Resistivity measurements on ultrathin (36 and 42 nm thick) YBCO films were performed with a novel pulsed-current technique that allows for application of current densities up to several MA/cm~2 with minimal self-heating even in the resistive state. Using current pulses as short as 50 ns we found that the onset of resistance in the Ⅰ-Ⅴ curves takes place at substantially higher current densities as compared to results achieved with the commonly used d.c. technique. Both the temperature dependence and the absolute values for such critical current measurements indicate that indeed the depairing current has been probed. The results are in quantitative agreement with the predictions from Ginzburg-Landau theory. Thus, our measurements can determine the ultimate thermodynamic limit for dissipation-less current transport in a high-temperature superconductor that is more than an order of magnitude larger than previous d.c. technique results.
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