Core-hole Effects Research Articles

Oxygen x-ray emission and absorption spectra as a probe of the electronic structure of strongly correlated oxides

We present synchrotron-excited oxygen x-ray $K$-emission spectroscopy ($\text{O}\text{ }K\ensuremath{\alpha}$ XES) and oxygen x-ray absorption spectroscopy ($\text{O}\text{ }1s$ XAS) spectra of transition-metal (TM) oxides MnO, CoO, and NiO. The comparison of oxygen $K$-emission and absorption spectra to valence band photoemission and bremsstrahlung isochromat spectra measurements shows that $\text{O}\text{ }1s$ XAS is not strongly influenced by the core hole effect, whereas the $\text{TM}\text{ }2p$ XAS significantly shifts to a lower energy. New and effective methods for determining the band gap and anion-to-cation cation charge-transfer energies of the oxides from the measured spectra are presented and applied, and the combination of O XAS and XES is shown to agree well with the results of numerical electronic structure methods applied to strongly correlated oxides. For MnO, the charge-transfer energy is found to be 6.6 eV and the band gap is 4.1 eV; for CoO, the values are 6.1 and 2.6 eV and for NiO, the values are 5.4 and 4.0 eV.

Effect of the core hole potential on the K-edge absorption and emission spectra of silicon and oxygen atoms in a quartz crystal

The K-edge absorption and emission spectra of silicon and oxygen atoms in a quartz crystal have been calculated by the full multiple scattering method. Clusters containing more than 100 atoms and the model muffin-tin potential were used. Good agreement with the experimental data was obtained. The effect of a core hole on the absorption spectrum was investigated.

Theory of core-hole effects in1score-level spectroscopy of the first-row elements

The 1s core-level excited spectra in LiF, BeO, cubic BN, CaB6, MgB2, SiC, diamond, and C3N4 were calculated using an ab initio pseudopotential plane wave method and a projector augmented wave reconstruction. Core-hole effects were investigated through a detailed examination of spectral differences between theoretical results from standard ground state calculations and from supercell calculations that included the core hole. A quantitative analysis reveals a relationship between core-hole strength and the valence charge population.

Large differences in the electronic structure and spectroscopic properties of three phases ofAlPO4fromab initiocalculations

The electronic structure, interatomic bonding, and spectroscopic properties of the three known phases of $\mathrm{Al}\mathrm{P}{\mathrm{O}}_{4}$ in trigonal, orthogonal, and monoclinic structures formed at different pressures were studied using the density functional method. These three crystals have the same chemical formula and the same percentages of different atomic species but distinct local atomic coordination which offers a unique opportunity to systematically investigate their structure-properties relationship. The $\mathrm{Al}\text{\ensuremath{-}}K$, $\mathrm{Al}\text{\ensuremath{-}}{L}_{3}$, $\mathrm{P}\text{\ensuremath{-}}K$, $\mathrm{P}\text{\ensuremath{-}}{L}_{3}$, and $\mathrm{O}\text{\ensuremath{-}}K$ x-ray absorption near edge structure (XANES) edges were calculated using a supercell approach including the effects of core hole. The large differences in these properties among the three crystals are demonstrated and explained in terms of the unique structural units present in each phase. These results are in contradiction to the prevailing notion of using fingerprinting for XANES spectra interpretation.

Ab initiocalculation of the O-K, N-K, Si-K,Si−L3, Y-K,Y−L3edges in the Y-Si-O-N system: A strategy for ELNES/XANES spectral modeling in complex materials

The electron energy-loss near-edge structure (ELNES) or the x-ray absorption near-edge structure (XANES) spectra of the anion (O, N) and cation (Si,Y) (O-K, N-K, Si-K, $\mathrm{Si}\text{\ensuremath{-}}{\mathrm{L}}_{3}$, Y-K, and $\mathrm{Y}\text{\ensuremath{-}}{\mathrm{L}}_{3}$) edges in 12 crystals of the Y-Si-O-N system have been calculated using an ab initio supercell method based on density functional theory in its local approximation. The collection consists of six binary ($\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Si}{\mathrm{O}}_{2}$, stishovite $\mathrm{Si}{\mathrm{O}}_{2}$, $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Si}}_{3}{\mathrm{N}}_{4}$, $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Si}}_{3}{\mathrm{N}}_{4}$, $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Si}}_{3}{\mathrm{N}}_{4}$, and ${\mathrm{Y}}_{2}{\mathrm{O}}_{3}$), three ternary (${\mathrm{Si}}_{2}{\mathrm{N}}_{2}\mathrm{O}$, ${\mathrm{Y}}_{2}{\mathrm{Si}}_{2}{\mathrm{O}}_{7}$, and ${\mathrm{Y}}_{2}\mathrm{Si}{\mathrm{O}}_{5}$), and three quaternary (${\mathrm{Y}}_{2}{\mathrm{Si}}_{3}{\mathrm{N}}_{4}{\mathrm{O}}_{3}$, ${\mathrm{Y}}_{4}{\mathrm{Si}}_{2}{\mathrm{O}}_{7}{\mathrm{N}}_{2}$, and ${\mathrm{Y}}_{3}{\mathrm{Si}}_{5}{\mathrm{N}}_{9}\mathrm{O}$) crystals. The calculations took into account the core-hole effect and included the explicit evaluation of the transition matrix elements. Based on the extensive data calculated, attempts were made to correlate the spectral features in the calculated spectra with the local structural environments of the atoms in the crystal. It is shown that with the exception of the fourfold tetrahedrally coordinated Si ion, there is no clear evidence of trends in the ELNES/XANES spectra that can be established to justify a so-called finger printing capability. In crystals with well-defined $\mathrm{Si}\text{\ensuremath{-}}{\mathrm{O}}_{4}$ units, very different Si-K edges were obtained. For the O-K and N-K edges, there are numerous counterexamples of such a correlation that can be traced to effects beyond the nearest neighbor coordination. In the Y-K and $\mathrm{Y}\text{\ensuremath{-}}{\mathrm{L}}_{3}$ edges, no meaningful correlations can be established because of the longer bond lengths and more ionic nature of the bonding. For simpler crystals where nonequivalent sites of a particular ion exist, the weighted sum of the spectra from individual sites should be used for comparison with experiment. These results indicate that for proper interpretation of the experimentally measured spectra in complex ceramics, or for ceramic materials containing microstructures, calculated spectra based on simpler crystals with similar coordination should not be used. It is argued that the proper way to understand these spectra would be to actually model the structure and then carry out the full spectral calculations on the constructed models.

Momentum-Specified OxygenKResonant Inelastic X-ray Scattering for Cuprates

The present study theoretically shows that oxygen K resonant inelastic X-ray scattering (O K RIXS) in cuprates gives momentum-resolved information on the occupied and unoccupied electronic states. By tuning the incident X-ray energy carefully, we can select the k point where the X-ray absorption process as well as the subsequent X-ray emission process actually takes place. The requirement for the momentum specification is that the core-hole potential effect in the intermediate state of the RIXS should be negligible. This requirement is fulfilled only for the O K RIXS in the case of cuprates.

Combined ab-initio and N-K, Ti-L2,3, V-L2,3 electron energy-loss near edge structure studies for TiN and VN films

AbstractIn this work we examined the electronic structure of well-defined TiN/VN bilayers grown on a MgO(100) substrate by both experimentation and calculation. The electron energy-loss spectra were taken in transmission electron microscopes equipped with and without a monochromator. The experimental studies reveal nearly stoichiometric phases with atomic ratios Ti/N of 1.1 ± 0.1 and V/N of 1.2 ± 0.1. The comparison of the electron energy-loss near-edge structures of the N-K, Ti-L2,3, and V-L2,3 spectra with the theoretically calculated partial density of states using density functional theory revealed that core hole effects must be included. The calculations indicate that the N-K edge is sensitive to vacancies in the sublattice of VN. Thus, the N-K edge may be used as a fingerprint when studying non-stoichiometric or diffusional phenomena.

Observation of long-range compositional fluctuations in glasses: Implications for atomic and electronic structure

Experimental evidence for long-range compositional fluctuations in glasses is given. The implications for electronic structure and stoichiometry-induced structural variations are analyzed. These fluctuations were discovered by examining the spatial dependence of inner shell near-edge absorption spectra obtained using a 50 nm diameter probe. This spectroscopy is sensitive to both angular and distance correlations in bonding. Comparisons with spectra from compositionally equivalent crystals, and multiple-scattering calculations which include core-hole effects are used to analyze the data.

OxygenK-edge XANES of germanates investigated using first-principles calculations

O K-edge x-ray absorption near-edge structure (XANES) spectra of {alpha}-quartz-type and rutile-type GeO{sub 2} polymorphs and of K{sub 2}Ge{sub 8}O{sub 17} have been analyzed using first-principles plane-wave pseudopotential calculations. XANES spectra have been calculated using supercell including core-hole effects and good agreement with experiment has been obtained. In the the case of GeO{sub 2} polymorphs, local density of empty states has been performed and peaks in the experimental spectra can be assigned to transitions involving hybridization of the O p orbitals with the Ge s, Ge p, Ge sp, and Ge d orbitals. Furthermore, peak positions in the theoretical spectra appear to be correlated with changes in the Ge-O-Ge angle as well as indirectly with the Ge coordination geometry. Analysis of O K-edge XANES spectra for individual O sites in K{sub 2}Ge{sub 8}O{sub 17} shows that oxygens shared between two fivefold Ge atoms or one fourfold and one fivefold Ge atom exhibit subtle shifts to lower energy of the peaks, which have been previously observed in alkali germanate glasses at and above the germanate anomaly.

Investigation of hole states near the Fermi level in Nb 1−xMg xB 2 by electron energy-loss spectroscopy and first-principles calculations

The fine structures of the electron energy-loss spectra (EELS) for the B-K edge have been examined in NbB 2 and superconducting Nb 0.75Mg 0.25B 2. The experimental results are analyzed based on the calculations of density functional theory (DFT) using the Wien2k code. The results of the EELS spectra and the angular decomposition of the density of states (DOS) reveal that both the B p z and B p x +p y states in NbB 2 have large weights at the Fermi energy due to intersheet covalent bonding with notable hybridization between the Nb 4d and B 2p states. This kind of hybridization also results in different core-hole behaviors for the B-K edge in two orthogonal crystallographic orientations. The best fit between experimental and theoretical data is achieved with consideration of the core-hole effect of the B 1 s states, in particular for the q⊥ c spectra. Analysis of the electronic structure of the Nb 1− x Mg x B 2 superconductors suggests that confinement of the intersheet covalent bonding is likely to be favorable for the improvement of superconductivity in this kind of materials.

Prediction of Strong Dichroism Induced by X Rays Carrying Orbital Momentum

We predict the presence of strong dichroic effects induced by x-ray beams carrying orbital angular momentum (OAM). Taking the difference between spectra obtained with positive and negative OAM states allows the separation of quadrupolar from dipolar transitions at, e.g., the transition-metal K edges, enabling the study of the unoccupied states in the absence of strong core-hole effects. We study the dependence of OAM-induced x-ray dichroism on different polarization vectors and derive sum rules relating the integrated intensity to ground-state hole densities. Calculations of spectral line shapes for cuprates, manganites, and ruthenates confirm the strong OAM-induced dichroism and indicate the potential of this new spectroscopy in the fields of orbital physics and magnetism.

The effect of hydrogen on the electronic and bonding properties of amorphouscarbon

A series of amorphous carbon and hydrogen-containing amorphous carbon structures hasbeen generated, by using classical Monte Carlo and molecular dynamics simulationtechniques, respectively. The Brenner empirical bond order potential is used to generatehydrogenated amorphous carbon while that by Tersoff is used for amorphous carbon. Theresulting structures are relaxed further using the density functional theory approach.Structures containing 15 at.% hydrogen are generated for various mass densitiesin order to investigate the effect of pressure changes on the properties of thehydrogenated amorphous carbon. The structures are analysed in terms of theirsp3/sp2 ratio. The density of states (DOS), energy-loss near-edge structure (ELNES)and x-ray photoelectron spectroscopy (XPS) calculations are performedwithin the first-principles methodology on the generated carbon systems. TheELNES and the C 1s energy calculations are performed taking into accountthe core–hole effect. Our calculations show that depending on the densityρ,hydrogenated amorphous carbon can be classified into three categories: polymeric at low densities(ρ≤2.0 g cm−3), graphitic at intermediatedensities (2.0 g cm−3<ρ≤2.4 g cm−3) and diamond-likeat high densities (ρ>2.4 g cm−3).

Theory and calculations of X-ray spectra: XAS, XES, XRS, and NRIXS

There has been dramatic progress in recent years both in calculations and in the interpretation of various X-ray spectroscopies, ranging from X-ray absorption and emission to elastic and non-resonant inelastic X-ray scattering. Here we review a number of recent developments focusing primarily on the ab initio real-space Green's function (RSGF) approach, which is applicable to arbitrary aperiodic materials. The RSGF approach is essentially a quasi-particle approximation which takes into account final state effects including many-body, core-hole effects, lifetime, and Debye–Waller factors. While generally adequate for deep core spectra, corrections due to local screening fields can be important, even far above an absorption edge. Such corrections require a theory beyond the independent-electron approximation, and can be calculated using a combination of the time-dependent density functional theory (TDDFT) and the Bethe–Salpeter equation. The combined approach makes possible efficient calculations of optical constants from the UV to X-ray energies.

Theoretical determination of the surface excitation parameter from X‐ray photoelectron spectroscopy

AbstractThe surface excitation parameter (SEP) is determined by analysis of X‐ray photoelectron spectroscopy (XPS) model developed by Simonsen, Yubero and Tougaard (Phys. Rev. B. 1997; 56: 1612). The model includes excitation due to the static core hole as well as the effect of the surface. The only input in the model is the dielectric function of the medium. More precisely, the SEP is extracted from the theoretical spectrum by calculating the ratio between the surface loss part of the spectrum and the elastic peak intensity. The angular distributions of the SEP are calculated for aluminium and silicon for photoelectrons of 1 and 3.4 keV energy. The SEP for XPS is found to be slightly (5–10%) smaller compared to that for reflection‐electron‐energy‐loss spectroscopy (REELS) calculated within the same model, and this difference is ascribed to the effect of the static core hole. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Electron spectroscopy of carbon materials: experiment and theory

We present a comparative spectroscopic study of carbon as graphite, diamond and C60 using C1s K-edge electron energy-loss spectroscopy (EELS), X-ray emission spectroscopy, and theoretical modelling. The first principles calculations of these spectra are obtained in the local density approximation using a self-consistent Gaussian basis pseudo-potential method. Calculated spectra show excellent agreement with experiment and are able to discriminate not only between various carbon hybridisations but also local variation in environment. Core-hole effects on the calculated spectra are also investigated. For the first time, the EEL spectrum of carbyne is calculated.

Report on the 42nd IUVSTA workshop ‘Electron scattering in solids: from fundamental concepts to practical applications’

A summary is given of the workshop entitled ‘Electron Scattering in Solids: from fundamental concepts to practical applications,’ which was held in Debrecen, Hungary, on July 4–8, 2004, under the sponsorship of the International Union of Vacuum Science, Technique, and Applications (IUVSTA). This workshop was held to review the present status and level of understanding of electron-scattering processes in solids, to identify issues of key importance (hot topics) in the light of the most recent scientific results, and to stimulate discussions leading to a deeper understanding and new solutions of current problems. This report contains summaries of presentations and discussions in sessions on elastic scattering of electrons by atoms and solids, inelastic scattering of electrons in solids, modeling of electron transport in solids and applications, and software. The principal areas of application include Auger-electron spectroscopy (AES), X-ray photoelectron spectroscopy, elastic-peak electron spectroscopy (EPES), reflection electron energy-loss spectroscopy (REELS), secondary-electron microscopy, electron-probe microanalysis (EPMA), and the use of coincidence techniques in electron-scattering experiments. A major focus of the workshop was determination of the inelastic mean free path of electrons for various surface spectroscopies, particularly corrections for surface and core-hole effects. Published in 2005 by John Wiley & Sons, Ltd.

Core-hole effect on dipolar and quadrupolar transitions ofSrTiO3andBaTiO3at TiKedge

Experimental pre-edge fine structures in the x-ray absorption spectra of $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ and $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ at Ti $K$ edge are successfully reproduced within the framework of first-principles one-electron theory using the full-potential augmented plane wave plus local orbitals method. The importance of the following two points has been demonstrated: (1) proper inclusion of a Ti $1s$ core-hole using a large supercell, and (2) consideration of both dipolar and quadrupolar transitions. The core-hole effect is found to be more significant for quadrupolar transitions that form the first two peaks of the pre-edge structure, i.e., $p1$ to $p2$. They correspond to ${t}_{2g}$ and ${e}_{g}$ bands of the core-holed Ti. The other two peaks, $p3$ and $p4$, are found to be entirely due to the dipolar transition.

Theoretical electron energy-loss spectroscopy and its application in materials research

Electron energy-loss near-edge fine structure (ELNES) of group-III nitrides is calculated using a pseudopotential plan wave method within the framework of density functional theory. Core-hole effect and supercell size influence are investigated. Based on our present and earlier work, a comprehensive understanding of theoretical ELNES application in materials research is demonstrated: interpreting experimental spectra, predicating theoretical reference spectra when reliable experimental spectra are not available, identifying ELNES-structure correlation and estimating the reliability of experimental spectra.

Bethe–Salpeter treatment of X-ray absorption including core-hole multiplet effects

We present a method to include multiplet effects and band structure effects simultaneously in an X-ray absorption spectrum. Theoretical results are presented and compared to experiment for the Ca 2p edge in calcium fluoride and the Ti 2p edge in rutile and strontium titanate.

Non-quasiparticle states in the core level spectra of ferromagnetic semiconductors and half-metallic ferromagnets

The Green's functions that determine x-ray spectra are calculated in the s-d exchange model of a saturated conducting ferromagnet in the presence of the core hole. A possibility to observe non-quasiparticle (NQP) states in the core level (x-ray absorption, emission and photoelectron) spectroscopy is demonstrated. It is shown that NQP contributions to resonant x-ray scattering spectra can be considerably enhanced by core hole effects in comparison with those to the density of states.