Vortex correlations in the superconducting transition of ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$${)}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ films in high magnetic fields

We determine the characteristic length scale for vortex correlations in ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Al}}_{\mathit{x}}$ ${)}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ films [0≤x≤0.03] by studying their current-voltage characteristics in fields 0.5 T≤B≤6 T. We find that critical scaling persists over 60% of the resistive transition (which widens from 3 to 30 K as doping concentration and field increase) and breaks down when the correlation length is 500–1250 Å, a value that does not depend on the intervortex spacing. This result suggests that pinning in as-made films of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ is dominated by extended rather than microscopic defects.