RESONANCE EFFECTS IN JOSEPHSON TUNNEL-JUNCTIONS WITH INTEGRATED TUNING STRUCTURES

Large-current steps have been found in the I/V characteristics of supe rconducting tunnel elements that are integrated within fixed tuning st ructures. The Josephson frequency at the steps corresponds to the oper ation frequencies of the different tuning structures at 345, 460, or 6 00 GHz. Therefore, these current steps are interpreted as resonance ef fects generated by an internal interaction of the Josephson oscillatio n with the tuning structure. The result of a spectrometer measurement around 350 GHz is in very good agreement with the resonance frequency obtained from the I/V characteristic. This confirms the interpretation of the current steps. A simple dc measurement is therefore sufficient to determine the actual resonance frequency of the tuning structure. It may be shifted with respect to the expected frequency, due to devia tions of the junction size or other parameters from their required val ues. The exact knowledge of the resonance frequency is important for t he application of these integrated tunnel elements as mixers in radioa stronomical receivers. In single junctions a splitting of the resonanc e step into several branches was observed, whereas in two-junction arr ays a splitting into two distinct states was found. Irregular switchin g appeared between these two states. The temperature and magnetic-fiel d dependence of the current steps was investigated.