Bulk electronic structure of YbInCu4 from photoemission:: A unique test of the single impurity model

The heavy fermion compound YbInCu4 presents a unique opportunity to test the single impurity model (SIM) owing to its isostructural phase transition at 42 K, whereby the Kondo temperature suddenly increases from 30 K above, to 400 K below the transition. Photoelectron spectroscopy (PES) at varying temperatures is well suited for this test, Previous researchers have claimed that PES probes only a subsurface region which differs from the bulk, thus rendering PES less than optimum for such a test. We present PES data in which a single-crystal sample was cooled through the phase transition only once in order to minimize defect formation. We find clear evidence of the bulk transition and no evidence for a subsurface region. The PES spectra do not agree with the predictions of the SIM and its approximate solution, the noncrossing approximation. We propose that only a periodic model is applicable to highly correlated, stoichiometric heavy fermion systems.