T_c degradation in cuprate superconductors from the resistivity of YBa₂(Cu_1-xM_x)₄O₈ for M=Fe and Ni

Abstract

Systematic measurements of the resistivity of the polycrystalline samples of the YBa₂(Cu_1−xM_x)₄O₈ system with M=Fe, Ni are reported. The resistivity behavior depends strongly on the dopant concentration. For the Fe 6-at. % sample, there is no metallic behavior in the resistivity, but for T > 50 K, the resistivity decreases very slowly with temperature. The behavior of the resistivity of the Fe 10-at. % and Ni 5-at. % samples is analyzed, and it is found that the variable-range hopping process may occur in these samples below about 30 K. While the Ni 5-at. % sample is found to show signs of the nearest-neighbor hopping process of conductivity above about 150 K, the Fe 10-at. % sample does not show such behavior up to 300 K. On this basis it is found that localization may not be a possible reason of the large T_c depression in the YBa₂(Cu_1−xM_x)₄O₈ system by the Fe and Ni dopants. We have examined the possibility of T_c degradation due to carrier-impurity potential scattering also by using a single-crystal analog of the polycrystalline sample. This analogy is justified to within a factor of less than 1.33. On the basis of this analogy we have found that potential scattering may not be a reasonable source of T_c degradation in the Ni- and Fe-doped samples of the system considered.