Probing bulk states of correlated electron systems by high-resolution resonance photoemission

Electron correlations are known to play an important role in determining th e unusual physical properties of a variety of compounds. Such properties in clude high-temperature superconductivity(1), heavy fermion behaviour(2,3) a nd metal-to-insulator transitions(4,5). High-resolution photoelectron spect roscopy (PES) provides a means of directly probing the electronic states (p articularly those near the Fermi level) in these materials, but the; short photoelectron mean free paths(6) (less than or equal to 5 Angstrom) associa ted with the low excitation energies conventionally used (less than or equa l to 120 eV) make this a surface-sensitive technique. Now that high-resolut ion PES is possible at much higher energies(7), with mean free paths as lon g as 15 Angstrom (ref. 6), it should become feasible to probe the bulk elec tronic states in these materials. Here we demonstrate the power of this tec hnique by applying it to the cerium compounds CeRu2Si2 and CeRu2. Previous PES studies of these compounds revealed very similar spectra for the Ce 4f electronic states(8-13), yet it is expected that such states should be diff erent owing to their differing degrees of hybridization with other valence bands. Our determination of the bulk Ce 4f electronic states of these compo unds resolves these differences.