The role of the voltage criterion in the interpretation of flux pinning in epitaxial films of YBa2Cu3O7

The determination of the critical current is usually based upon a voltage criterion that is used as an indication that the critical state has been reached in the superconducting regime. Resistive measurements of the I-V characteristics are carried out on patterned thin films of epitaxially grown and c-axis-oriented YBa2Cu3O7. Bridges with widths ranging between 1 and 100 μm and lengths between a few μm and 7 mm are characterized. The sensitivity of the measurement is better than 10−9 V. The volume pinning force is evaluated for different voltage criteria. Although the absolute values of the volume pinning force change with the value of the criteria, the field and temperature scaling is not affected. They agree with the expectations of the flux-line sheer model. The value obtained for the width of the weakly pinning channels is independent of the chosen criterion. Finally, model calculations on the basis of a modified flux-line shear mechanism are carried out that are in good agreement with the experimentally measured I-V characteristics.