Coherence effects above T-c in YBa2Cu4O8

High temperature superconductors (HTSC) consist of CuO2 layers which are se parated by charge reservoirs (CuO-chains). They show strongly anisotropic p hysical parameters. Lower doped samples reveal metallic conductivity inside the layers but insulating temperature dependence for the out-of-plane dire ction. The resistivities of highly doped samples are metallic for all three directions. Evidence is presented that the temperature dependence for the resistivity perpendicular to the CuO2 layers is related to the charge reser voirs in between the planes. The metallic out-of-plane resistivity might be traced back to a metallic conduction channel due to the charge reservoirs. (C) 2000 Published by Elsevier Science Ltd. All rights reserved.