RESPONSE OF PINNED FLUX VORTICES TO LOW-FREQUENCY FIELDS

The susceptibility of type II superconductors in low-frequency alternating current fields has been measured using a phase sensitive detector. Penetrations of magnetic field of between 300 Å and 500 μm can be measured and if it is assumed that a critical state exists the field profile across the specimen can be plotted on this scale. The response is linear at low amplitude and is consistent with a model based on the reversible oscillation of vortices in harmonic wells. It is not consistent with the signal predicted from 'flux creep'. The effective potential wells are found to be harmonic up to their edges and only about 25 Å in radius. The resolution is sufficient to show that the surface current is not a critical state, but can be described by a single row of vortices pinned by the boundary of the specimen.