Abstract:
Systematic measurements of the resistivity of the polycrystalline samples of the YBa(2)(Cu(1−x)M(x))(4)O(8) system with M=Fe, Ni are reported. The resistivity behavior depends strongly on the dopant concentration. For the Fe 6-at. percent sample, there is no metallic behavior in the resistivity, but for T greater than 50 K, the resistivity decreases very slowly with temperature. The behavior of the resistivity of the Fe 10-at. percent and Ni 5-at. percent 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. percent sample is found to show signs of the nearest-neighbor hopping process of conductivity above about 150 K, the Fe 10-at. percent 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 YBa(2)(Cu(1−x)M(x))(4)O(8) system by the Fe and Ni dopants. We have examined the possibility of Tc 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.
