Ab initio multiplet plus cumulant approach for correlation effects in x-ray photoelectron spectroscopy

Abstract

The treatment of electronic correlations in open-shell systems is among the most challenging problems of condensed matter theory. Current approximations are only partly successful. Ligand field multiplet theory (LFMT) has been widely successful in describing intra-atomic correlation effects in x-ray spectra, but typically ignores itinerant states. The cumulant expansion for the one electron Green's function successfully describes shake-up effects but ignores atomic multiplets. More complete methods are computationally problematic. Here we show that separating the dynamic Coulomb interactions into local and longer-range parts yields an efficient, nearly ab initio multiplet + cumulant approach that accounts for both local atomic multiplet-splittings and charge-transfer shake-up satellites. An application to α-Fe2O3 (hematite) yields very good agreement with XPS experiment, including the broad 9 eV satellites and distributed background features missing from previous approaches.

Main and Satellite Features in the Ni 2p XPS of NiO

“…This generality may be in contrast to the application of the cumulant Green's function methodology. 35−37 While the method has been used to describe the main and satellite features of the Fe 2p XPS of Fe 2 O 3 , 35,37 the authors 37 suggest that "... various improvements are desirable, especially for more strongly correlated systems like NiO. "…”

“…A quite different approach to the theoretical study of XPS, based on a cumulant Green's function methodology, has been taken in refs 35−37. The authors describe this method as nearly ab initio, 35 and it has been able to describe both the main and satellite XPS features for a few oxides. However, the authors do not analyze the nature of the XPS peaks in terms of the kinds of configurations that contribute to them which is the major focus of our present study.…”