Pinning parameters in the PbTl system

Abstract

Measurements are reported of the reversible penetration of acripple signals into superconducting PbTl alloys. Removal of the surface barrier allows measurements to be made near the upper critical field H_c2 , which is the region in which it is possible to make comparisons with theory. Near H_c2 the depth tends to infinity as (1-B/B_c2)-112, which is in agreement with theory. At lower fields agreement is not good. Measurements are also reported of the distance the vortex lattice must be moved to unpin vortices. This gives a direct measurement of the relative strengths of pinning forces and elastic forces.