Observation of Low-Energy Excitations in Lanthanide Materials by High-Resolution Photoemission

Abstract

One of the fundamental motivations for electron spectroscopic studies is to deduce ground state properties of electronic systems from their excitation spectra. Among the high-energy spectroscopies photoemission has become very popular because it is commonly considered as one of the most direct methods to probe the density of the electronic states (DOS) in solids. However, when the correlation among the band electrons increases, the relationship between measured spectra and DOS is no longer straightforward¹. In the extreme case of the 4f-states, which are sufficiently localized to participate only very weakly to bands, the simple single-particle picture becomes almost meaningless and fails to account for many properties involving their excitations. In particular, at low temperatures, the quasi-equilibrium methods (specific heat, susceptibility, transport properties) applied to the study of systems containing lanthanide elements, reveal very unusual manifestations, like Kondo-, mixed -valence, or heavy-Fermion behaviour². The single-impurity model of Anderson, allowing the mixing of one localized 4f-state with extended states, has proven to yield a first approach to important aspects of this problem ^3,4,5. it has been used to calculate the spectral functions of high-energy spectroscopies.