HIGH-FREQUENCY FLUX DYNAMICS IN SINGLE-CRYSTAL ND1.85CE0.15CUO4

Flux dynamics in a Nd1.85Ce0.15CuO4 single crystal is investigated in the 20 kHz-3 MHz frequency range by using a mutual-inductance techniqu e. We show that the superconducting ac response is due to thermally ac tivated flux motion; from the dc field, ac amplitude, and frequency de pendencies of this ac response, we extract the effective energy barrie rs U(T, B, J) = U0(1-T/T(c))B-0.66ln(J(c)/J). Significantly, these ene rgies account for the time-dependent response of these superconductors over ten decades of time. As a consequence of them, the ac response i s found to be strongly nonlinear, i.e., the ac susceptibility depends on the driving-field amplitude for ac fields as small as 10(-6) T supe rposed to dc fields up to 2 T. The ac transition is well described by a critical-state model with a frequency-dependent effective critical c urrent J(c)(T, B, f) stemming from the thermally activated flux motion . Our analysis allows us to obtain the temperature and frequency depen dencies of J(c)(T, B, f). J(c)(f) follows a power law which is shown t o be in agreement with the observed logarithmic U(J) dependence. Final ly, isothermal cuts carried out for different driving-field amplitudes are used to explore the onset and limits of the nonlinear response.