Upper-critical fields of YBa2Cu3O7-δ epitaxial thin films with variable oxygen deficiency δ

Fluctuation analysis in the limit of high magnetic fields was performed on three epitaxial thin films of YBa2Cu3O7-δ for various oxygen deficiencies δ<0.3. On the 90-K plateau, the three-dimensional (3D) limit yielded an Hc2(T) slope of -1.7 T/K for H parallel c, consistent with previous observations of transport and magnetic properties. Moreover, the 3D scaling showed better convergence than the 2D scaling, which gave relatively low values of Hc2. In contrast, the transitions were not adequately described by either scaling for Tc off the 90-K plateau; it is speculated that this is due to an extrinsic broadening of the transitions, possibly due to the lack of a complete percolation path of the ortho-I phase (δ=0)