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 CuO$_{ 2}$ layers as a function of the oxygen density $y$ in (Ca$_{ x}$ La$_{ 1-x}$)(Ba$_{ 1.75-x}$La$_{ 0.25+x}$)Cu$_{ 3}$O$_{ y}$. 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.
Categories: cond-mat.supr-con cond-mat.str-el