Universal critical behavior in single crystals and films of YBa2 Cu3 O7-δ

We have studied the normal-to-superconducting phase transition in optimally doped YBa2 Cu3 O7-δ in zero external magnetic field using a variety of different samples and techniques. Using dc transport measurements, we find that the dynamical critical exponent z=1.54±0.14, and the static critical exponent ν=0.66±0.10 for both films (when finite-thickness effects are included in the data analysis) and single crystals (where finite-thickness effects are unimportant). We also measured thin films at different microwave frequencies and at different powers, which allowed us to systematically probe different length scales to avoid finite-thickness effects. dc transport measurements were also performed on the films used in the microwave experiments to provide a further consistency check. These microwave and dc measurements yielded a value of z consistent with the other results, z=1.55±0.15. The neglect of finite-thickness, finite-current, and finite-frequency effects may account for the wide ranges of values for ν and z previously reported in the literature. © 2009 The American Physical Society