Critical charge dynamics of superconducting LSCO thin films probed by complex microwave spectroscopy: Anomalous changes of the universality class by hole doping

TL;DR

This study investigates the critical charge dynamics in LSCO thin films across various doping levels, revealing a novel 2D-

Contribution

It uncovers a new 2D-

Findings

Identified three distinct universality classes: 2D-XY, 3D-XY, and a novel 2D-U in overdoped LSCO.

Discovered a double change in the dimensionality of critical charge dynamics with hole doping.

Confirmed the intrinsic nature of the 2D-U behavior through systematic measurements.

Abstract

We study the critical charge dynamics of the superconducting to the normal-state transition for LSCO thin films with a wide range of the Sr concentration, by measuring the frequency-dependent excess parts of the complex microwave conductivity, which is induced by the superconducting fluctuations. We present a dynamic scaling analysis of the complex fluctuation conductivity, which includes the information on the universality class and the dimensionality of the critical charge dynamics as a function of the Sr concentration, the film thickness and the magnetic field. In our previous study (H. Kitano et al., Phys. Rev. B 73, 092504 (2006).), the 2D-XY critical dynamics for underdoped LSCO and the 3D-XY critical dynamics for optimally doped LSCO were reported. In this study, we observed a novel two-dimensional unknown critical charge dynamics for overdoped thin films from x=0.17 to 0.20, which is clearly distinguished from the 2D-XY critical dynamics. Through the systematic measurements by changing the film thickness or by applying small magnetic field, it was confirmed that this unusual behavior, which is referred as 2D-"U" below, was not induced by the finite size effect but was intrinsic to the overdoped LSCO. Thus, it was found that the critical behavior in the phase diagram of LSCO is classified into the following three types; (i) 2D-XY for underdoped region, (ii) 3D-XY for optimally doped region, and (iii) 2D-"U" for overdoped region. In other words, the dimensionality in the critical charge dynamics is changed twice with hole doping. We discuss possible origins of such anomalous dimensional crossovers with hole doping, including an interpretation based on the possible existence of a hidden quantum critical point near the optimally doped region.