INFLUENCE OF HC1 ON THE IRREVERSIBILITY LINE IN YBA2CU3O7 THIN-FILMS

The irreversibility line of a type-II superconductor separates the H-T plane into a region above the line where the critical current is zero and the magnetic properties are reversible, and a region below where a critical current is observed and the magnetic properties are irrever sible. Although there is great controversy over the various models pro posed to explain these observations, they all depend on the existence of magnetic-flux quanta in the sample. This implies that the irreversi bility line cannot cross H(c1)(T). In bulk samples, assuming that the observed functional forms are maintained, this would be expected to oc cur much too close to T(c) to be observed. However, for films thinner than the London penetration depth, when the field is applied parallel to the film surface, H(c1) can be greatly enhanced. We report the resu lts of measurements of the irreversibility line in thin YBa2Cu3O7 film s that clearly show deviation from the power-law behavior observed for bulk samples. This deviation is in good agreement with calculations o f the enhanced H(c1), which can be as large as several tesla for films thinner than approximately 250 angstrom.