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