We have studied the field, frequency, and amplitude dependence of the ac susceptibility on a single crystal of the electron-doped superconductor ${\mathrm{Pr}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4\mathrm{-}\mathit{y}}$. We show that a thermally activated flux motion over effective energy barriers U(T,H)=${\mathit{U}}_0$(1-T/${\mathit{T}}_{\mathit{c}}$)${\mathit{H}}^{\mathrm{-}0.6}$, with ${\mathit{U}}_0$≊40 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.

• Received 12 November 1992

DOI:https://doi.org/10.1103/PhysRevB.47.15250