Transport Critical Current-Density And Dimensional Crossover In Superconducting Nb/Nbzr Multilayers

In order to understand the nature of the mixed state, the critical current density J(c) of Nb/NbZr multilayers was measured as a function of magnetic field Hand of the angle between Hand the film plane at the temperatures where the superconducting order parameter is localized in the Nb layers. In the parallel fields, which are perpendicular to the current J, the J(c)-H curves show a peak at H=H(max). The observed angular dependence of J(c) below and above H(max) can be explained by the 3D stepwise vortex model and the 2D pancake vortex model, respectively. These results indicate that a dimensional crossover in the mixed state occurs at H(max). This crossover may be the cause of the field dependence of J(c) which is an increasing function of H below H(max).