Evidence of thermal nucleation of two-dimensional point vortices in single-crystal Nd1.85Ce0.15Cu2O4-y superconductors

We report detailed magnetic measurements in H parallel to the c-axis direction on a single crystal of Nd1.85Ce0.15Cu2O4-y. The magnetization data indicate that the entrance field is determined by surface barriers. At low temperatures, the penetration field Hp(T) can be fit with Hp=Hcexp(-T/T0); at temperature close to Tc, Hp(T) is linear in T. The date can be qualitatively understood by a model where two-dimensional point vortices are thermally nucleated near the surface in weakly coupled layered superconductors.