Comment to: Correlation Between Superfluid Density and T_{c} of Underdoped YBa2Cu3O6+x Near the Superconductor-Insulator Transition

TL;DR

This paper critiques previous measurements of superfluid density in underdoped YBa2Cu3O6+x, clarifying that their data do not contradict the expected relationship between Tc and penetration depth near the superconductor-insulator transition.

Contribution

It demonstrates that the data from Zuev et al. do not challenge the linear Tc-penetration depth relationship because their observed Tc reduction is not linked to increased anisotropy.

Findings

Zuev et al.'s data do not support a breakdown of the linear relationship.

The observed Tc reduction is not associated with increased anisotropy.

The data do not fall into the regime where the 2D-QSI transition theory applies.

Abstract

Zuev et al. reported measurements of the in-plane magnetic penetration depth of supposedly underdoped films of YBa2Cu3O6+x, with Tc from 6 to 50 K. They argue that their data is inconsistent with a linear relationship between Tc and the inversw of the T=0 in-plane penetration depth squared, as expected for a two dimensional quantum superconductor to insulator (2D-QSI) transition where Tc vanishes and the anisotropy diverges. It is shown that the data of Zuev et al. do not fall onto the regime where the linear relationship applies, because the observed reduction of Tc is not associated with an increase of the anisotropy.