ELECTRICAL-CONDUCTION ACROSS CUO2 PLANES IN COPPER-OXIDE SUPERCONDUCTORS

We have investigated the out-of-plane resistivity (rho-c) of a number of copper oxides with various carrier concentration systematically, using well-characterized single crystals. The out-of-plane conduction is strongly dependent on crystal structure, and in contrast to the metallic T-linear in-plane resistivity, rho-c is non-metallic in most superconducting compounds, suggesting an unconventional conduction mechanism. The most remarkable observation is that rho-c decreases with increasing carrier concentrations much more rapidly than rho-ab.