PHOTOEMISSION AND THE ELECTRONIC-PROPERTIES OF HEAVY FERMIONS - LIMITATIONS OF THE KONDO MODEL

The electronic properties of Yb-based heavy fermions have been investigated by means of high resolution synchrotron radiation photoemission and compared with predictions of the Kondo model. The Yb heavy fermion photoemission spectra show significant disagreement with the Kondo model predictions (as calculated within the Gunnarsson-Schonhammer computational method). Moreover, the Yb heavy fermion photoemission spectra give strong indications of core-like characteristics and compare favorably to purely divalent Yb metal and core-like Lu 4f levels. The heavy fermions YbCu2Si2, YbAgCu4 and YbAl3 were measured and shown to have lineshapes much broader and deeper in binding energy than predicted by the Kondo model. The lineshape of the bulk component of the 4f emission for these three heavy fermion materials was compared with that from Yb metal and the Lu 4f levels in LuAl3. The heavy fermion materials show no substantive spectroscopic differences from 4f levels observed in Yb metal and LuAl3. Also, the variation with temperature of the 4f lineshape was measured for Yb metal and clearly demonstrates that phonon broadening plays a major role in 4f level lineshape analysis and must be accounted for before considerations of correlated electron resonance effects are presumed to be at work.