based on a nonequilibrium population of quasiparticles created during branch crossing in the depairing region. The branch-crossing relaxation time ${\tau }_Q\mathrm{}\left(0\right)\mathrm{}$ for our tantalum films was estimated to be ∼ 0.65 × ${10}^{-10\mathrm{}}$ sec.