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

TL;DR

This study investigates the critical behavior of the phase transition in optimally-doped YBa2Cu3O7−δ, revealing consistent critical exponents across different samples and measurement techniques, and clarifying discrepancies in previous reports.

Contribution

The paper provides a comprehensive analysis of the critical exponents in YBa2Cu3O7−δ, demonstrating the universality of critical behavior and addressing prior inconsistencies by considering finite-size effects.

Findings

Critical exponents z ≈ 1.55 and ν ≈ 0.66 were determined.

Finite-thickness and frequency effects influence previous measurements.

Consistent critical behavior observed across crystals and films.

Abstract

We have studied the normal-to-superconducting phase transition in optimally-doped YBa$_2$Cu$_3$O$_{7-\delta}$ in zero external magnetic field using a variety of different samples and techniques. Using DC transport measurements, we find that the dynamical critical exponent $z=1.54\pm0.14$, and the static critical exponent $\nu=0.66\pm0.10$ 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, $z=1.55\pm0.15$. The neglect of finite-thickness, finite-current, and finite-frequency effects may account for the wide ranges of values for $\nu$ and $z$ previously reported in the literature.