Study of quasi-particle conduction in small non-equilibrium superconductors

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Quasi-particles injected into superconductor do not recombine immediately but exist in a non-equilibrium state for a short time. We have studied physics of non-equilibrium superconductivity using new method employing nano-fabrication technology. We have found or clarified following issues.(1)If non-vanishing accumulation of quasi-particle charge, a charge imbalance, occurs, excess current due to charge imbalance is predicted to be observed in a normal tunnel junction connected to the non-equilibrium superconductor. We observed bias-voltage dependence of the excess current for the first time. We found that the current was unchanged against bias-voltage, as expected from the theory of tunneling current.(2)We have performed spectroscopic measurements of the charge imbalance using a Josephson junction device consisted of superconductors with different magnitude of energy gap. The energy distribution of charge imbalance state was determined experimentally. We found that the distribution had structures similar to initial distribution of the injected charge imbalance. This result was interpreted to mean that the energy relaxation time is the same order of magnitude of the relaxation time of the charge imbalance.