Photoemission spectroscopy of correlated transition-metal compounds in the charge-transfer regime

An overview is given on the effects of strong d-d Coulomb repulsion on the electronic structure of charge-transfer (CT)-type 3d transition-metal (TM) compounds, especially of 3d TM chalcogenides. Mean-field effects determine the ground-state properties of the system including magnetic and orbital ordering whereas correlation effects are necessary to correctly describe excitation properties such as photoemission spectral weight distribution. Because of the strong d-d repulsion even in the metallic state of CT-type compounds, the overall photoemission spectral line shapes remain relatively unchanged across metal-insulator transitions. In the vicinity of the Fermi level, high-resolution photoemission spectra exhibit unusual line shapes and spectral weight transfer, which implies strong renormalization of quasi-particles due to electron correlation.