Charge imbalance observed in voltage-biased superconductor-normal tunnel junctions

We have studied the tunneling current of a superconductor-insulator-normal tunnel junction connected to a nonequilibrium superconductor in which nonvanishing charge imbalance is present, by injection experiments using multiterminal devices fabricated with electron beam lithography. We found that the tunneling current could be separated into excess current, due to charge imbalance, and normal current, which showed clear and different dependences on the bias voltage, as expected from the theory of charge imbalance [M. Tinkham, Phys. Rev. B 6, 1747 (1972)]. We also found in the analysis of the normal current that nonequilibrium distributions were created in the normal electrode; they were presumably caused by excitation due to hot electrons that tunneled from the nonequilibrium superconductor.