Interplanar coupling, induced superconductivity, and van Hove singularity in high-T-c cuprates

The interplanar coupling between the superconducting CuO2 planes in RBa(2)Cu(3)O(7-delta) is controlled by changing the size of the rare earth and by changing the oxygen deficiency in the chains while keeping the hole concentration on the planes fixed at optimum doping by means of Ca substitution on the R site. The irreversibility field H-in(T), thermopower, penetration depth from muon spin rotation, and the c-axis coherence length from fluctuation conductivity all indicate superconductivity extends onto the CuO1-delta chains by the proximity effect, giving an increased interlayer coupling equivalent to a halving of the interplanar distance. The insensitivity of T-c to interlayer coupling and the absence of a peak in the density of states determined from Y NMR seriously erodes the van Hove singularity scenario for the high-T-c cuprates.