Noise voltages produced by flux motion in superconductors

The transport of magnetic flux in type 1 and type 2 superconductors under the influence of a transport current gives rise to a noise voltage with power spectrum W f superposed on the d.c. voltage (V). The authors report measurements of W f for cold-rolled strips and discs of type 1 and type 2 InPb alloys. For the type 2 strips a new phenomenon has been observed: peaks in W f which are very sensitive to current density and magnetic field. The general features of the W f measurements are compared, where possible, with various model of flux motion. For the type 1 strips with leads of different separation a similar dependence of W f on frequency f is found, a result which cannot be interpreted according to any of the simple random pulse models.