J/sub c/(H) crossover in YBCO thick films and Bi2223/Ag tapes with columnar defects

We report a study of critical current density J/sub c/ as a function of temperature T, magnetic field H, and film thickness d in a series of high quality YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) films. A crossover in the thickness dependence of J/sub c/(H) curves was observed for T>50 K, where thinner film carries significantly lower J/sub c/(H) than thicker film at high fields, while the opposite was observed at low fields. This J/sub c/(H) crossover behavior seems to be a general feature for a superconducting system with a spectrum of pinning density, whose trend opposes to the effective volume faction for J/sub c/. This generalization is supported by examining J/sub c/(H) crossover behaviors observed earlier in the Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10+/spl delta//(Bi2223)/Ag tapes irradiated with 5.8 GeV Pb ions, where enhanced J/sub c/(H) at high fields is attributed to the density increase in the columnar flux pinning centers.