High-frequency flux dynamics in single-crystal Nd1.85Ce0.15CuO4.

Flux dynamics in a ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4}$ single crystal is investigated in the 20 kHz--3 MHz frequency range by using a mutual-inductance technique. We show that the superconducting ac response is due to thermally activated flux motion; from the dc field, ac amplitude, and frequency dependencies of this ac response, we extract the effective energy barriers U(T,B,J)=${\mathit{U}}_{0}$(1-T/${\mathit{T}}_{\mathit{c}}$)${\mathit{B}}^{\mathrm{\ensuremath{-}}0.66}$ln(${\mathit{J}}_{\mathit{c}}$/J). Significantly, these energies account for the time-dependent response of these superconductors over ten decades of time. As a consequence of them, the ac response is found to be strongly nonlinear, i.e., the ac susceptibility depends on the driving-field amplitude for ac fields as small as ${10}^{\mathrm{\ensuremath{-}}6}$ T superposed to dc fields up to 2 T. The ac transition is well described by a critical-state model with a frequency-dependent effective critical current ${\mathit{J}}_{\mathit{c}}$(T,B,f) stemming from the thermally activated flux motion. Our analysis allows us to obtain the temperature and frequency dependencies of ${\mathit{J}}_{\mathit{c}}$(T,B,f). ${\mathit{J}}_{\mathit{c}}$(f) follows a power law which is shown to be in agreement with the observed logarithmic U(J) dependence. Finally, isothermal cuts carried out for different driving-field amplitudes are used to explore the onset and limits of the nonlinear response.