Two types of Hc2(T) dependences in Bi2Sr2Ca1−xYxCu2O8+δ with different Yttrium content

We reanalyze the magnetization data collected on Bi2Sr2Ca1−xYxCu2O8+δ samples [G.C. Kim, M. Cheon, H. Kim, Y.C. Kim, D.Y. Jeong, Phys. Rev. B 72 (2005) 64525] and argue that the method, which was used for the analysis of equilibrium magnetization data, is not adequate to the experimental situation. As a result, the temperature dependences of the upper critical field Hc2(T) and the magnetic field penetration depth λ(T), obtained in this work, are misinterpreted. Using a different approach to analysis, we 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 important observation is that the Hc2(T) dependences for two samples with different levels of doping are qualitatively different.