Direct measurements of the critical current density in YBa2Cu3O7–<i>x</i> single crystals

The critical current densities Jc are measured directly in single crystals of HTS compounds YBa2Cu3O7–x near the superconducting transition temperature Tc. Extrapolation of the temperature dependence Jc(T) to low-temperature region (∼ 77 K) makes it possible to obtain very high values of the current density Jc (∼ 106 A/cm2) in these systems, which confirms the preliminary estimates of Jc obtained from the magnetic data. It is also shown that in HTS single crystals, the external magnetic field suppresses the critical current to a lesser extent than in polycrystals. Measurements of the critical current at various temperatures and magnetic fields have been used to estimate the second critical field in these compounds (Hc2 ∼ 35 T), whose order of magnitude is the same as the typical value for a wide range of HTS compounds.

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