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 (CaₓLa_1-x)(Ba_1.75-xLa0.25+x)Cu₃Oy (CLBLCO) regarding the superexchange (J) dependence of the maximum superconductivity (SC) critical temperature Tcᵐᵃˣ(J); RE and x are implicit variables. This is done by measuring the N\'eel 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 TN 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 Tcᵐᵃˣ and TNᵐᵃˣ vary considerably in the undoped limit and a positive correlation between the two quantities was observed.