Electrical Study of 123 HTSC (77 to 400 K) with Varying Fraction of Y. Explanation of the Normal State Behaviour

Abstract

By varying the fraction n of yttrium in copper oxide based ceramic superconductors (nRE)BaCuO (RE = Y/Dy) it is found that below a certain n the dc percolation path through superconducting grains is lost and the material behaves insulator-like with variable range hopping mode of electrical transport (In e cc Experiments also show that the variation of resistivity e with temperature T in the normal state for samples (n 2 0.25) exhibiting a superconducting transition is far from linear as the temperature is raised. By an extension of Mott's original ideas to polaron hopping with decreased localisation it is found that In e should vary as T in satisfactory agreement with experiment. Thus, the so-called "anomalous temperature variation of resistivity"in the metal-like normal statc as well as in the insulator-like state with increased disorder is explained within a single framework of hopping of localised carriers.

Current transfer and initial dissipation in high- superconductors

“…which obey, at least in a limited temperature interval at high temperatures, a linear dependence ρ ∝ T , resembling scattering by Debye phonons in ordinary metals) are either superconducting or insulating at low temperatures. The conduction mechanism in such 'metallic' samples may also involve percolation of various origins [137,138,141]. Indeed, a very weak saturation-type dependence of the superconducting critical temperature on charge carrier density (instead depending parabolically [142]), seems to indicate [137] the presence of effective phase separation [143] into nominally metallic (and below T c superconducting) and dielectric components.…”