Use of whiskers as model systems for studying surface barriers in type II superconductors

A range of micron-sized Hall probe arrays has been used to investigate the Bean-Livingston surface barrier in single crystal whiskers of the type 11 superconductor Bi_2Sr_2CaCu_2O_8_+_#delta#. It is shown that the magnetic properties of the whiskers are dominated by surface barriers, and hysteresis loops exhibit behaviour entirely consistent with such a system, showing very small magnetisation on the return leg. Measurements of the temperature dependence of the field of first flux penetration indicate that above approximately 5mT the vortex lines decouple into stacks of two-dimensional vortex pancakes, which surmount the surface barrier through thermal activation, in agreement with recent models of giant flux creep over surface barriers. Once inside the whisker, the vortices collect in the centre to produce a dome-shaped magnetic flux profile, which is observed experimentally. Careful measurements of the initial leg of the hysteresis loop show a sawtooth structure, which is interpreted as evidence of a hierarchy of stable vortex states. Field-cooled measurements exhibit a sharp drop in the local magnetic induction, which is interpreted as a boundary between a regime dominated by surface barriers and a regime dominated by the inter-vortex repulsion. Transport measurements have also been carried out, and the results support the idea that the transport properties of the whiskers are strongly affected by surface barriers. (author)Available from British Library Document Supply Centre-DSC:DXN032859 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo