Ac response of the vortex system in a Pr1.85Ce0.15CuO4-y single crystal

We have studied the field, frequency, and amplitude dependence of the ac susceptibility on a single crystal of the electron-doped superconductor Pr1.85Ce0.15CuO4-y. We show that a thermally activated flux motion over effective energy barriers U(T,H)=U0(1-T/Tc)H-0.6, with U040 meV, can account for the dc-field and frequency effects. The amplitude dependence of the ac response in this superconducting system is discussed and it is argued that it is a signature of the critical state.