Pinning size effects in critical currents of superconducting films

Abstract

The thickness dependence of the critical current densityJc(d) in films due to the two-dimensional–three-dimensional (2D–3D) pinning crossover at lowmagnetic fields is addressed, taking into account the spatial correlation of pinning centres, thecombined effect of bulk and surface pinning, and the effect of thermal fluctuations. The wholeJc(d) curve for both the 3D and 2D pinning regimes, and the crossover thicknessdc are calculated using a dynamic approach for a random pinning potentialcharacterized by the Gaussian correlation functions. It is shown thatthe spatial correlation of pinning centres can significantly increasedc as compared to uncorrelated point pins, and the competition betweenpinning and thermal fluctuations gives rise to a nonmonotonic dependence ofJc(d). Theaccount of multiscale spatial correlations in a uniform pinning nanostructure can result in behaviour ofJc(d) similar to thatobserved on YBa2Cu3O7−δ. Theultimate limit of Jc is discussed.