Iterative partial equalization of orbital electronegativity – a rapid access to atomic charges

The last decade witnessed significant progress in angle-resolved photoemission spectroscopy (ARPES) and its applications. Today, ARPES experiments with 2-meV energy resolution and $0.2\ifmmode^\circ\else\textdegree\fi{}$ angular resolution are a reality even for photoemission on solids. These technological advances and the improved sample quality have enabled ARPES to emerge as a leading tool in the investigation of the high-${T}_{c}$ superconductors. This paper reviews the most recent ARPES results on the cuprate superconductors and their insulating parent and sister compounds, with the purpose of providing an updated summary of the extensive literature. The low-energy excitations are discussed with emphasis on some of the most relevant issues, such as the Fermi surface and remnant Fermi surface, the superconducting gap, the pseudogap and $d$-wave-like dispersion, evidence of electronic inhomogeneity and nanoscale phase separation, the emergence of coherent quasiparticles through the superconducting transition, and many-body effects in the one-particle spectral function due to the interaction of the charge with magnetic and/or lattice degrees of freedom. Given the dynamic nature of the field, we chose to focus mainly on reviewing the experimental data, as on the experimental side a general consensus has been reached, whereas interpretations and related theoretical models can vary significantly. The first part of the paper introduces photoemission spectroscopy in the context of strongly interacting systems, along with an update on the state-of-the-art instrumentation. The second part provides an overview of the scientific issues relevant to the investigation of the low-energy electronic structure by ARPES. The rest of the paper is devoted to the experimental results from the cuprates, and the discussion is organized along conceptual lines: normal-state electronic structure, interlayer interaction, superconducting gap, coherent superconducting peak, pseudogap, electron self-energy, and collective modes. Within each topic, ARPES data from the various copper oxides are presented.

https://arxiv.org/pdf/cond-mat/0208504.pdf

Angle-resolved photoemission studies of the cuprate superconductors

Ferrous (Fe2+) and ferric (Fe3+) compounds were investigated by XPS to determine the usefulness of calculated multiplet peaks to fit high-resolution iron 2p3/2 spectra from high-spin compounds. The multiplets were found to fit most spectra well, particularly when contributions attributed to surface peaks and shake-up satellites were included. This information was useful for fitting of the complex Fe 2p3/2 spectra for Fe3O4 where both Fe2+ and Fe3+ species are present. It was found that as the ionic bond character of the iron —ligand bond increased, the binding energy associated with either the ferrous or ferric 2p3/2 photoelectron peak also increased. This was determined to be due to the decrease in shielding of the iron cation by the more increasingly electronegative ligands. It was also observed that the difference in energy between a high-spin iron 2p3/2 peak and its corresponding shake-up satellite peak increased as the electronegativity of the ligand increased. The extrinsic loss spectra for ion oxides are also reported; these are as characteristic of each species as are the photoelectron peaks. Copyright © 2004 John Wiley & Sons, Ltd.

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Investigation of multiplet splitting of Fe 2p XPS spectra and bonding in iron compounds

Before we start, we need a working definition for MOPAC. The following description has been used many times to describe MOPAC: MOPAC is a general-purpose, semiempirical molecular orbital program for the study of chemical reactions involving molecules, ions, and linear polymers. It implements the semiempirical Hamiltonians MNDO, AM 1, MINDO/3, and MNDOPM3, and combir_es the calculations of vibrational spectra, thermodynamic quantities, isotopic substitution effects, and force constants in a fully integrated program. Elements parameterized at the MNDO level include H, Li, Be, B, C, N, O, F, A1, Si, P, S, C1, Ge, Br, Sn, Hg, Pb, and I; at the PM3 level the elements H, C, N, O, F, A1, Si, P, S, C1, Br, and I are available. Within the electronic part of the calculation, molecular and localized orbitals, excited states up to sextets, chemical bond indices, charges, etc. are computed. Both intrinsic and dynamic reaction coordinates can be calculated. A transition-state location routine and two transition-state optimizing routines are available for studying chemical reactions.

MOPAC: A semiempirical molecular orbital program

Advances in the development of novel cobalt Fischer-Tropsch catalysts for synthesis of long-chain hydrocarbons and clean fuels.

High-resolution gold-valence-band photoemission spectra were obtained by the use of monochromatized $\mathrm{Al} K\ensuremath{\alpha}$ radiation and a single-crystal specimen. After background and scattering corrections were made, the results were compared directly with broadened theoretical density-of-states functions. The following conclusions were drawn: (i) Relativistic band-structure calculations are required to fit the spectrum. (ii) Both the Korringa-Kohn-Rostoker calculation of Connolly and Johnson and the relativistic-augmented-plane-wave calculation by Christensen and Seraphin give density-of-states results that (after broadening) follow the experimental curve closely. (iii) Of the theoretical functions available to date, those with full Slater exchange agree best with experiment (perhaps because of a cancellation of errors). Fractional ($\frac{2}{3} or \frac{5}{6}$) exchange gives $d$ bands that are too wide. (iv) Eastman's 40.8-eV ultraviolet photoemission spectrum is similar to the x-ray spectrum, suggesting little dependence on photon energy above 40 eV. (v) Both (ii) and (iv) imply an absence of strong matrix-element modulation in the photoemission spectrum of gold.

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High-Resolution X-Ray Photoemission Spectrum of the Valence Bands of Gold

Polycrystalline samples of iron‐containing ferrichrome A, a cyclic hexapeptide obtained from the fungus Ustillago sphaerogena, have been investigated by paramagnetic resonance. Spectra were obtained at several temperatures between 300° and 1°K; a prominent line of 400‐Oe width located at g=4.3 was observed at all temperatures, while at 1°K additional resonances at g values of 9.6, 1.3, and 1.0 were observed. The spectra are interpreted by assuming a spin Hamiltonian containing crystal‐field terms large compared with the Zeeman splittings; the crystal‐field situation is intermediate between the case of axial symmetry, with H=D[Sz2−(1/3)S(S+1)]+gβS·H and a model proposed by Castner, Newell, Holton, and Slichter to explain certain iron resonances occurring at g=4.3, with H=E(Sx2−Sy2)+gβS·H. We have computed g values, energy eigenvalues, and eigenfunctions to be expected for the region between these two extremes, and the results should be useful in interpreting similar spectra due to iron situated in strong c...

Paramagnetic Resonance of Fe3+ in Polycrystalline Ferrichrome A

The electronic structure of SrTiO3 and some simple related oxides (MgO, Al2O3, SrO, TiO2)

The M\"ossbauer effect of ${\mathrm{Fe}}^{57}$ in the metalloprotein ferrichrome $A$ (FA) has been investigated at temperatures between 0.98 and 300\ifmmode^\circ\else\textdegree\fi{}K; in some cases the absorber was placed in an external field of 18 kOe, oriented perpendicular to the propagation direction. The polycrystalline paramagnetic sample exhibited temperature-, field-, and concentration-dependent magnetic hyperfine structure. At 0.98\ifmmode^\circ\else\textdegree\fi{}K in zero field, a well-resolved six-line spectrum was observed. As the temperature was raised, the pattern coalesced until at 300\ifmmode^\circ\else\textdegree\fi{}K only a single broad non-Lorentzian line was visible. The effect of an applied field was, at all temperatures, to noticeably sharpen the hfs pattern; and at 77\ifmmode^\circ\else\textdegree\fi{}K or below eight lines were easily resolved. Changes of the spectra with temperature and field are attributed to electronic relaxation, and a model is used to calculate relaxation spectra. The model employs the modified Bloch equations as rate equations to describe fluctuations of an effective hyperfine field. The iron-iron separation was increased by dilution of some of the samples in ethyl alcohol. As is well known from ESR work, spin-spin interactions decrease on dilution: a sharpening of the spectrum resulted. The importance of using solutions in studying M\"ossbauer spectra of biologically interesting ${\mathrm{Fe}}^{3+}$ compounds is demonstrated. Some of the consequences of paramagnetic hyperfine interactions for M\"ossbauer spectroscopy are discussed. Differences between the effective hyperfine field and the interaction $A\mathbf{I}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{S}$ are noted, as are certain analogies between M\"ossbauer and optical spectroscopy, under the $A\mathbf{I}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{S}$ interaction. From previous ESR work the electronic spin Hamiltonian for FA was found to be of the form $D[{{S}_{z}}^{2}\ensuremath{-}\frac{1}{3} S(S+1)]+E[{{S}_{x}}^{2}\ensuremath{-}{{S}_{y}}^{2}]$. Rather complex hyperfine spectra are predicted for ${\mathrm{Fe}}^{3+}$ with this spin Hamiltonian, and representative spectra are illustrated. It is shown that these spectra, expected in the absence of spin-spin interaction, would be sufficient to determine the ratio $\frac{E}{D}$ of the spin-Hamiltonian parameters; this illustrates the complementary nature of ESR and M\"ossbauer spectra of the same system.

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Paramagnetic Hyperfine Structure and Relaxation Effects in Mössbauer Spectra: Fe 57 in Ferrichrome A

https://escholarship.org/content/qt9944610k/qt9944610k.pdf?t=p21o67

D. A. Shirley

Papers

High-Resolution X-Ray Photoemission Spectrum of the Valence Bands of Gold

Paramagnetic Resonance of Fe3+ in Polycrystalline Ferrichrome A

The electronic structure of SrTiO3 and some simple related oxides (MgO, Al2O3, SrO, TiO2)

Paramagnetic Hyperfine Structure and Relaxation Effects in Mössbauer Spectra: Fe 57 in Ferrichrome A

The effect of atomic and extra-atomic relaxation on atomic binding energies☆