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Published online by Cambridge University Press: 28 February 2011
Measurements of the critical current density Jc for ultrathin YBa2Cu307‐X (YBCO) films are presented.A high Jc value at 77K in zero field of 2xl06A/cm2 was obtained in a 30nm thick film and even a 5nm thick film could carry a supercurrent of 3.5x105 A/cm2 at 4.2K in zero field. A very weak field‐dependence of Jc was observed when the field was in the (a,b) plane showing greatly depressed dissipation.