A systematic study of the role of Nd/Ba disorder on the superconducting properties of Nd_(1+x)Ba_(2-x)Cu_3O_(7-δ)

摘要

The role of Nd/Ba disorder on the superconducting properties of Nd_(1+x)Ba_(2-x)Cu_3O_(7-δ) epitaxial thin films has been systematically studied. The temperature dependence of the ab-plane penetration depth of highly c-axis epitaxial samples having different Nd/Ba composition has been measured by an inverted microstrip resonator technique. Stoichiometric films (x = 0) exhibit a linear penetration depth at low temperature, while Nd-rich films (0 < x < 0.3) show a T~2 law. We find that for low Nd excess it is possible to obtain a reasonable fit of the data in the full temperature range using the d-wave picture, with the assumption that Nd at Ba site plays the role of unitary scattering center. At high concentration, the low temperature data can be instead explained supposing that excess Nd acts as impurity with intermediate strength. The dependence of the critical temperature and of the zero temperature superfluid density on disorder have been also studied. It is found that these two quantities are correlated in a way analogous to what reported for YBCO films and single crystals having Zn, Ni impurities at Cu site. An attempt to explain the results using a s-wave approach, as in the two-gap model, is able to reproduce the data but gives results quantitatively conflicting. On the contrary a self-consistent picture within a d-wave framework can be obtained taking into account the effect of localization and suppression of the order parameter at impurity sites in the case of both low and high impurity content.