Flux pinning and flux flow studies in superconductors using flux flow noise techniques. Progress report, April 1-December 30, 1979

Flux flow noise power spectra were investigated, and information obtained through such spectra is applied to describe flux flow and pinning in situations where volume pinning force data is also available. In one case, the application of noise data to PB/sub 80/In/sub 20/ samples after recovery and after high temperature annealing is discussed. This work is consistent with a recent model for flux flow noise generation. In the second case we discuss experiments designed to change the fluxoid transit path length, which according to the model should affect both the noise amplitude and the parameter ..cap alpha.. specifying the longest subpulse times in terms of the average transit time, tau/sub c/. Transient flux flow voltages when a current is switched on after field cycling a Pb/sub 60/In/sub 40/ sample have been discovered. Noise spectra have been measured during the transient. These observations are discussed along with a simple model which fits the data. A surprising result is that the transient decay times increase with the applied current. Other characteristics of Pb/sub 60/In/sub 40/ after cold working are also discussed

Flux pinning and flux flow studies in superconductors using flux flow noise techniques. Progress report, April 1, 1977--December 17, 1977

A model has been developed for the flux flow state in type II superconductors in which flux motion is interrupted by local interactions between pinning centers and moving fluxoids. This model allows us to predict the functional forms for the flux flow noise power spectrum and for the dc voltage-current characteristics. Experimental results conform quite closely to the predictions of this model. Measurements of noise spectra, voltage-current characteristics, and volume pinning force curves have been used to study flux flow and local pinning interactions in situations in which pinning is dominated by: (1) normal metal precipitates, (2) grain boundaries, and (3) a combination of dislocations and grain boundaries. In each case, the metallurgical state of the sample controls the measured results. The greater sensitivity of noise spectra as compared to bulk critical current measurements in obtaining a detailed picture of flux flow is emphasized

Flux pinning and flux flow studies in superconductors using flux flow noise techniques. Progress report, April 1-December 30, 1979

Flux flow noise power spectra were investigated, and information obtained through such spectra is applied to describe flux flow and pinning in situations where volume pinning force data is also available. In one case, the application of noise data to PB/sub 80/In/sub 20/ samples after recovery and after high temperature annealing is discussed. This work is consistent with a recent model for flux flow noise generation. In the second case we discuss experiments designed to change the fluxoid transit path length, which according to the model should affect both the noise amplitude and the parameter ..cap alpha.. specifying the longest subpulse times in terms of the average transit time, tau/sub c/. Transient flux flow voltages when a current is switched on after field cycling a Pb/sub 60/In/sub 40/ sample have been discovered. Noise spectra have been measured during the transient. These observations are discussed along with a simple model which fits the data. A surprising result is that the transient decay times increase with the applied current. Other characteristics of Pb/sub 60/In/sub 40/ after cold working are also discussed

FLUX PINNING AND FLUX FLOW NOISE IN Pb80IN20 SAMPLES EXHIBITING A DOUBLE PEAK IN THE VOLUME PINNING FORCE

Des alliages de Pb80In20 correctement préparés avec une haute densité de dislocations montrent que la force d'ancrage en volume possède deux pics en fonction du champ magnétique, et que la puissance de bruit du mouvement de flux possède un pic aigu. Le recuit à la température ambiante provoque une importante réduction du bruit, la disparition du pic de la puissance de bruit et un seul pic dans la force d'ancrage en volume. Nous pouvons expliquer ces caractéristiques, en tenant compte de la structure métallurgique de nos échantillons.Pb80In20 alloys, when properly prepared with a high dislocation density, show two volume pinning force peaks as a function of magnetic field, and also a sharp peak in the flux flow noise power. Room temperature annealing produces a significant reduction in the noise, a disappearance of the peak in the noise power, and a single peak in the volume pinning force. These features can be explained in terms of the metallurgical structure of the samples