Systematic measurements of the resistivity of the polycrystalline samples of the ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$${\mathit{M}}_{\mathit{x}}$${)}_4$${\mathrm{O}}_8$ system with M=Fe, Ni are reported. The resistivity behavior depends strongly on the dopant concentration. For the Fe 6-at. % sample, there is no metallic behavior in the resistivity, but for T>50 K, the resistivity decreases very slowly with temperature. The behavior of the resistivity of the Fe 10-at. % and Ni 5-at. % samples is analyzed, and it is found that the variable-range hopping process may occur in these samples below about 30 K. While the Ni 5-at. % sample is found to show signs of the nearest-neighbor hopping process of conductivity above about 150 K, the Fe 10-at. % sample does not show such behavior up to 300 K. On this basis it is found that localization may not be a possible reason of the large ${\mathit{T}}_{\mathit{c}}$ depression in the ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$${\mathit{M}}_{\mathit{x}}$${)}_4$${\mathrm{O}}_8$ system by the Fe and Ni dopants. We have examined the possibility of ${\mathit{T}}_{\mathit{c}}$ 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 ${\mathit{T}}_{\mathit{c}}$ degradation in the Ni- and Fe-doped samples of the system considered.

• Received 12 August 1994

DOI:https://doi.org/10.1103/PhysRevB.51.539