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

注入超导体的准粒子不会立即复合，而是在短时间内以非平衡状态存在。我们用纳米制造技术的新方法研究了非平衡超导电性。我们发现或澄清了以下问题：(1)如果准粒子电荷的非零积累，即电荷不平衡，预计在连接到非平衡超导体的正常隧道结中将观察到由于电荷不平衡而产生的过剩电流。我们首次观察到了超额电流的偏压依赖关系。我们发现，正如隧道电流理论所预期的那样，电流不随偏压变化。(2)我们利用由不同带隙大小的超导体组成的约瑟夫森结装置进行了电荷不平衡的光谱测量。实验测定了电荷不平衡态的能量分布。我们发现，该分布具有类似于注入电荷不平衡的初始分布的结构。这一结果被解释为能量弛豫时间与电荷不平衡的弛豫时间相同的数量级。

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.