Normal-state conductivity between CuO₂ planes in copper oxide superconductors

Abstract

THE high-transition-temperature (high- T_c) copper oxide superconductors are highly anisotropic in their electrical properties; this is one of the key concerns for applications of these materials, and may also provide a clue to the mechanism of the superconductivity. Although the resistivity in the CuO₂ planes, ρ^ab, shows a metallic temperature dependence, the resistivity parallel to the c axis (ρ_c) has been reported to be both non-metallic^1,2 and metallic^3,4. Here we present systematic data for ρ_c in a number of high-T_c materials, obtained from well characterized single crystals. Both the magnitude and the temperature dependence of ρ_c are strongly dependent on crystal structure, and on the concentration of charge carriers. We find that ρ_c is non-metallic (dρ_c/dT< 0) in most superconducting compounds, suggesting an unconventional conduction mechanism. Fully oxygenated YBa₂Cu₃O_∼7 (with T_c≈90K) is the only exception, with a relatively small ρ_c and positive dρ_c/d T that may arise from the crystal structure specific to this material.