Critical microwave-conductivity fluctuations across the phase diagram of superconducting La$_{2-x}$Sr$_x$CuO$_4$ thin films

TL;DR

This study investigates microwave conductivity near the critical temperature in LSCO thin films, revealing a doping-dependent shift from 2D-XY to 3D-XY universality classes, which enhances understanding of the phase diagram in high-$T_c$ cuprates.

Contribution

It provides the first direct microwave evidence of a doping-dependent change in the universality class of superconducting fluctuations in LSCO thin films.

Findings

Underdoped LSCO exhibits 2D-XY behavior.

Overdoped LSCO shows 3D-XY critical exponents.

Doping influences the universality class of superconducting fluctuations.

Abstract

We report a systematic study of the dynamic microwave conductivity near $T_c$ for La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) thin films with $x$=0.07 to 0.16. The strong frequency dependence of the phase stiffness together with scaling analysis of the ac fluctuating conductivity of superconductivity provide direct evidence for the 2D-XY behavior of nearly decoupled CuO$_2$ planes in underdoped LSCO ($x$=0.07 and 0.12). On the other hand, the critical exponents for slightly overdoped LSCO ($x$=0.16) were found to agree with those for the relaxational 3D-XY model, indicating that the universality class in LSCO is changed by hole doping. The implication of these results for the phase diagram of high-$T_c$ cuprates is discussed.