Magnetic field expulsion in superconducting granular ceramics and in polymer/superconductor composites

Abstract
The magnetic interaction between a permanent magnet and superconducting ceramics such as YBa₂Cu₃O_7-δ and Bi_1,6Pb_0,4Sr₂Ca₂Cu₃O10+δ depend on the superconducting state of each phase and on the junctions between grains. In the case of polymer/superconductor composites, screening effects depend on the volume fraction of superconductor. Measurements of the evolution of the levitation force (F=A/d^γ) as a function of the interaction distance d are used to characterize the effective response of the ceramics or composites to the magnetic flux penetration. Some of the abnormal variations of the exponent ε and of the term "A" ( in F=A/d^γ) could be reinterpreted in terms of a change in superconducting regime. Other observed variations of γ should be due to the variation of the effective field from the cylindrical magnet.