High-Resolution Photoemission Studies of the Low-Energy Excitations in Heavy-Fermion Systems

Abstract

This paper presents a qualitative discussion of the heavy-electron excitations within the single-impurity model. For lanthanide atoms it shows that the 4f states which are weakly hybridized with band states acquire at low temperature a metallic-like character concentrated in a narrow energy range around EF (Kondo peak). In different Ce systems we demonstrate that high-resolution (<20 meV) photoemission can probe directly these heavy-electron states which have in the spectra a very low intensity. Comparisons with spectra measured with a conventional resolution illustrate the necessity to resolve these low-energy excitations. The observed temperature dependence of the Kondo peak reflects the change from metallic-like to atomic-like character of these 4f states. The study of an Yb alloy reveals that the 4f hybridization in heavy rare earths can be as strong as in Ce systems but that it is responsible in this case for a regime of intermediate valence.