Influence of Hc1 on the irreversibility line in YBa2Cu3O7 thin films

Abstract

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 irreversible. Although there is great controversy over the various models proposed to explain these observations, they all depend on the existence of magnetic-flux quanta in the sample. This implies that the irreversibility line cannot cross Hc1(T). In bulk samples, assuming that the observed functional forms are maintained, this would be expected to occur much too close to Tc to be observed. However, for films thinner than the London penetration depth, when the field is applied parallel to the film surface, Hc1 can be greatly enhanced. We report the results of measurements of the irreversibility line in thin YBa2Cu3O7 films that clearly show deviation from the power-law behavior observed for bulk samples. This deviation is in good agreement with calculations of the enhanced Hc1, which can be as large as several tesla for films thinner than ∼250. © 1993 The American Physical Society.