Zero-field superconducting phase transition obscured by finite-size effects in thick YBa₂Cu₃O_7-δ films

We report on the normal-superconducting phase transition in thick YBa ₂Cu₃O_7-δ films in zero magnetic field. We find significant finite-size effects at low currents even in our thickest films (d=3200 Å). Using data at higher currents, we can unambiguously find T_c and ¿, and show z=2.1±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≤500 Å), 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), v, 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.