Upper-critical fields of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ 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 ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ for various oxygen deficiencies δ<0.3. On the 90-K plateau, the three-dimensional (3D) limit yielded an ${\mathit{H}}_{\mathit{c}2}$(T) slope of -1.7 T/K for H∥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 ${\mathit{H}}_{\mathit{c}2}$. In contrast, the transitions were not adequately described by either scaling for ${\mathit{T}}_{\mathit{c}}$ 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).