Critical-doping universality for cuprate superconductors: Oxygen nuclear-magnetic-resonance investigation of (CaxLa1-x)(Ba1.75-xLa0.25+x)Cu3Oy

The critical doping levels in cuprates, where the ground state changes its nature (from an antiferromagnet to a spin glass to superconductor to metal), are not universal. We investigate the origin of these critical doping variations by measuring the in-plane oxygen p(sigma) hole density in the CuO2 layers as a function of the oxygen density y in (CaxLa1-x)(Ba1.75-xLa0.25+x)Cu3Oy (CLBLCO). This is done using the oxygen 17 nuclear quadrupole resonance parameter nu(Q). We compare compounds with x=0.1 and 0.4 which have significant critical y variations and find that these variations can be explained by a change in the efficiency of hole injection into the p(sigma) orbital. This allows us to generate a unified phase diagram for the CLBLCO system across the entire doping range, with no adjustable parameters.