Flux pinning in Pb-Na: a peak-effect superconductor

Abstract
The response to an AC magnetic field of a type II superconducting single-crystal Pb-Na cylindrical rod has been observed as a function of longitudinal DC fields. The results show two different regimes of flux pinning within the specimen. At low fields in the mixed state the specimen is largely reversible in the bulk and all pinning appears to be at the surface. In a field close to Hc2, there is a peak region where bulk pinning rapidly dominates. Evidence for these regimes is discussed.