Flux-creep activation energies at the grain boundaries of Y-Ba-Cu-O superconductors in the characteristic frequency band

Measurements on the temperature and frequency dependence of the complex ac susceptibility of sintered Y_1.01Ba_1.95Cu_2.97O_x are reported. The data are used to obtain the flux-creep activation energies at the grain boundaries in the characteristic frequency band. The found nonlinear frequency dependence of these energies can be approximated differently in the low, characteristic, and high frequency bands. The relaxation frequency of thermal fluctuation of the vortex lattice (2 3×10⁴ Hz) and the surface current density (J_s=0–10 A/cm²) have been estimated.