Y89NMR Study of the Pauli Susceptibility of the CuO2Planes inYBa2Cu3O6+x

The negative Knight shift measured at 293 K in $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ is found to decrease progressively for decreasing oxygen content and to vanish below $x\approx 0.4$, in the insulating phases. The corresponding decrease of the macroscopic susceptibility is found to be dominated by the variation of the Pauli susceptibility of the Cu${\mathrm{O}}_2$ planes. These results imply that the chain contribution to the density of states at the Fermi level is not significant enough to explain the high $T_c$. It is also concluded that strong coupling between charge and spin degrees of freedom occurs in this material.