A Study on Transport Mechanism in High-Temperature Superconductor Josephson Junctions and Their Application to 60K-Electronics

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The purpose of this work is to understand the transport mechanisms in Josephson junctions based on high-temperature superconductors, leading to 'usable' Josephson junctions operating at 60 K.We study electrical properties of Yba_2Cu_3O_7 ramp-edge Josephson junctions with PrBa_2Cu_30_7-based barriers having different resistivities. Experimental results show that the transport mechanism of the pair particles is direct tunneling, whereas that of quasipartioles is resonant tunneling in addition to the direct tunneling. We also find that the high density of the localized states in the barriers reduces the quality parameter, IcRn products, where Ic and Rn are a critical current and a junction resistance, respectively. In fact, high IoRn values are obtained up to 6mV in the junctions with highly resistive Ga-doped PBCO barriers.We face the several drawbacks to the development of the usable junctions. The most serious one is the large spread of the junction parameters such as Ic, Rn. We find that the deposition of the barriers or counter-electrodes increase the spread. To break this situation, we have developed the Josephson junctions without artificial barriers and succeeded in the reduction of the spread.