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

Electron correlations are known to play an important role in determining the unusual physical properties of a variety of compounds. Such properties include high-temperature superconductivity1, heavy fermion behaviour2,3 and metal-to-insulator transitions4,5. High-resolution photoelectron spectroscopy (PES) provides a means of directly probing the electronic states (particularly those near the Fermi level) in these materials, but the short photoelectron mean free paths6 (⩽5 Å) associated with the low excitation energies conventionally used (⩽120 eV) make this a surface-sensitive technique. Now that high-resolution PES is possible at much higher energies7, with mean free paths as long as 15 Å (ref. 6), it should become feasible to probe the bulk electronic states in these materials. Here we demonstrate the power of this technique 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 states8,9,10,11,12,13, yet it is expected that such states should be different owing to their differing degrees of hybridization with other valence bands. Our determination of the bulk Ce 4f electronic states of these compounds resolves these differences.