High, magnetic-field-insensitive transport critical currents in unoriented polycrystalline thin films of Tl 2CaBa 2Cu 2O y
We report critical current measurements for unoriented polycrystalline films of Tl 2CaBa 2Cu 2O y. At 77 K these films show critical current densities ranging from 10 000 to 110 000 A/cm 2, with a median value of 50 000 A/cm 2. Moreover, the best films' critical currents at 77 K drop less than 50% in an applied magnetic field of 1 T, and less than a factor of 20 at 6 T. At 4 K, critical current densities over 10 6A/cm 2 with less than 15% drop at 6 T have been obtained. The zero-field temperature dependence of the critical current from 4 to 95 K is well described as quadratic. High-field magnetization loops on the films at 76 K show a striking collapse in hysteresis below 0.6 T. Our results indicate that, although the grains in these films are Josephson-coupled, the strength of the coupling is greatly improved over other high- Tc granular systems.