Modulation of I-V characteristics of YBCO films and tunnel junctions under microwave irradiation and tunnel current injection

We present the measurements on the I-V characteristic modulation of YBCO films under microwave irradiation and tunnel current injection. In the case of tunnel current injection through a Pb(or Au)/MgO/YBCO junction, the critical current I/sub c/ of a YBCO film decreases drastically with the increase in tunnel current, and a normal state appears when the injection current reaches a certain value smaller than I/sub c/, leading to a current gain greater than one. The I-V characteristics of a YBCO film under both current injection and external microwave irradiation (fa=11 GHz) show strong modulation behavior. We find that the above modulation is mainly caused by the Cooper-pair breaking effect due to injected quasiparticles with energy greater than 2/spl Delta/ and not by heating.<
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