Depairing current and superconducting transition of YBCO at intense pulsed currents



The critical current density of a superconductor as a function of temperature and magnetic field is a key quantity for the design of superconductor applications. Tremendous efforts are currently undertaken to enhance the critical current in technical materials by improving the pinning forces. The ultimate thermodynamic limit for carrying a superconducting current is named the depairing current and usually cannot be measured directly. Our measurements are carried out using short-pulse currents with about 50 ns duration in order to avoid thermal heating and eventually destruction of the samples that result inevitably from such high dissipation. In addition, ultrathin films of YBa2Cu3O7–x are used, patterned to microbridges to allow for an effective heat transfer into the MgO substrate. The depairing current is directly measured and is in good accordance with the values predicted by Ginzburg-Landau theory. In intense currents the critical temperature is not only shifted to lower temperatures, as seen in conventional d.c. measurements, but the shape of the transition changes significantly. The latter effect is attributed to a reduction of the fluctuation paraconductivity with increasing electric field and is compared to our recent theoretical work. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)