Depth-dependence of electron screening, charge carriers and correlation: Theory and experiments

Core-level Photoemission Spectroscopy (PES) has played a very important role in our understanding of the electronic structure of correlated transition metal and rare-earth compounds. The appearance of strong satellite structures accompanying the main PES spectra in correlated systems is well known, and systematic variations in the position and intensities of these satellites provide us with important clues to their electronic structures. In spite of these successes, the surface sensitivity of PES has often led to controversies regarding surface versus bulk electronic structure, and hence, hard X-ray photoelectron spectroscopy (HAXPES) is very important and promising. HAXPES is a bulk sensitive probe of the electronic structure due to its ability to overcome surface sensitivity of conventional PES. Unlike soft X-ray PES, 2p core-level HAXPES have shown additional well-screened features with significant intensity at the low binding energy side of the main peak. These features were explained well by the configuration-interaction model including a screening channel derived from coherent states near Fermi energy. Here, we review these advances and examine the application of HAXPES to studies of the strongly correlated electron systems, especially for 3d transition metal compounds. The details of the well-screened features are also discussed.