Flux-Transport Noise in Type-II Superconductors

Abstract

The transport of magnetic flux in type-II superconductors under the influence of a transport current is shown to give rise to a noise voltage, superposed on the dc voltage. This flux-flow noise is analogous to shot noise limited by transit-time effects, and is caused by the motion of discrete units of flux between the edges of the superconductor. If the flux is in a circular motion, so that no transit of flux between the edges takes place, no flux-flow noise can be detected. It is shown from measurements of the frequency spectrum of the noise voltage in vanadium foils that the dc voltage is built up of voltage pulses generated during flux motion, whose duration is equal to the transit time of the flux units. Because of interaction with pinning centers, a fraction of the flux does not take part in the motion. This fraction is found to increase with decreasing temperature and current. It increases with magnetic field in cold-rolled vanadium that exhibits a resistance minimum, but decreases with field in annealed vanadium, where such a minimum is not present. The moving flux units are bundles of flux lines. The average number of flux quanta in a bundle depends on pinning conditions, as is shown for vanadium foils and an indium-thallium crystal. This number decreases with increasing temperature, current, and magnetic field. It is also found to fluctuate owing to interaction with the pinned fraction. At high fields and currents, flux transport in annealed material is in the form of single flux quanta. When the power dissipation in the specimen is increased, a noise voltage with approximately $\frac{1}{f^b}$ frequency dependence is found, with $2<b<\mathrm{}3\mathrm{}$. This flicker noise, as we call it, is shown to be generated by temperature fluctuations in the foil. These fluctuations are caused by nucleate boiling of liquid helium, and they virtually disappear below the helium $\lambda$ point. Some flicker noise is left there, due to pressure variations above the liquid.