Upper critical field in electron-doped superconductor with nonstoichiometric disorder near antiferromagnetic-superconducting phase boundary

摘要

Using the resistivity method it was found that temperature dependence of the upper critical field for underdoped Nd_(1.86)Ce_(0.14)Cu〇_(4+δ) have an anomalous upward curvature of H_(c2)(T) dependence and can be consistently explained by the two-band/two-gap model of a dirty superconductor. Near antiferromagnetic-superconducting-phase boundary the critical temperature remains constant with the change of the disorder parameter and the slope of Bc2 increases with increasing of the disorder parameter. This behavior is completely different from dependencies for pure superconducting phase at optimal doping region. This difference may indicate the change of the type of the paring: from the predominance of the anisotropic s-wave component (may be due to unstable competition between antiferromagnetic (AF) and superconducting (SC) regions) in underdoped (x=0.14) region to the prevalence of d-wave part in optimal doped regions (x=0.15) because of residual spin fluctuations.