Photoemission experiments on YbInCu4: Surface effects and temperature dependence

We present temperature-dependent photoemission (PES) measurements on single -crystalline YbInCu4, a stoichiometric material that shows an isostructural first-order valence transition at T(a)approximate to 60 K. The results of the analysis of the PES data show peculiar differences to other, bulk-sensi tive measurements: (a) a lack of the sharp first-order valence transition a nd (b) the Yb valence - determined directly from the 4f intensity ratio lie s substantially below x-ray absorption (XAS) results over the whole investi gated temperature range. We discuss these discrepancies with particular att ention to surface effects and conclude that the PES results are due to a di storted near-surface region which differs in its physical properties signif icantly from the volume material.[S0163-1829(98)08843-2].