Slow crossover in YbXCu4 (X=Ag, Cd, In, Mg, Tl, Zn) intermediate-valence compounds

We compare the results of measurements of the magnetic susceptibility χ(T), the linear coefficient of specific heat γ(T)=C(T)/T, and 4f occupation number n_f(T) for the intermediate-valence compounds YbXCu₄ (X=Ag, Cd, In, Mg, Tl, Zn) to the predictions of the Anderson impurity model, calculated in the noncrossing approximation (NCA). The crossover from the low temperature Fermi-liquid state to the high-temperature local-moment state is substantially slower in the compounds than predicted by the NCA; this corresponds to the ``protracted screening'' recently predicted for the Anderson lattice. We present results for the dynamic susceptibility, measured through neutron-scattering experiments, to show that the deviations between theory and experiment are not due to crystal-field effects, and we present x-ray-absorption fine-structure results that show the local crystal structure around the X atoms is well ordered, so that the deviations probably do not arise from Kondo disorder. The deviations may correlate with the background conduction electron density, as predicted for protracted screening.