Comment on "Hole concentration dependence of penetration depth and upper critical field in Bi2Sr2(Ca,Y)Cu2O8 extracted from reversible magnetization"

Abstract

We argue that the method, which was used for the analysis of equilibrium magnetization data in the original publication, is not adequate to the experimental situation. As a result, the temperature dependencies of the upper critical field Hc2(T) and the magnetic field penetration depth lambda(T), obtained in this work, are incorrect. Using a different approach, we reanalyze the presented experimental data and demonstrate that the normalized Hc2(T) curves are rather different from those presented in the original publication and do not follow predictions of the Werthamer-Helfand-Hohenberg theory. Another interesting observation is that the Hc2(T) dependencies for two samples with close levels of doping are rather different.