Superconducting transition of multiconnected Josephson network

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Abstract
The response of superconducting Tc with multiconnected structure against a small ac magnetic field is studied in terms of complex susceptibility. It has been revealed that the fundamental susceptibility χ1′−iχ1″ is very sensitive to the amplitude h0 of the ac field; i.e., the superconducting transition width is appreciably broadened as h0 increases, but the onset temperature of the transition holds its value. In addition, χ1″ with respect to temperature has an asymmetric peak. We propose a phenomenological model on the assumption that the multiconnected network behaves like a single loop as a whole due to the coherent nature of the specimen. It has been revealed that the main features of observed characters are well reproduced by this model. Since our model predicts higher harmonics in susceptibility, the third harmonic response of the specimen against a small ac magnetic field is also studied. Measurements with respect to temperature are simultaneously performed for the amplitude of third harmonic susceptibility ‖χ3‖ and χ1″. Observations show that ‖χ3‖ is proportional to χ1″ and forms a peak in the temperature region of the superconducting transition. This finding is in good agreement with our prediction from the weakly connected loop model. Peculiar instability in susceptibility curves of transition (h0≳2 Oe) is first observed.