Photoemission from YbInCu4:: testing the single impurity model

The electronic properties of single crystal YbInCu4 and YbAgCu4 have been investigated by means of high resolution photoelectron spectroscopy. A first order, isostructural phase transition for YbInCu4 at 42K leads to changes in the Kondo temperature, of more than an order of magnitude. This phase transition and accompanying Kondo temperature change provide the most direct test of the single impurity model to date. The photoemission results are incongruous with the single impurity model predictions for temperature dependence, binding energy and 4f occupancy, encouraging a re-evaluation of the single impurity model applicability.