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MnKβ x-ray emission spectra provide a direct method to probe the effective spin state and charge density on the Mn atom and is used in an experimental study of a class of Mn oxides. Specifically, the MnKβ line positions and detailed spectral shapes depend on the oxidation and the spin state of the Mn sites as well as the degree of d covalency/itinerancy. Theoretical calculations including atomic charge and multiplet effects, as well as crystal-field splittings and covalency effects, are used as a guide to the experimental results. Direct comparison of the ionic system MnF2 and the covalent system MnO reveals significant changes due to the degree of covalency of Mn within atomic-type MnKβ simulations. Moreover, comparisons of measurement with calculations support the assumed high spin state of Mn in all of the systems studied. The detailed shape and energy shift of the spectra for the perovskite compounds, LaMnO3 and CaMnO3, are, respectively, found to be very similar to the covalent Mn3+−Mn2O3 and Mn4+−MnO2 compounds thereby supporting the identical Mn-state assignments. Comparison to the theoretical modeling emphasizes the strong covalency in these materials. Detailed MnKβ x-ray emission results on the La1−xCaxMnO3 system can be well fit by linear superpositions of the end member spectra, consistent with a mixed-valent character for the intermediate compositions. However, an arrested Mn-valence response to the doping in the x0.3 range is found. No evidence for Mn2+ is observed at any x values seemingly ruling out proposals regarding Mn3+ disproportionation.

We report on a systematic analysis of x-ray photoelectron spectroscopy (XPS) core- and valence-level spectra of clean and well-characterized iron oxide films, i.e., α−Fe2O3, γ−Fe2O3, Fe3−δO4, and Fe3O4. All iron oxide films were prepared epitaxially by NO2-assisted molecular-beam epitaxy on single crystalline MgO(100) and α−Al2O3(0001) substrates. The phase and stoichiometry of the films were controlled precisely by adjusting the NO2 pressure during growth. The XPS spectrum of each oxide clearly showed satellite structures. These satellite structures were simulated using a cluster-model calculation, which could well reproduce the observed structures by considering the systematic changes in both the Fe 3d to O 2p hybridization and the d−d electron-correlation energy. The small difference in the satellite structures between α−Fe2O3 and γ−Fe2O3 resulted mainly from changes in the Fe-O hybridization parameters, suggesting an increased covalency in γ−Fe2O3 compared to α−Fe2O3. With increasing reduction in the γ−Fe2O3−Fe3O4 system, the satellite structures in XPS became unresolved. This was not only due to the formation of Fe2+ ions, but also to nonhomogeneous changes in the hybridization parameters between octahedral and tetrahedral Fe3+ ions.

State-of-the-art electronic-structure calculations based on the local-density approximation (LDA) to the density functional fail to reproduce the insulating antiferromagnetic ground state in the parent compounds of the high-temperature oxide superconductors. Similar problems have been observed earlier in classical transition-metal oxides such as FeO, CoO, and NiO. In this work we present the method which delivers the correct insulating antiferromagnetic ground state in the correlated oxides preserving other properties as well as the efficiency of the standard LDA. The method embeds the relevant (for a given system of electrons) part of the Hubbard Hamiltonian into the Kohn-Sham LDA equation. The resulting Hamiltonian attempts to fix two intrinsic problems of the LDA: the deficiency in forming localized (atomiclike) moments and the lack of discontinuity of the effective one-particle potential when going from occupied to unoccupied states. We present the detailed study of La2CuO4 and LaCuO3. In the case of La2CuO4 the energy gap and the value of the localized magnetic moment in the stable insulating antiferromagnetic solution are in good agreement with experiment. We compare our results with the standard local spin density approximation calculation and multiband Hubbard model calculations, as well as with results of spectroscopy: inverse photoemission, valence photoemission, and x-ray absorption at the K edge of oxygen. In the case of LaCuO3 such an extensive comparison is limited due to the limited data available for this compound. We discuss, however, the electric and magnetic properties and the insulator-metal-insulator transitions upon increase of oxygen deficiency.

We present band-structure and cluster-model calCulatiOns Of LaCoO3 in the low-spin phase. The purpose of these calculations is to contrast and complement the results and conclusions of recent spectroscopic studies. The total density of states (DOS) is compared to the photoemission spectrum; the agreement is very good except for the many-body satellites which appear at higher binding energies. The unoccupied O p DOS reproduces fairly well the O 1s x-ray-absorption spectrum; the main discrepancy appears in the Co 3d region and is attributed to core-hole effects. The ground state predicted by the cluster-model calculation is highly covalent and contains mainly 62% of t2g6 (1A1) and 36% of t2g6e(g)(2E)L. The first (one-electron) removal state has more 3d6L than 3d5 character whereas the first addition state is almost completely dominated by the 3d7 state. This means that low-spin LaCoO3 is in the charge-transfer regime and the optical band pp is of the p-d type. The Co 3d contribution to the photoemission spectrum calculated with the cluster-model reproduces not only the leading peaks but also the many-body satellites. The main drawback in this case is the absence of the spectral weight coming from the O 2p bands.

With the use of many-particle theory general equations are derived for the six fundamental spectra, which are special linear combinations of the spin-polarized photoemission spectra measured with left-, Z-, and right-circularly polarized radiation. These fundamental spectra, which are the isotropic spectrum, spin spectrum, magnetic circular dichroism, spin-orbit spectrum, anisotropic spectrum, and anisotropic spin spectrum give the correlation between the spin, orbital, and quadrupole momenta in the ground state and spin and orbit of the hole created after photoemission. For emission from an incompletely filled shell the integrated intensities of the fundamental spectra are proportional to the expectation values of the number of electrons, spin magnetic moment, orbital magnetic moment, the alignment between orbital and spin magnetic moment, the quadrupole moment, and the correlation between quadrupole and spin magnetic moment, respectively. This can be used to study the magnetic anisotropy of localized magnetic systems. We calculated the fundamental spectra for the 4f photoemission of the rare-earth ions and show that the multiplet structure displays a strong polarization dependence in agreement with our analysis in terms of correlations of moments and the rules for the integrated intensities. Finally we present the relation between the polarization effects in inverse photoemission and normal photoemission and we discuss the magnetic dichroism in metallic iron, cobalt, and nickel.

We present soft-x-ray absorption spectra (XAS) of LaCoO3 taken at different temperatures (80-630 K). The shape of the multiplets in the Co 2p XAS spectra conveys information on the symmetry and spin of the ground state. The 0 Is XAS spectra are related to unoccupied metal bands through covalent mixing. The changes in the spectra taken at different temperatures provide information on the spin-state transition in this compound. At low temperature, 80 and 300 K, the material is in a highly covalent low-spin state. The main contribution to the ground state in this case is given by t2g6 (1A1) with an occupancy of 0.56. At higher temperature, 550 and 630 K, the results indicate a gradual transition to a mixed-spin state. The main contribution to the high-spin part of the mixture is given by t2g4e(g)2(5T2) with an occupancy of 0.71. There is no evidence of charge disproportionation in the temperature range 80-630 K. The O 1s XAS spectra reflect important changes in the unoccupied Co 3d bands across the spin-state transition.

We predict very strong peak enhancements in core-level photoemission at the resonance with a deeper core-level absorption. This allows a straightforward interpretation of core-level spectroscopy in two ways: (a) photoemission peaks can be resolved in resonance with different x-ray absorption peaks, and (b) x-ray absorption peaks can be resolved using partial electron yield at constant binding energy. This can be used to study covalency effects in transition-metal, rare-earth, and actinide compounds. An example is given for nickel metal using a cluster calculation including multiplet structure.

The electronic structure of Li(x)Co(1-x)O (0.01 less-than-or-equal-to x less-than-or-equal-to 0.2), LiCoO2, and Co3O4(1% Li) has been investigated using x-ray photoemission spectroscopy (XPS), bremsstrahlung isochromat spectroscopy (BIS), and x-ray-absorption spectroscopy. The experimental results are compared with model cluster calculations. We find that the CoO band gap is of an intermediate character, between Mott-Hubbard-like and charge-transfer-like. The first ionization state of CoO is therefore of strongly mixed Co 3d and O 2p character. Its local symmetry corresponds to 3T1g, similar to an intermediate-spin Co3+ state. For x less-than-or-equal-to 0.2 the local Co electronic structure is similar to that of CoO. However, LiCoO2 has a strongly reduced Co-O interatomic distance, resulting in a ligand field strong enough to stabilize a Co3+ low-spin ground state. LiCoO2 is an insulator with a gap of 2.7 eV. From a comparison of the XPS and BIS CoO spectra to the cluster calculations, we find values for U (= 5.3 eV), DELTA (= 5.5 eV), and (pd-sigma) (= 1.3 eV).

With the aim of disentangling the role of the constituent blocks in the high-${T}_{c}$ cuprate superconductors we have measured x-ray absorption spectra at the $\text{Cu}\text{ }{L}_{3}$ edge on $({\text{Cu}}_{1\ensuremath{-}\ensuremath{\delta}}{\text{C}}_{\ensuremath{\delta}}){\text{Ba}}_{2}\phantom{\rule{0.1em}{0ex}}\text{Cu}{\text{O}}_{x}$ (BCO), ${\text{CaCuO}}_{2}$ (CCO), and BCO/CCO superconducting superlattices obtained by stacking in a sequence the two blocks. By observing the polarization dependence we could discriminate between the in-plane and out-of-plane hole densities. The results strongly indicate that charge is redistributed across the interfaces from the BCO charge reservoir block to the CCO infinite layer block. In the superconducting samples a relatively high density of out-of-plane holes is still present, whereas when the CCO layer thickness is excessive the holes appear to have preferential in-plane orientation and superconductivity is suppressed.

We have used resonant soft x-ray scattering to study the effects of discommensuration on the hole Wigner crystal (HC) in the spin ladder Sr14-xCaxCu24O41 (SCCO). As the hole density is varied the HC forms only with the commensurate wave vectors L-L=1/5 and L-L=1/3; for incommensurate values it "melts." A simple scaling between L-L and temperature is observed, tau(1/3)/tau(1/5)=5/3, indicating an inverse relationship between the interaction strength and wavelength. Our results suggest that SCCO contains hole pairs that are crystallized through an interplay between lattice commensuration and Coulomb repulsion, reminiscent of the "pair density wave" scenario.

We have succeeded in growing epitaxial films of rocksalt VOx on MgO(001) substrates. The oxygen content as a function of oxygen flux was determined using O-18(2)-Rutherford backscattering spectrometry and the vanadium valence using x-ray-absorption spectroscopy. The upper and lower stoichiometry limits found are similar to those known for bulk material (0.8

Ultrafast electron delocalization induced by a femtosecond laser pulse is a well-known process in which electrons are ejected from the ions within the laser pulse duration. However, very little is known about the speed of electron localization out of an electron gas in correlated metals, i.e., the capture of an electron by an ion. Here, we demonstrate by means of pump-probe x-ray techniques across the Eu $L_3$ absorption edge that an electron localization process in the EuNi$_2$(Si$_{0.21}$Ge$_{0.79}$)$_2$ intermetallic material occurs within a few hundred femtoseconds after the optical excitation. Spectroscopy and diffraction data collected simultaneously at low temperature and for various laser fluences show that the localization dynamics process is much faster than the thermal expansion of the unit cell along the c direction which occurs within picoseconds. Nevertheless, this latter process is still much slower than pure electronic effects, such as screening, and the subpicosecond timescale indicates an optical phonon driven origin. In addition, comparing the laser fluence dependence of the electronic response with that found in other intermediate 4f valence materials, we suggest that the electron localization process observed in this Eu-based correlated metal is mainly related to changes in the 4f hybridization. The observed ultrafast electron localization process sparks fundamental questions for our understanding of electron correlations and their coupling to the lattice.