Universal critical behavior in single crystals and films of YBa${}_2$Cu${}_3$O${}_{7-\hat{I}´}$

Abstract

We have studied the normal-to-superconducting phase transition in optimally-doped YBa${}_2$Cu${}_3$O${}_{7-\hat{I}´}$ in zero external magnetic field using a variety of different samples and techniques. Using DC transport measurements, we find that the dynamical critical exponent $z=1.54\pm 0.14$, and the static critical exponent $\hat{I}½=0.66\pm 0.10$ 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, $z=1.55\pm 0.15$. The neglect of finite-thickness, finite-current, and finite-frequency effects may account for the wide ranges of values for $\hat{I}½$ and $z$ previously reported in the literature.

13 pages and 12 figures