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We have performed valence-band resonant photoemission spectroscopy across the transition-metal (TM) ${L}_{3}$ edge of ${\mathrm{Rh}}_{2}{\mathrm{O}}_{3}, {\mathrm{RuO}}_{2}$, and ${\mathrm{MoO}}_{2}$. This technique allows us to access important information on the partial contribution of each respective TM. We show that an extended cluster model, coupled with transitions between discrete and continua states, describes well the overall resonance behavior as well as the experimental constant initial-state (CIS) spectra. Moreover, we analyze the resonant enhancement of the main lines and satellite structures to discuss the influence of the different charge regimes, the TM--O $2p$ hybridization, and additional charge screenings to the CIS signal. Finally, we argue that this approach can be extended to the study of other transition-metal oxides.

We study the electronic structure of the ${\mathrm{Sr}}_{2}{\mathrm{YRuO}}_{6}$ and ${\mathrm{Sr}}_{2}{\mathrm{YRu}}_{0.75}{\mathrm{Ir}}_{0.25}{\mathrm{O}}_{6}$ compounds using x-ray (resonant) photoemission and absorption spectroscopies. The experimental results are interpreted with first-principles calculations, which give a good agreement with all the spectra. These results show that, although the spin-orbit coupling does not induce orbital anisotropies in these systems, it is responsible for the reduction of magnetic moments within the the ${\mathrm{Ir}}^{5+}{\text{O}}_{6}$ octahedra, weakening the magnetic ordering in the substituted system. Finally, our findings support the idea that the canting of ${\mathrm{Ru}}^{5+}$ magnetic moments actually plays an important role in the stabilization of the intriguing magnetic ordering in the ${\mathrm{Sr}}_{2}{\mathrm{YRuO}}_{6}$ compound.

We report on the study of magnetic properties of the ${\mathrm{La}}_{1.5}{\mathrm{Ca}}_{0.5}{\mathrm{CoIrO}}_{6}$ double perovskite. Via ac magnetic susceptibility we have observed evidence of weak ferromagnetism and reentrant spin glass behavior on an antiferromagnetic matrix. Regarding the magnetic behavior as a function of temperature, we have found that the material displays up to three inversions of its magnetization, depending on the appropriate choice of the applied magnetic field. At low temperature, the material exhibits exchange bias effect when it is cooled in the presence of a magnetic field. Also, our results indicate that this effect may be observed even when the system is cooled at zero field. Supported by other measurements and also by electronic structure calculations, we discuss the magnetic reversals and spontaneous exchange bias effect in terms of magnetic phase separation and magnetic frustration of ${\mathrm{Ir}}^{4+}$ ions located between the antiferromagnetically coupled Co ions.

We studied the electronic structure of ${\mathrm{CaMnO}}_{x}$ $(2.66l~xl~3.00)$ using x-ray photoemission and O $1s$ x-ray-absorption spectroscopy. Analyzing the spectra of the two end members with the configuration-interaction cluster model, we have determined all the main parameters of the electronic structure. We have found that the ground states of ${\mathrm{CaMnO}}_{2.5}$ and ${\mathrm{CaMnO}}_{3}$ are highly covalent, with approximately the same number of ligand holes per oxygen atom: 0.22--0.23. The main separations between the Mn $3d$ bands closest to the Fermi level are of the order of 3 eV and the band gaps are of the charge-transfer type $(Ug\ensuremath{\Delta})$. The main effects of the oxygenation are the disappearance of the occupied ${e}_{g\ensuremath{\uparrow}}$ band and an approximately rigid shift of about 1 eV to lower energies of all the other Mn $3d$ bands. Finally, we discuss the most probable causes for the absence of a metallic phase in ${\mathrm{CaMnO}}_{x}.$

We have measured photoemission and oxygen $1s$ x-ray absorption spectra of the ferromagnetic metal ${\mathrm{SrRuO}}_{3}$ and compared them with a first-principles band-structure calculation. The overall distribution of Ru $4d$ and O $2p$ spectral weight is in good agreement with that predicted by the band-structure calculation. However, the observed spectral line shape of the Ru $4d$ band is spread over a wide energy range and the emission intensity at the Fermi level is weakened compared to the band-structure calculation. This implies the importance of electron correlation in the Ru oxide.

We have studied the effects of hole doping on the electronic structure of La${}_{1\ensuremath{-}x}$Sr${}_{x}$CoO${}_{3}$ by photoemission and x-ray-absorption spectroscopy. The Co 2$p$ core-level and the valence-band spectra show charge-transfer satellites, which have been more obviously observed in 3$p$-3$d$ resonant-photoemission spectra. By Sr substitution for La in LaCoO${}_{3}$, the valence-band spectra do not show rigid-band behavior but change systematically and reflect the semiconductor-to-metal transition which occurs with hole doping. Only small changes with $x$ have been observed in the resonant-photoemission spectra. Combined with configuration-interaction cluster-model calculations, this observation suggests that the intermediate-spin state is realized in the ferromagnetic phase.

We have studied the electronic structure of ${\mathrm{LaCoO}}_{3}$ by photoemission spectroscopy and x-ray absorption spectroscopy (XAS). The Co 2p core-level and valence-band photoemission spectra display satellite structures indicating a strong electron-correlation effect. The Co 2p core-level photoemission, the valence-band photoemission, and the O 1s XAS spectra have been analyzed using a configuration-interaction cluster model for the initial-state configurations of the low-spin (LS: $^{1}\mathrm{A}_{1}$), intermediate-spin (IS: $^{3}\mathrm{T}_{1}$) and high-spin (HS: $^{5}\mathrm{T}_{2}$) states and their mixtures. The ground state of ${\mathrm{LaCoO}}_{3}$ in the LS state is found to have heavily mixed ${\mathrm{d}}^{6}$ and ${\mathrm{d}}^{7}$L- character, reflecting the strong covalency. The magnetic susceptibility has been analyzed for various level orderings of the LS, IS, and HS states. From the analyses of the photoemission spectra and the magnetic susceptibility data, the temperature-induced paramagnetism in ${\mathrm{LaCoO}}_{3}$ above \ensuremath{\sim}90 K is most likely due to a gradual LS-to-IS transition.

The electronic structure of ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{MnO}}_{3}$ has been studied by photoemission and O 1s x-ray-absorption spectroscopy. Spectra of the Mn 2p core levels and the valence bands for ${\mathrm{LaMnO}}_{3}$ and ${\mathrm{SrMnO}}_{3}$ have been analyzed using a configuration-interaction cluster model. The ground state of ${\mathrm{LaMnO}}_{3}$ is found to be mixed ${\mathit{d}}^{4}$ and ${\mathit{d}}^{5}$L states and that of ${\mathrm{SrMnO}}_{3}$ to be heavily mixed ${\mathit{d}}^{3}$ and ${\mathit{d}}^{4}$L states, reflecting their strong covalency. The character of the band gap of ${\mathrm{LaMnO}}_{3}$ is of the p-to-d charge-transfer type while that of ${\mathrm{SrMnO}}_{3}$ has considerable p-p character as well as p-d character. Holes doped into ${\mathrm{LaMnO}}_{3}$ mainly of oxygen p character are coupled antiferromagnetically with the ${\mathit{d}}^{4}$ local moments of the ${\mathrm{Mn}}^{3+}$ ions and become itinerant, thus aligning the Mn moments ferromagnetically. The changes in the electronic structure with carrier doping are not of the rigid band type: By La substitution for ${\mathrm{SrMnO}}_{3}$, the so-called in-gap spectral weight (of ${\mathit{e}}_{\mathit{g}\mathrm{\ensuremath{\uparrow}}}$ symmetry) appears with its peak located 1--2 eV below the Fermi level and grows in intensity with increasing La concentration, while the spectral intensity of the ${\mathit{e}}_{\mathit{g}\mathrm{\ensuremath{\uparrow}}}$ states above the Fermi level decreases, showing a transfer of spectral weight from the unoccupied to the occupied ${\mathit{e}}_{\mathit{g}\mathrm{\ensuremath{\uparrow}}}$ states with electron doping. Meanwhile, the intensity at the Fermi level remains low even in the metallic phase (0.2\ensuremath{\lesssim}x\ensuremath{\lesssim}0.6). The energy shifts of core-level peaks and valence-band features with x suggest a downward shift of the Fermi level with hole doping, but the shift is found to be very small in the metallic phase. The importance of the orbital degeneracy of the ${\mathit{e}}_{\mathit{g}\mathrm{\ensuremath{\uparrow}}}$ band and possible orbital fluctuations in the ferromagnetic phase are pointed out.

We discuss the possibility of an intermediate-spin ground state for a ${\mathit{d}}^{5}$ (${\mathit{d}}^{6}$) system. The intermediate-spin state is stabilized by the relative stability of the ligand hole state that it hybridizes with. Using atomic multiplet calculations we showed that an intermediate-spin ground state is possible for ${\mathrm{Co}}^{4+}$ (${\mathit{d}}^{5}$) when the ${\mathit{d}}^{6}$L state dominates the ground state. From a comparison of the experimental Co 2p x-ray absorption spectroscopy spectrum with the calculated one we assume an intermediate-spin ground state for ${\mathrm{SrCoO}}_{3}$. The intermediate-spin ground state is a highly symmetrical state with high-spin Co ${\mathit{d}}^{6}$ ions on each site. Each oxygen then contributes 1/3 hole which is antiferromagnetically coupled to both neighboring Co ions. In this way the itinerant oxygen holes couple the high-spin Co ${\mathit{d}}^{6}$ ions ferromagnetically. With this model of oxygen holes that introduce ferromagnetic correlations we can also explain the spin-glass behavior for slightly doped ${\mathrm{LaCoO}}_{3}$.

We present and discuss photoemission (PES) and O 1s x-ray-absorption spectra (XAS) of ${\mathrm{Ti}}_{4}$${\mathrm{O}}_{7}$ taken at different temperatures in the range 50--300 K. The PES taken at 300 K show Ti 3d bands at the Fermi level and O 2p bands at higher binding energies. The Ti 3d bands shift approximately 0.25 eV towards higher binding energies in the low-temperature semiconducting phase (50 K). The O 1s XAS are related, via the corresponding metal-oxygen hybridization, to the unoccupied electronic states in the conduction band. The XAS taken at 300 K reflect Ti 3d bands at threshold and Ti 4sp bands at higher photon energies. The Ti 3d bands are split by crystal-field effects into ${\mathit{t}}_{2\mathit{g}}$ and ${\mathit{e}}_{\mathit{g}}$ subbands and shift approximately 0.45 eV towards higher photon energies in the low-temperature semiconducting phase (80 K). The XAS are basically temperature independent in both the low-temperature semiconducting phase and in the high-temperature metallic phase. The main changes in the O 1s XAS appear rather suddenly at the semiconductor-metal transition around 150 K.

We studied the electronic structure of SrRuO${}_{3}$ using several spectroscopic techniques. These include (resonant) photoemission, x-ray absorption, and optical conductivity. The experimental results were interpreted using an extended cluster model, which takes into account electron correlation and the Ru 4$d$-O 2$p$ covalence. The analysis shows that this material is in the negative charge transfer regime, where the ground state is dominated by the $4{d}^{5}\underline{L}$ configuration with an occupation of 47$%$. This is mainly due to the relatively large crystal field and exchange splitting in the Ru 4$d$ states. The electronic structure of SrRuO${}_{3}$ is strongly influenced by the Ru 4$d$-O 2$p$ hybridization. Thus, the oxygen states should be explicitly considered in the analysis of the physical properties of this system. However, correlation effects are also important in this system giving rise to the coherent peak in the valence band spectra.

We present band-structure and cluster-model calculations of ${\mathrm{LaCoO}}_{3}$ 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 ${\mathit{t}}_{2\mathit{g}}^{6}$${(}^{1}$${\mathit{A}}_{1}$) and 36% of ${\mathit{t}}_{2\mathit{g}}^{6}$${\mathit{e}}_{\mathit{g}}$${(}^{2}$E)L. The first (one-electron) removal state has more 3${\mathit{d}}^{6}$L than 3${\mathit{d}}^{5}$ character whereas the first addition state is almost completely dominated by the 3${\mathit{d}}^{7}$ state. This means that low-spin ${\mathrm{LaCoO}}_{3}$ is in the charge-transfer regime and the optical band gap 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.

The oxygen 1s x-ray-absorption spectra of ${\mathrm{SrTiO}}_{3}$ and ${\mathrm{TiO}}_{2}$, in both the rutile and anatase crystal structure, are analyzed using the oxygen p-projected density of states of ground-state band-structure calculations. Good agreement is found and it is concluded that multielectron effects, transition matrix elements, and the core-hole potential present only small, largely undetectable, influences on the spectral shape. From the site- and symmetry-projected density of states the rutile peaks could be assigned to the 3d band (4--8 eV), antibonding oxygen 2p states (10--18 eV), and the titanium 4sp band (20--25 eV). For anatase the titanium 4sp band is shifted to lower energy by about 5 eV, which can be related to the lower density of anatase. From differences in the crystal structure it is argued that the core-hole potential is considerably more effective in perovskite ${\mathrm{SrTiO}}_{3}$ than in both ${\mathrm{TiO}}_{2}$ crystal structures. This is in accordance with the experimental findings.

The electronic structure of ${\mathrm{La}}_{\mathit{x}}$${\mathrm{Sr}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{TiO}}_{3}$ has been studied by photoemission and x-ray absorption spectroscopy. Electron doping through La substitution induces a new photoemission feature of Ti 3d character within the band gap of ${\mathrm{SrTiO}}_{3}$; the Fermi level is located near the bottom of the conduction band of ${\mathrm{SrTiO}}_{3}$. The experimental results are incompatible with the rigid-band behavior predicted by one-electron band theory or Hubbard-model calculations.

The controlled-valence properties of ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{FeO}}_{3}$ and ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{MnO}}_{3}$ are studied by means of soft-x-ray absorption spectroscopy. A comparison between the transition-metal 2p spectra and atomic-multiplet calculations is used to determine the 3d count. The O 1s spectrum is used to characterize changes in unoccupied states that contain oxygen p character. The results indicate that the holes induced by substitution for both series are located in states of mixed metal 3d--oxygen 2p character. The ground state of ${\mathrm{LaFeO}}_{3}$ is mainly 3${\mathit{d}}^{5}$ and becomes 3${\mathit{d}}^{5}$L (where L denotes a ligand hole) in the ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{FeO}}_{3}$ series for low Sr concentration. The main component of the ground state of ${\mathrm{LaMnO}}_{3}$ is 3${\mathit{d}}^{4}$ and becomes a mixture of 3${\mathit{d}}^{3}$ and 3${\mathit{d}}^{4}$L in the ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{MnO}}_{3}$ series. The trends in controlled- valence properties of similar oxides across the transition-metal series can be rationalized within the framework of the Zaanen-Sawatzky-Allen model.

Core-level and valence-band optical spectra provide important information on highly correlated systems. The former corresponds to transitions from the core level to the conduction band, and it is usually related to the unoccupied electronic structure. The later corresponds to transitions from the valence band to the conduction band, and it is sometimes described in terms of the joint density of states. These spectra are usually treated separately due to the differences in the experimental and theoretical methods. We present here a combined description of the core-level and valence-level optical spectra of Mott-Hubbard compounds. In particular, we studied the $\text{O}\text{ }1s$ x-ray absorption and the optical-conductivity spectra of ${\text{SrVO}}_{3}{\text{-CaVO}}_{3}{\text{-LaVO}}_{3}{\text{-YVO}}_{3}$. The experimental data were analyzed using an extended $p\text{\ensuremath{-}}d$ cluster model solved by an exact diagonalization method. The results show that correlation effects alone cannot account for the experimental structures, and that crystal-field effects and exchange interactions are necessary to explain the spectra. We also show that there is a correspondence between the features in the charge-transfer region of both spectra.

We investigate experimentally and theoretically the origin of magnetism in nanocrystalline-undoped ceria films. Our results are consistent with density-functional calculations showing that both oxygen and cerium vacancies lead to ferromagnetism. Although the number of cerium vacancies is smaller than that of oxygen vacancies, the relatively high magnetic moment of the former might result in a substantial contribution to the magnetic properties. Diluted defect magnetism is suggested as a universal feature of ferromagnetism in nonmagnetic oxides without magnetic impurities.

We studied the electronic structure of the ${\text{SrVO}}_{3}$ compound using an extended cluster model. The calculation is compared to the photoemission and x-ray absorption/bremsstrahlung isochromat spectra. The calculated spectral weight correctly describes the experimental features in the ${\text{SrVO}}_{3}$ material. The results indicate that the $\text{O}\text{ }2p$ states are essential to explain the data and must be included explicitly in any minimal model of this compound.

We present the O 1s and V 2p absorption edges of ${\mathrm{VO}}_{2}$ at room temperature and at T=120 \ifmmode^\circ\else\textdegree\fi{}C, i.e., below and above the metal-insulator transition temperature ${\mathit{T}}_{\mathit{c}}$\ensuremath{\approxeq}67 \ifmmode^\circ\else\textdegree\fi{}C. The O 1s spectra show a clear splitting of 1 eV in the unoccupied ${\mathit{d}}_{\mathrm{?}}$ band below ${\mathit{T}}_{\mathit{c}}$. Because the V 2p absorption edges are dominated by the V 2p-3d interaction in the final state, these spectra show minor changes that cannot be easily correlated with the changes in the unoccupied density of states.

We studied the Mott-Hubbard series ${\text{SrVO}}_{3}{\text{-CaVO}}_{3}{\text{-LaVO}}_{3}{\text{-YVO}}_{3}$ with high-energy photoemission. The features in the experimental spectra were interpreted using cluster model calculations. The valence-band photoemission results show very interesting trends across the Mott-Hubbard series: (i) From ${\text{SrVO}}_{3}$ to ${\text{CaVO}}_{3}$, the spectral weight is transferred from the coherent to the incoherent feature. (ii) From ${\text{CaVO}}_{3}$ to ${\text{LaVO}}_{3}$, the coherent structure disappears, opening the insulating band gap. (iii) Finally, from ${\text{LaVO}}_{3}$ to ${\text{YVO}}_{3}$, the bandwidth of the remaining incoherent feature further decreases. There is also a considerable $\text{V}\text{ }3d$ spectral weight contribution mixed in the $\text{O}\text{ }2p$ band region in all cases. These results suggest that the $\text{O}\text{ }2p$ states play an important role in the Mott-Hubbard transition. Some of the changes in the spectra are unexpected and cannot be explained by the current Mott-Hubbard theories. The calculation reproduces not only the coherent and incoherent structures in the $\text{V}\text{ }3d$ band but also the main features in the $\text{O}\text{ }2p$ band. In addition, the same model explains the charge-transfer satellites observed in the $\text{V}\text{ }2p$ core-level spectra.

We studied the electronic structure of the ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ compound using cluster model calculations. The calculation included the reduced ${\mathrm{V}}^{5+}$ symmetry and the strong $\mathrm{V}\phantom{\rule{0.2em}{0ex}}3d--\mathrm{O}\phantom{\rule{0.2em}{0ex}}2p$ covalence effects. The many-body effects are taken into account using the configuration interaction method. The ground state of ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ is highly covalent and dominated by the $3{d}^{1}\underset{}{L}$ $(^{1}A_{1})$ configuration, where $\underset{}{L}$ denotes a ligand hole. The ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ material is in the charge-transfer regime and the band gap is due to $p\text{\ensuremath{-}}d$ transitions. The first removal state is given by the $3{d}^{0}\underset{}{L}$ $(^{2}A_{1})$ configuration, and the first addition state is formed by the $3{d}^{1}$ $(^{2}E)$ configuration. The results of the cluster model are in good agreement with band structure calculations. The calculation results are also in good agreement with photoemission and x-ray absorption spectra. The $\mathrm{V}\phantom{\rule{0.2em}{0ex}}2p$ core-level spectra exhibit many-body effects despite the nominal $3{d}^{0}$ occupancy. These effects and the reduced ${\mathrm{V}}^{5+}$ symmetry are crucial to describe the electronic structure of ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$.

We report the $\mathrm{Ni}\phantom{\rule{0.2em}{0ex}}2p$ x-ray photoelectron spectra of NiO thin films grown on different oxide substrates, namely, $\mathrm{Si}{\mathrm{O}}_{2}$, ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$, and MgO. The main line of the $\mathrm{Ni}\phantom{\rule{0.2em}{0ex}}2p$ spectra is attributed to the bulk component, and the shoulder at $1.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ higher binding energies to the surface component. The spectra of the NiO thin films show strong differences with respect to that of bulk NiO. The energy separation between the main peak and the shoulder increases with the substrate covalence. This indicates the strong covalent interactions between the NiO thin films and the oxide substrates, and reflects changes in the bonding at the interface from a more ionic to a more covalent interaction. These conclusions are supported by cluster model calculations with a reduced $\mathrm{O}\phantom{\rule{0.2em}{0ex}}2p\text{\ensuremath{-}}\mathrm{Ni}\phantom{\rule{0.2em}{0ex}}3d$ hybridization.

We have studied the polarization dependence of the Cu 2{ital p} absorption spectra in a (Bi{sub 0.84}Pb{sub 0.16}){sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} single crystal. At normal incidence ({bold E}{parallel}{ital xy} plane) we found an intense peak at 932.0 eV, which becomes much weaker and shifts 500 meV toward lower energies at grazing incidence ({bold E}{parallel}{ital z} axis). Also, at grazing incidence a structured absorption step appears starting at 935.8 eV. The interpretation of these results is discussed.

We studied the electronic structure of the filling-controlled $\mathrm{Y}\mathrm{V}{\mathrm{O}}_{3}$ and $\mathrm{Ca}\mathrm{V}{\mathrm{O}}_{3}$ compounds using a cluster model. The calculations included additional non-local screening channels (a coherent screening in metallic $\mathrm{Ca}\mathrm{V}{\mathrm{O}}_{3}$ and a Mott-Hubbard screening in insulating $\mathrm{Y}\mathrm{V}{\mathrm{O}}_{3}$). The cluster model results are in good agreement with previous photoemission and X-ray absorption spectra. In $\mathrm{Ca}\mathrm{V}{\mathrm{O}}_{3}$, the coherent structure is mostly due to a coherent screened $3{d}^{1}\underset{}{C}$ state, whereas the incoherent feature is mainly related to a ligand screened $3{d}^{1}\underset{}{L}$ state. In $\mathrm{Y}\mathrm{V}{\mathrm{O}}_{3}$, the incoherent feature remains mainly related to a ligand screened $3{d}^{2}\underset{}{L}$ state, but the Mott-Hubbard screened $3{d}^{2}\underset{}{D}$ state appears at higher energies. The opening of the band gap is related to the transfer of spectra weight, from the $3{d}^{1}\underset{}{C}$ state in metallic $\mathrm{Ca}\mathrm{V}{\mathrm{O}}_{3}$ to the $3{d}^{2}\underset{}{D}$ in insulating $\mathrm{Y}\mathrm{V}{\mathrm{O}}_{3}$. The band gap in insulating $\mathrm{Y}\mathrm{V}{\mathrm{O}}_{3}$ scales mainly with the charge-transfer energy $\ensuremath{\Delta}$. Further, the larger orthorhombic distortion in $\mathrm{Y}\mathrm{V}{\mathrm{O}}_{3}$ also favors the insulating state. The results show that $\mathrm{Ca}\mathrm{V}{\mathrm{O}}_{3}$ is closer to the charge-transfer than to the Mott-Hubbard regime. On the other hand, the $\mathrm{Y}\mathrm{V}{\mathrm{O}}_{3}$ compound is in a heavily O $2p\text{\ensuremath{-}}\mathrm{V}$ $3d$ mixed intermediate regime. The lowest energy charge fluctuations explain the changes in the Hall resistance ${R}_{H}$.

We report experimental and theoretical evidence of surface effects in the Ni $2p$ x-ray photoemission spectra (XPS) of NiO. The Ni $2{p}_{3∕2}$ surface-enhanced XPS of a NiO sample show a relative enhancement of the intensity of the known satellite at $1.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ higher binding energy from the main line, indicating a considerable surface contribution of this satellite. The results are discussed in terms of bulk-octahedral and surface-pyramidal Ni symmetries. Other contributions, like nonlocal screening effects, cannot be neglected.

We studied the changes in the electronic structure of ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ using a cluster model. The calculations included fluctuations from the coherent band in the metallic phase and nonlocal Mott-Hubbard fluctuations in the insulating phase. The incoherent structure is mostly related to the usual ligand screening channel $(3{d}^{2}\underset{}{L})$. The coherent peak in the metallic phase corresponds to coherent band fluctuations $(3{d}^{2}\underset{}{C})$. However, The Mott-Hubbard screened state in the insulating phase $(3{d}^{2}\underset{}{D})$ appears at higher energies. The transfer of spectral weight from the $3{d}^{2}\underset{}{L}$ to the $3{d}^{2}\underset{}{D}$ state produces the opening of the band gap. The photon energy dependence of the spectra is partly related to the relative $\mathrm{V}\phantom{\rule{0.2em}{0ex}}3d\char21{}\mathrm{O}\phantom{\rule{0.2em}{0ex}}2p$ cross sections. The calculations reproduce also the experimental changes observed in the $\mathrm{V}\phantom{\rule{0.2em}{0ex}}1s$ core-level spectra. The above results suggest that the Mott-Hubbard transition in ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ requires a multiband model.

The electronic structure of $\mathrm{NiO}$ nanometric planar islands on highly oriented pyrolitic graphite has been studied by means of the $\mathrm{Ni}\phantom{\rule{0.3em}{0ex}}2p$ and $\mathrm{O}\phantom{\rule{0.3em}{0ex}}1s$ x-ray absorption spectra. The $\mathrm{O}\phantom{\rule{0.3em}{0ex}}1s$ spectrum of the early stages of growth shows a double peak at threshold which is attributed to a splitting of the unoccupied $\mathrm{Ni}\phantom{\rule{0.3em}{0ex}}{e}_{g}$ states. This spectrum is compared to previous results for $3\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ $\mathrm{NiO}$ nanoparticles which also show a rather similar splitting of the $\mathrm{Ni}\phantom{\rule{0.3em}{0ex}}{e}_{g}$ states. This splitting observed in the $\mathrm{O}\phantom{\rule{0.3em}{0ex}}1s$ spectra of the $\mathrm{NiO}$ nanostructures is caused by the reduced symmetry of the Ni ions at the surface. Cluster model calculations for a high-spin ${\mathrm{Ni}}^{2+}$ ion performed in both octahedral (bulk) and pyramidal (surface) symmetries confirm this interpretation. This new interpretation of the surface electronic structure of $\mathrm{NiO}$ suggests a revision of the accepted concepts concerning this system.

We studied the changes in the electronic structure of $\mathrm{V}{\mathrm{O}}_{2}$ across the metal-insulator transition. The main technique was cluster model calculations with nonlocal screening channels. The calculation included a screening from a coherent state at the Fermi level in the metallic phase, and a screening from a Hubbard charge fluctuation within the V-V dimmer in the insulating phase. The calculation results are compared to previous photoemission and x-ray absorption spectra. The coherent feature at the Fermi level in the metallic phase is due to the coherent screening. But the Hubbard screened state in the insulating phase appears at higher energies opening the band gap. The changes in the electronic structure of $\mathrm{V}{\mathrm{O}}_{2}$ are thus related to the nonlocal screening channels.

We have performed photoemission and soft x-ray absorption studies of pyrochlore-type Ru oxides, namely, the filling-control system Sm$_{2-x}$Ca$_x$Ru$_2$O$_7$ and the bandwidth-control system Sm$_{2-x}$Bi$_x$Ru$_2$O$_7$, which show insulator-to-metal transition with increasing Ca and Bi concentration, respectively. Core levels and the O 2$p$ valence band in Sm$_{2-x}$Ca$_x$Ru$_2$O$_7$ show almost the same amount of monotonous upward energy shifts with Ca concentration, which indicates that the chemical potential is shifted downward due to hole doping. The Ru 4$d$ band in Sm$_{2-x}$Ca$_x$Ru$_2$O$_7$ is also shifted toward the Fermi level ($E_F$) with hole doping and the density of states (DOS) at $E_F$ increases. The core levels in Sm$_{2-x}$Bi$_x$Ru$_2$O$_7$, on the other hand, do not show clear energy shifts except for the Ru 3$d$ core level, whose line shape change also reflects the increase of metallic screening with Bi concentration. We observe pronounced spectral weight transfer from the incoherent to the coherent parts of the Ru 4d $t_{2g}$ band with Bi concentration, which is expected for a bandwidth-control Mott-Hubbard system. The increase of the DOS at $E_F$ is more abrupt in the bandwidth-control Sm$_{2-x}$Bi$_x$Ru$_2$O$_7$ than in the filling-control Sm$_{2-x}$Ca$_x$Ru$_2$O$_7$, in accordance with a recent theoretical prediction. Effects of charge transfer between the Bi 6$sp$ band and the Ru 4$d$ band are also discussed., Comment: 11 pages, 6 figures

We studied the relation between the electronic structure and the physical properties of ${\mathrm{Sr}}_{3}\mathrm{Fe}M{\mathrm{O}}_{7}$ ($M=\mathrm{Fe}$, Co). The main technique used in the study was Fe $2p$, Co $2p$, and O $1s$ x-ray-absorption spectroscopy. The experimental spectra were analyzed in terms of cluster model and band-structure calculations. The analysis of the spectra shows that both ${\mathrm{Sr}}_{3}{\mathrm{FeFeO}}_{7}$ and ${\mathrm{Sr}}_{3}{\mathrm{FeCoO}}_{7}$ are in the negative-charge-transfer regime (this means that their ground states are highly covalent and contain considerable O $2p$ hole character). The Fe ions are in a high-spin state and the Co ions in an intermediate-spin state stabilized by strong hybridization. The electronic structure helps to explain the trends in the electrical conductivity and the observed magnetic moments. Further, the relatively large O $2p$ hole weight in the ground state explains also the absence of Jahn-Teller distortions. ${\mathrm{Sr}}_{3}{\mathrm{FeCoO}}_{7}$ illustrates ferromagnetism and magnetoresistance in a two-dimensional negative-charge-transfer material.

The antiferromagnetic-to-ferromagnetic transition in $\mathrm{Sr}{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{O}}_{3}$ $({x}_{c}\ensuremath{\simeq}0.15)$ induced by Co doping has been studied by magnetic circular x-ray dichroism. The orbital and spin magnetic moments of the Fe and Co $3d$ states under the magenetic field of $2\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ are found to show different $x$ dependences: The spin polarization of Fe $3d$ gradually increases with Co concentration; on the other hand, a large spin polarization of Co $3d$ is induced already in the antiferromagnetic phase, indicating that the Co moment is nearly fully alligned already in the antiferromagnetic phase. This suggests that the alignment of the Fe magnetic moment in $\mathrm{Sr}{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{O}}_{3}$ is induced by interaction with doped Co magnetic moment. Possible formation of ferromagnetic Co rich region is discussed.

We studied the changes in the electronic structure of $\mathrm{Li}\mathrm{Co}{\mathrm{O}}_{2}$ induced by 10% Ga substitution. The technique used in the study was $\mathrm{Co}\phantom{\rule{0.3em}{0ex}}2p$ and $\mathrm{O}\phantom{\rule{0.3em}{0ex}}1s$ X-ray absorption spectroscopy. The spectra were analyzed using atomic multiplet and band structure calculations. The $\mathrm{Co}\phantom{\rule{0.3em}{0ex}}2p$ spectra show that the Co ions in $\mathrm{Li}\mathrm{Co}{\mathrm{O}}_{2}$ are in a trivalent ${\mathrm{Co}}^{3+}$ low-spin state, and that the electronic structure of the Co ions is not greatly affected by 10% Ga substitution. The Co ions remain in the trivalent ${\mathrm{Co}}^{3+}$ low-spin state in chemically deintercalated ${\mathrm{Li}}_{x}{\mathrm{Co}}_{0.9}{\mathrm{Ga}}_{0.1}{\mathrm{O}}_{2}$. This shows that the Co ions in deintercalated ${\mathrm{Li}}_{x}{\mathrm{Co}}_{0.9}{\mathrm{Ga}}_{0.1}{\mathrm{O}}_{2}$ are not oxidized to a tetravalent ${\mathrm{Co}}^{4+}$ state. The $\mathrm{O}\phantom{\rule{0.3em}{0ex}}1s$ spectra are dominated by transitions to mixed $\mathrm{O}\phantom{\rule{0.3em}{0ex}}2p\ensuremath{-}\mathrm{Co}\phantom{\rule{0.3em}{0ex}}3d$ states at threshold. The transitions to $\mathrm{O}\phantom{\rule{0.3em}{0ex}}2p\ensuremath{-}\mathrm{Ga}\phantom{\rule{0.3em}{0ex}}4sp$ states appears well above threshold due to the larger hybridization. The $\mathrm{O}\phantom{\rule{0.3em}{0ex}}1s$ spectra of deintercalated ${\mathrm{Li}}_{x}{\mathrm{Co}}_{0.9}{\mathrm{Ga}}_{0.1}{\mathrm{O}}_{2}$ show an increase in the absorption intensity at threshold. The increased $\mathrm{O}\phantom{\rule{0.3em}{0ex}}1s$ absorption intensity indicates that the $\mathrm{O}$ ions are oxidized in deintercalated ${\mathrm{Li}}_{x}{\mathrm{Co}}_{0.9}{\mathrm{Ga}}_{0.1}{\mathrm{O}}_{2}$. The results and conclusions obtained for deintercalated ${\mathrm{Li}}_{x}{\mathrm{Co}}_{0.9}{\mathrm{Ga}}_{0.1}{\mathrm{O}}_{2}$ are similar to those for ${\mathrm{Li}}_{x}{\mathrm{Co}\mathrm{O}}_{2}$; this shows that the Ga substitution does not significantly affect the charge transfer mechanism in ${\mathrm{Li}}_{x}{\mathrm{Co}\mathrm{O}}_{2}$.

We studied the changes in the electronic structure of ${\mathrm{LaNiO}}_{3\ensuremath{-}\ensuremath{\delta}}$ across the metal-insulator transition. The technique used in the study was mainly $\mathrm{O} 1s$ x-ray absorption spectroscopy (XAS). The experimental spectrum of ${\mathrm{LaNiO}}_{3.00}$ was analyzed in terms of a cluster-model calculation. The spectrum of ${\mathrm{LaNiO}}_{3.00}$ presents a sharp peak at a threshold that corresponds to ${3d}^{8}L\stackrel{\ensuremath{\rightarrow}}{}c{3d}^{9}$ transitions. Analysis of this peak indicates that the charge carriers in ${\mathrm{LaNiO}}_{3.00}$ contain considerable oxygen character. The intensity of this peak decreases in the spectrum of ${\mathrm{LaNiO}}_{2.75}$ and disappears almost completely for ${\mathrm{LaNiO}}_{2.50}.$ This suggests that the metal-insulator transition is related to the disappearance of the charge carriers and the ensuing band-gap opening. The peak in the ${\mathrm{LaNiO}}_{2.75}$ compound is split due to the presence of two nonequivalent crystallographic sites. The metal-insulator transition in this compound is tentatively attributed to potential disorder between these two sites.