NORMAL-STATE CONDUCTIVITY BETWEEN CUO2 PLANES IN COPPER-OXIDE SUPERCONDUCTORS

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, rho-ab, shows a metallic temperature dependence, the resistivity parallel to the c axis (rho-c) has been reported to be both non-metallic 1,2 and metallic 3,4. Here we present systematic data for rho-c in a number of high-T(c) materials, obtained from well characterized single crystals. Both the magnitude and the temperature dependence of rho-c are strongly dependent on crystal structure, and on the concentration of charge carriers. We find that rho-c is non-metallic (d-rho-c/dT < 0) in most superconducting compounds, suggesting an unconventional conduction mechanism. Fully oxygenated YBa2Cu3O approximately 7 (with T(c) almost-equal-to 90 K) is the only exception, with a relatively small rho-c and positive d-rho-c/dT that may arise from the crystal structure specific to this material.