Investigation of Photo-Response in Micro-structure Device of Photo-conducting Semiconductor and Superconductor

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In order to investigate the influence of illumination on the superconductivity proximity effect between superconductor and photo-conducting semiconductor, we have made the mesoscopic devices from a pair of Pb, In, Au multi-layr superconducting films on indium doped Pb_<1-x>Sn_xTe (x=0.25) semiconductor single crystal. The electron mobility is very high and the amount of persistent photoconductivity is large in indium doped Pb_<1-x>Sn_xTe, which indicates that the penetration length of superconductivity into semiconductor is long and could be varied widely. We measured the superconducting characteristic of these devices under illumination. In the devices with the gap width between the superconductor films below 0.2-0.3mum, the hysteresis in the current-voltage curve is observed at 4.2K, which is a typical characteristic of Josephson junction. Further the critical current (the sudden increase point of resistance) increases with increaseing illumination. From these results, we succeed in changing the superconducting characteristic of the mesoscopic device by illumination.Using an AC resistance bridge, which was purchased by this grant, we have also measured magnetic field dependence of capacitance in a two dimensional electron system (GaAs/AlGaAs heterostructure) in the quantum Hall regime. It has been found that the bottom values of capacitance at the Hall plateaus are determined not by the gate area but by the edge length of the gate. These results can not be explained by the conventional interpretation of magnetocapacitance, where the capacitance is directly related to the density of states of a two dimensional electron gas. We suggest a model that the bottom values are determined by the area of edge states. From this model, the widths of edge states were estimated and compared with the recent theory.