FLUXON DYNAMICS AND HIGHER HARMONICS OF AC SUSCEPTIBILITY IN HTSC

Measurements of fundamental and higher harmonics of a.c. susceptibilit y on polycrystalline high-T-c superconductors were carried out using a new method, involving a 2-channel dynamic analyser, allowing 20 harmo nics in phase and out of phase to be measured at the same time. The cr itical-state model, used for describing the higher-harmonics dependenc e on temperature at fixed d.c. and a.c. magnetic fields, did not expla in the experimental results. A frequency analysis indeed suggested tha t an effect of the fluxon dynamics takes place. Simple considerations lead to the conclusion that really Thermally Activated Flux Flow (TAFF ), Flux Creep (FC) and Flux Flow (FF) would have a big role in determi ning the higher harmonics of a.c. susceptibility. Measurements on low- T-c superconductors showed the occurrence of the same effect.