T-C DEGRADATION IN CUPRATE SUPERCONDUCTORS FROM THE RESISTIVITY OF YBA2(CU(1-X)M(X))(4)O-8 FOR M=FE AND NI

Systematic measurements of the resistivity of the polycrystalline samples of the YBa2(Cu1-xMx)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 Tc depression in the YBa2(Cu1-xMx)4O8 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 Tc degradation in the Ni- and Fe-doped samples of the system considered. © 1995 The American Physical Society.