Relevance of magnetism to cuprate superconductivity: Lanthanides versus charge-compensated cuprates

We address what seemed to be a contradiction between the lanthanide series REBa2Cu3Oy (RE123) and the charge-compensated series (CaxLa1-x)(Ba1.75-x La0.25+x)CU3Oy (CLBLCO) regarding the superexchange (J) dependence of the maximum superconductivity (SC) critical temperature T-c(max)(J); RE and x are implicit variables. This is done by measuring the Neel temperature and the temperature dependence of the magnetic order parameter for RE = Nd, Sm, Eu, Gd, Dy, Yb, Y, and for Y(BaSr)Cu3Oy, at various very light dopings. The doping is determined by thermopower and the magnetic properties by muon spin rotation. We find that the normalized-temperature dependence of the order parameter is identical for all RE123 in the undoped limit (with the exception of Gd123) implying identical out-of-plane magnetic coupling. The extrapolation of T-N to zero doping suggests that, despite the variations in ionic radii, J varies too weakly in this system to test the relation between SC and magnetism. This stands in contrast to CLBLCO, where both T-c(max) and T-N(max) vary considerably in the undoped limit and a positive correlation between the two quantities was observed.