Local singlet for CuO andNd2CuO4

Photoemission 3d spectra according to the Anderson impurity model and experiment are compared for CuO and ${\mathrm{Nd}}_2$ ${\mathrm{CuO}}_4$. While good agreement between theory and experiment is obtained for CuO, the calculated local singlet has much more weight than observed for ${\mathrm{Nd}}_2$ ${\mathrm{CuO}}_4$. This suggests an appreciable difference between the low-energy excitations in CuO and ${\mathrm{Nd}}_2$ ${\mathrm{CuO}}_4$, which is surprising due to the similarities of the compounds, both having formally divalent Cu coordinated to four oxygen atoms in an approximate square. This difference is not predicted by the impurity Anderson model.