Temperature-invariant valence-band 4fphotoemission features in the heavy-fermion compoundYbAl3

A high-resolution photoemission (PES) study of the heavy-fermion compound ${\mathrm{YbAl}}_3$ has been undertaken to test the qualitative and quantitative predictions of the single-impurity model. Corroborating data on ${\mathrm{YbAgCu}}_4$ and ${\mathrm{YbCu}}_2$ ${\mathrm{Si}}_2$ are also presented. For Yb-based heavy fermions the model predicts a fully occupied Kondo resonance so that its width, intensity, and temperature dependence should be directly accessible by PES. Due to the existence of temperature- and/or contamination-dependent surface features, which are not dealt with by the model, a rigorous normalization and curve-fitting procedure was required to extract the bulk 4f line shape. For the particular temperature cycling method employed by us, we do not observe any temperature dependence in the peak that had been previously identified as the Kondo resonance. We find that both the binding energy and width of this peak are significantly larger than theory predicts. It is also found that the trends in 4f line shape and surface shift between ${\mathrm{YbAl}}_3$ and ${\mathrm{LuAl}}_3$ are consistent with the trends seen between the 4f core levels of elemental Yb and Lu. Similar results are also found in ${\mathrm{YbCu}}_2$ ${\mathrm{Si}}_2$ and ${\mathrm{YbAgCu}}_4$ for similar cycling methods. Recent observations of temperature dependence in these systems are most easily understood in terms of surface phenomena.