Thermally activated flux motion in Nd1.85Ce0.15CuO4-y

Flux motion in a Nd1.85Ce0.15CuO4-y single crystal has been studied by using ac susceptibility experiments under a bias dc field for H parallel to the c axis. It is found that the irreversibility line is given by H(T)=H(0)(1-t)2. Analysis of the susceptibility curves within the framework of a thermally activated flux motion allows the extraction of the activation energies U(T,B). It turns out that U(T=0,B=1T)a40 meV, which is notably larger than the energies typically found in p-type high-temperature superconductors. Single-vortex and collective-pinning mechanisms by oxygen vacancies are investigated to account for the observed field dependence of the activation energies. The dependence of energy absorption on the ripple ac field is also investigated.