•K.-P. Weiss¹, K. Grube², S. I. Schlachter¹, H. Leibrock¹, C. Meingast², W. H. Fietz¹, B. Obst¹, Th. Wolf², P. Schweiss² und H. Wühl^3
1Forschungszentrum Karlsruhe, ITP , 76021 Karlsruhe, Germany
²Forschungszentrum Karlsruhe, IFP , 76021 Karlsruhe, Germany
³Universität Karlsruhe, IEKP, 76128 Karlsruhe, Germany

We performed resistivity and ac-susceptibility measurements on detwinned RBa₂Cu₃O_x (R123, R = Nd, Y) single crystals as a function of oxygen content x and pressure. In our experiment we are able to distinguish three different states with increasing temperature: First the superconducting state is observed. Above T_c, we enter the normal state with the signature of the pseudogap. At temperatures higher than 200 K, oxygen ordering in the CuO chains begins to change the charge carrier concentration in the CuO₂ planes preventing a systematic analysis of the resistivity. We have investigated the influence of different hole concentrations, structural changes, and the oxygen order on T_c and the normal state resistivity of R123. The comparison between chemical doping and pressure application shows that at least for temperatures below 200 K the normal state and superconducting properties of R123 are mainly determined by the charge carrier concentration in the CuO₂ planes.