Dynamic Scaling and Two-Dimensional High-Tc Superconductors

TL;DR

This paper investigates the critical behavior of two-dimensional high-Tc superconductors, analyzing various scaling theories and experimental data to determine the true nature of their phase transitions.

Contribution

The study critically evaluates existing scaling theories and proposes a new criterion to conclusively identify phase transitions in 2D superconductors.

Findings

Data can be scaled according to the modified transition over a range of temperatures.

The universal dynamic critical exponent may be an artifact of data analysis flexibility.

A new criterion is proposed for definitive evidence of phase transitions.

Abstract

There has been ongoing debate over the critical behavior of two-dimensional superconductors; in particular for high Tc superconductors. The conventional view is that a Kosterlitz-Thouless-Berezinskii transition occurs as long as finite size effects do not obscure the transition. However, there have been recent suggestions that a different transition actually occurs which incorporates aspects of both the dynamic scaling theory of Fisher, Fisher, and Huse and the Kosterlitz-Thouless-Berezinskii transition. Of general interest is that this modified transition apparently has a universal dynamic critical exponent. Some have countered that this apparent universal behavior is rooted in a newly proposed finite-size scaling theory; one that also incorporates scaling and conventional two-dimensional theory. To investigate these issues we study DC voltage versus current data of a 12 angstrom thick YBCO film. We find that the newly proposed scaling theories have intrinsic flexibility that is relevant to the analysis of the experiments. In particular, the data scale according to the modified transition for arbitrarily defined critical temperatures between 0 K and 19.5 K, and the temperature range of a successful scaling collapse is related directly to the sensitivity of the measurement. This implies that the apparent universal exponent is due to the intrinsic flexibility rather than some real physical property. To address this intrinsic flexibility, we propose a criterion which would give conclusive evidence for phase transitions in two-dimensional superconductors. We conclude by reviewing results to see if our criterion is satisfied.