Resistivity of multiphase high-T c superconductors

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Abstract
Resistivity versus temperature (ρ‐T) behavior of high‐Tc superconductors in the normal state was recently modeled semiempirically according to conventional electron hopping theory for mixed‐valence metal oxides. One important feature of this work was the interpretation of a resistivity minimum (i.e., the transition between insulator and metal) in the temperature regime near Tc, based on the decrease of activation energies Ehop with increases in magnetic dilution reported in the NiO and CuO systems. Quantitative accuracy has now been established by fitting ρ‐T data of the La2−xSrxCuO4 and YBa2Cu3O7 high‐Tc compounds, which serve as the basis for interpreting the varying slopes of multiphase superconductors. Calculated curves fitted to reported experimental data show that the metallic slope (∂ρ/∂T≳0 above Tc) and the ρ‐axis intercept of the linear extrapolation may be used to estimate the volume ratio of effective superconducting phase to normal phase within a particular specimen. From the application of this theory, it is determined that: (i) ρ‐axis intercepts of single‐phase superconductors are proportional to Ehop(1 − x)/x; (ii) normal phases cause increases in ρ, but decreases in slope of ρ(T)/ρ(300); and (iii) metallic slopes may be achieved with less than 10% effective volume of superconducting phase.