Probing electron correlations in solids with Auger-photoelectron coincidence spectroscopy

Auger photoelectron coincidence spectroscopy (APECS) has emerged as a powerful tool for probing electron-electron correlation in solids. When a photoexcited core hole decays via a core-valence-valence (CVV) Auger transition, two valence holes are created in the final state. Consequently, the line shape of the Auger electron spectrum is influenced by both Coulomb and exchange interactions between the two holes. By simultaneously detecting the photoelectron and Auger electron emitted from a single photoionization event, APECS measurements place constraints on the two final state holes, enabling one to independently measure Coulomb and exchange correlation energies. 
This review highlights the key advantages of APECS over other electron spectroscopic techniques, including separating overlapping spectral features, removing the background signal from inelastically scattered electrons, enhanced surface sensitivity and site specificity and cites examples of how these attributes can be used to investigate the properties of solids in an unprecedented manner. To aid in understanding APECS data and assist in experimental design, a phenomenological model for the probability of electron pair emission as a function of the photo- and Auger electron kinetic energies and emission angles is presented. The model is applied to a hypothetical solid, demonstrating how the contribution to APECS spectrum of final states with different spin configurations depends on these experimental parameters. A summary of results obtained by performing one-dimensional (as a function of Auger [EA] or photoelectron [EP] kinetic energy), two-dimensional (parallel detection of EA and EP) and angle-resolved (as a function of EA at specific photo- and Auger electron emission angles) APECS measurements is presented. In particular, measurements of ferromagnetic metals and antiferromagnetic transition metal oxides demonstrate how angle-resolved (AR-) APECS is a powerful tool for independently measuring Coulomb and exchange correlation energies. Finally, potential future applications of APECS and further developments of this experimental technique are discussed.