Strongly correlated electron systems: Photoemission and the single-impurity model

We present high-resolution, angle-resolved photoemission spectra for Ce-based and U-based strongly correlated electron systems. The experimental results are irreconcilable with the long-accepted single-impurity model, which predicts a narrow singlet state, in close proximity to the Fermi energy, whose linewidth and binding energy are a constant determined by a characteristic temperature $T_K$ for the material. We report that both $4f$ and $5f$ photoemission features disperse with crystal momentum at temperatures both above and below $T_K;$ these are characteristics consistent with narrow bands but not with the single-impurity model. Inclusion of the lattice must be considered at all temperatures. Variants of the periodic Anderson model are consistent with this approach.