Two types of Hc2( T) dependences in Bi 2Sr 2Ca 1− xY xCu 2O 8+ δ with different Yttrium content

Abstract

We reanalyze the magnetization data collected on Bi 2Sr 2Ca 1− x Y x Cu 2O 8+ 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 H c2( 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 H c2( 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 H c2( T) dependences for two samples with different levels of doping are qualitatively different.