Chemical bonding and many-body effects in site-specific x-ray photoelectron spectra of corundum V 2 O 3 J. C. Woicik, 1 M. Yekutiel, 2 E. J. Nelson, 1 N. Jacobson, 2 P. Pfalzer, 3 M. Klemm, 3 S. Horn, 3 and L. Kronik 2, * 1 National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA 2 Department of Materials and Interfaces, Weizmann Institute of Science, Rehovoth 76100, Israel 3 Institut für Physik, Universtitätsstrasse 1, 86159 Augsburg, Germany Received 24 January 2007; revised manuscript received 24 July 2007; published 1 October 2007 Site-specific x-ray photoelectron spectroscopy together with density functional theory calculations based on the local density approximation have identified the chemical bonding, single-particle matrix element, and many-body effects in the x-ray photoelectron spectrum of corundum V 2 O 3 . Significant covalent bonding in both the upper and lower lobes of the photoelectron spectrum is found, despite the localized nature of the V 3d electrons that are responsible for the Mott behavior. We show that the approximate treatment of correlation dominates the discrepancy between theory and experiment in the near-Fermi-edge region and that many-body effects of the photoemission process can be modeled by Doniach-Šunjić J. Phys. C 3, 285 1970 asymmetric loss. Correlation effects govern the relative intensity and energy position of the higher level electron bands, and many-body effects dominate the “tail” region of both the upper and lower lobes of the photoemission spectrum. DOI: 10.1103/PhysRevB.76.165101 PACS numbers: 79.60.i, 71.30.h, 78.70.Ck I. INTRODUCTION Vanadium sesquioxide, V 2 O 3 , is a highly correlated solid that has been studied intensively for over three decades. At room temperature, V 2 O 3 is a paramagnetic metal possessing a trigonal corundum crystal structure. Upon cooling to 160 K, it undergoes a first order phase transition to an insulating antiferromagnet with monoclinic symmetry. 1 This transition is often considered a prototypical Mott-Hubbard metal-insulator transition. 2,3 However, more recent work suggests a more refined model to account for covalent bond- ing between the V 3d and O 2p states. 4,5 Photoelectron spectroscopy PES of the paramagnetic V 2 O 3 phase has been used intensively in an attempt to un- derstand the underlying electronic structure in general, hy- bridization trends in particular, and their relation to the metal-insulator transition. 5–13 Unfortunately, interpretation of the PES data is not straightforward. Ultraviolet photoelectron spectroscopy UPS is highly surface sensitive, and its results may deviate from the bulk electronic structure. X-ray photo- electron spectroscopy XPS is more sensitive to the true bulk electronic structure owing to the larger escape depth of the photoelectrons. Indeed, recent high-resolution XPS studies 12,13 have detected a prominent near-Fermi-level peak that was not revealed by earlier UPS investigations. 6,8,11 However, the interpretation of XPS data is, in general, also complicated. First, matrix element effects; i.e., the modula- tion of the photoelectron spectrum by the transition prob- abilities between the initial, bound-state valence electrons and the final, plane-wave continuum-state photoelectrons, are pronounced in XPS. 14 Second, the analysis of XPS data is more complicated than suggested by a single-particle de- scription due to many-electron effects in the photoemission process. 14,15 In this paper, we employ site-specific XPS SSXPS, 16 where XPS data are collected under conditions of x-ray Bragg diffraction, for a direct experimental determination of the contribution of the V and O atoms to the overall elec- tronic structure of V 2 O 3 in its metallic paramagnetic phase. By contrasting these measurements with first principles cal- culations based on density functional theory DFT within the local density approximation LDA, 17 we quantify the extent to which matrix element effects dominate the photo- electron spectra. By comparing the SSXPS results with ad- ditional high-resolution near-Fermi-edge XPS measurements as well as core-level ones, we identify the signature of many- body effects in the photoemission process and show the im- portance of both Doniach-Šunjić 18 electron shakeup across the Fermi edge and the approximate treatment of correlation in LDA in determining the relative intensity and energy po- sition of the upper electron bands. II. EXPERIMENTAL AND COMPUTATIONAL DETAILS The experiment was performed at the National Synchro- tron Light Source, Brookhaven National Laboratory, at the National Institute of Standards and Technology tender x-ray spectroscopy facility beamline X24A. A double crystal monochromator was operated with Si111 crystals and high- resolution photoelectron spectra were obtained with a hemi- spherical electron analyzer. An atomically clean, stoichio- metric single-crystal corundum V 2 O 3 0001 surface was prepared by annealing to 725 ° C in ultrahigh vacuum after which low-energy electron diffraction showed a sharp 1  1 pattern. Photoelectron spectra were recorded at two different photon energies within the photon-energy width E  0.6 eV of the corundum V 2 O 3 101 ¯ 4 Bragg backre- flection condition h  2286 eV; they were recorded with the photon beam incident to the 0001 surface at 51°. Photon energies were chosen to maximize the electric-field intensity on either the V or the O atomic planes. Partial V and O valence spectra were then computed by taking linear combinations of the spectra recorded within the width of the V 2 O 3 crystal x-ray rocking curve. Valence spectra were also recorded at photon energy 5 eV below the Bragg condi- PHYSICAL REVIEW B 76, 165101 2007 1098-0121/2007/7616/1651016 165101-1