The zero-field superconducting phase transition obscured by finite-size effects in thick $\mathbf{Y}\mathbf{B}{\mathbf{a}}_{\mathbf{2}}\mathbf{C}{\mathbf{u}}_{\mathbf{3}}{\mathbf{O}}_{\mathbf{7}-\mathbf{\delta }}$ films

We report on the normal-superconducting phase transition in thick $\mathrm{Y}\mathrm{B}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{7-\delta }$ films in zero magnetic field. We find significant finite-size effects at low currents even in our thickest films ($d = 3200$\mathring{A})\mathrm{.}Usingdataathighercurrents,wecanunambiguouslyfind$T_c$and$z$,andshow$z = 2.1 \pm 0.15$, as expected for the three-dimensional XY model with diffusive dynamics. The crossover to two-dimensional behavior, seen by other researchers in thinner films ($d\le 500$ \AA), obscures the three-dimensional transition in both zero field and the vortex-glass transition in field, leading to incorrect values of $T_c$ (or $T_g$), $\nu$, and $z$. The finite-size effects, usually ignored in thick films, are an explanation for the wide range of critical exponents found in the literature.