TEMPERATURE-INVARIANT VALENCE-BAND-4F PHOTOEMISSION FEATURES IN THE HEAVY-FERMION COMPOUND YBAL3

A high-resolution photoemission (PES) study of the heavy-fermion compo und YbAl3 has been undertaken to test the qualitative and quantitative predictions of the single-impurity model. Corroborating data on YbAgC u4 and YbCu2Si2 are also presented. For Yb-based heavy fermions the mo del predicts a fully occupied Kondo resonance so that its width, inten sity, and temperature dependence should be directly accessible by PES. Due to the existence of temperature- and/or contamination-dependent s urface features, which are not dealt with by the model, a rigorous nor malization and curve-fitting procedure was required to extract the bul k 4f line shape. For the particular temperature cycling method employe d by us, we do not observe any temperature dependence in the peak that had been previously identified as the Kondo resonance. We find that b oth the binding energy and width of this peak are significantly larger than theory predicts. It is also found that the trends in 4f line sha pe and surface shift between YbAl3 and LuAl3 are consistent with the t rends seen between the 4f core levels of elemental Yb and Lu. Similar results are also found in YbCu2Si2 and YbAgCu4 for similar cycling met hods. Recent observations of temperature dependence in these systems a re most easily understood in terms of surface phenomena.