Nonuniversal temperature dependencies of the low-frequency ac magnetic susceptibility in high-T-c superconductors

The complex ac magnetic susceptibilities (chi(n) = chi(n)' + i chi(n)") of high-T-c superconductors in absence of de fields have been studied by numer ically solving the nonlinear diffusion equation for the magnetic flux, wher e the diffusivity is determined by the resistivity. In our approach the par allel resistor model between the creep and flux flow resistivities is used, so that the crossover between different flux dynamic processes (thermally activated flux flow, flux creep, flux flow) can naturally arise. For this r eason we remark that, as the frequency increases, the presence of a differe nt nonlinearity in different regions of the I-V characteristic determines n onuniversal temperature dependencies of the chi(n), i.e., the chi(n) are fo und to be not universal functions of a frequency- and temperature-dependent single parameter. Moreover, the actual frequency-dependent behavior is als o shown to be strictly related to the particular pinning model chosen for t he simulations. Indeed, for large values of the reduced pinning potential ( U/KT greater than or equal to 220) and for increasing frequency, a transiti on has been observed between dynamic regimes dominated by creep and flux fl ow processes. On the other hand, for smaller reduced pinning potentials, a transition from the thermally activated flux flow (Taff) to the flow regime occurs. In qualitative agreement with available experimental data but in c ontrast with previously used simpler models, the amplitude of the peak of t he imaginary part of the first harmonic is shown to be frequency dependent. Moreover the frequency dependence of its peak temperature shows large disc repancies with approximated analytical predictions. Finally, the shapes of the temperature dependencies of the higher harmonics are found to be strong ly affected by the frequency. [S0163-1829(99)02217-1].