Flux creep in superconductors

Measurements of flux creep have been undertaken for a variety of superconductors´ to determine the factors that control the temperature and magnetic field regimes where the material transforms from a strong-pinning rigid-flux lattice material to the region where the flux lattice is highly mobile. Analysis of the data in terms of a single effective pinning potential, U_eff, using the Beasley model indicates that all of the superconductors measured have a gradual transition from the strong pinning behavior at low temperature to weak pinning behavior at high temperature. There is a narrow band of fields and temperatures in the H-T plane where U_eff/kT rises from 2 to 100. The position of this band changes considerably from material to material. It is shown that there are some striking similarities and some clear trends in the pinning potentials when they are plotted on a reduced temperature scale. The highly anisotropic Tl-based and Nd-based copper oxides are very similar even though their T_c values differ by a factor of 5. In the progression from Nb-Ti to the cubic Bi-oxide to the anisotropic Cu-oxide, there is a clear trend to a wider region of high flux creep