Anomalous Critical Alternating Currents of Superconducting Wires

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Anomalous transitions from the superconducting to the normal state are presented. The specimen used is superconducting Nb-Ti-Zr ternary alloy wire about 3 cm long. Alternating currents of 20 to 500 Hz are applied to the specimen in liquid Helium. The wave memory system is used to observe both currents and voltages across the specimen. The current transition characteristics can be classified into three: types; (1) I_c=I_m=I_q, (2) I_c>I_m->I_q, (3) I_c≫I_m〜I_q, where I_c is the critical direct current, I_m the peak value of the alternating current just before the transition, and I_q the instantaneous alternating current which actually drives the superconducting state into the normal state. Stabilized superconductors show the normal transition of type (1). Anomalous transitions are found to occur at surface magnetic fields of several thousand oersteds in unstable superconductors, where the magnetic flux often jumps. These findings shed some light on areas which have impeded wider application of superconducting materials.
社団法人応用物理学会の論文
1976-12-05