ac susceptibility of high-temperature superconductors

By numerically solving the flux creep equation, we have investigated the temporal and spatial evolution of the field profiles in a high-temperature superconducting slab immersed in an ac magnetic field together with a dc bias magnetic field, for the situation where the flux creep activation barrier U depends explicitly on current density j as U(j)=(${\mathit{U}}_0$/μ)[(${\mathit{j}}_{\mathit{c}}$/j${)}^{\mathrm{\mu }}$-1]. The fundamental ac susceptibilities of the slab as a function of temperature for different dc bias magnetic fields ${\mathit{B}}_{\mathit{d}}$, ac magnetic field amplitudes ${\mathit{B}}_{\mathrm{ac}}$, and frequencies f have been derived in a unified picture, which reproduce many of the features exhibited by experiments. We have shown that the frequency-independent critical-state model breaks down in explaining these results, which however, can be well described by means of flux creep. We have also shown that part of the loss in the high- temperature superconductors is due to flux creep. © 1996 The American Physical Society.