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In order to clarify the controversial issue of the topological nature in a mixed-valent Kondo system, ${\mathrm{SmB}}_{6}$, we have explored the surface states on the nonpolar (110) surface of ${\mathrm{SmB}}_{6}$, employing both angle-resolved photoemission spectroscopy (ARPES) experiment and ab initio density-functional theory (DFT) band calculations. Based on ARPES spectroscopic fingerprints and the DFT surface band structures, we ascribe the observed spectral weights at $\overline{X}$ and $\overline{Y}$ on the (110) surface Brillouin zone to topological surface states (TSSs) of ``topological insulator (TI)'' nature and of ``topological crystalline insulator (TCI)'' nature, respectively. With varying the chemical potential, the double Dirac cones of the TCI nature exhibit a Lifshitz transition of Fermi surfaces with intriguing spin textures. We have also examined the TSSs on the nearly nonpolar (111) surface of ${\mathrm{SmB}}_{6}$ in connection with a recently reported ARPES result and proposed a way to probe the Dirac points that are buried in the bulk-projected bands.

The electronic structure of a possible topological Kondo insulator of CeRhAs has been investigated by employing temperature $(T)$-dependent angle-resolved photoemission spectroscopy (ARPES). Fermi surfaces (FSs) and band structures are successfully measured for three orthogonal crystallographic planes. The measured Fermi-edge states are found to have a three-dimensional (3D) character, contradictory to the proposed topological surface states of the $2\text{D}$ character. The measured FSs due to the Ce $4f$ electrons agree well with those from the density functional theory band structures unfolded into the reduced Ce-only unit cell. The theoretically predicted hourglass-type bulk bands along $\overline{X}\overline{S}\overline{X}$ are not clearly resolved, in spite of the evident existence of those band features. $T$-dependent ARPES measurements reveal that the coherent Ce $4f$ states, having two pseudogap structures of ${\mathrm{\ensuremath{\Delta}}}_{1}\ensuremath{\sim}80$ meV and ${\mathrm{\ensuremath{\Delta}}}_{2}\ensuremath{\sim}30$ meV, persist to remain above 200 K, in agreement with the high Kondo temperature of CeRhAs.

The electronic structures of Prussian blue analog $({\mathrm{Rb}}_{x}{\mathrm{Ba}}_{y}){\mathrm{Mn}}_{[3\ensuremath{-}(x+2y)]/2}[\mathrm{Fe}{(\mathrm{CN})}_{6}]$ cyanides have been investigated by employing soft x-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) at the Fe and Mn $L$ $(2p)$ edges. The measured XAS spectra have been analyzed with the configuration-interaction (CI) cluster model calculations. The valence states of the Fe and Mn ions are found to be ${\mathrm{Fe}}^{2+}\text{\ensuremath{-}}{\mathrm{Fe}}^{3+}$ mixed valent, with an average valency of $v(\mathrm{Fe})\ensuremath{\sim}2.8$ and nearly divalent $({\mathrm{Mn}}^{2+})$, respectively. Our Mn/Fe $2p$ XMCD study supports that ${\mathrm{Mn}}^{2+}$ ions are in the high-spin states while ${\mathrm{Fe}}^{2+}\text{\ensuremath{-}}{\mathrm{Fe}}^{3+}$ ions are in the low-spin states. The Fe and Mn $2p$ XAS spectra are found to be essentially the same for $80\ensuremath{\le}T\ensuremath{\le}300$ K, suggesting that a simple charge transfer upon cooling from ${\mathrm{Fe}}^{3+}\text{\ensuremath{-}}\mathrm{CN}\text{\ensuremath{-}}{\mathrm{Mn}}^{2+}$ to ${\mathrm{Fe}}^{2+}\text{\ensuremath{-}}\mathrm{CN}\text{\ensuremath{-}}{\mathrm{Mn}}^{3+}$ does not occur in $({\mathrm{Rb}}_{x}{\mathrm{Ba}}_{y}){\mathrm{Mn}}_{[3\ensuremath{-}(x+2y)]/2}[\mathrm{Fe}{(\mathrm{CN})}_{6}]$. According to the CI cluster model analysis, it is necessary to take into account both the ligand-to-metal charge transfer and the metal-to-ligand charge transfer in describing Fe $2p$ XAS, while the effect of charge transfer is negligible in describing Mn $2p$ XAS. The CI cluster model analysis also shows that the trivalent ${\mathrm{Fe}}^{3+}$ ions have a strong covalent bonding with the $\mathrm{C}\ensuremath{\equiv}\mathrm{N}$ ligands and are under a large crystal-field energy of $10Dq\ensuremath{\sim}3$ eV, in contrast to the weak covalency effect and a small $10Dq\ensuremath{\sim}0.6$ eV for the divalent ${\mathrm{Mn}}^{2+}$ ions.

The electronic structures of a charge density wave (CDW) system $R{\mathrm{Te}}_{2}$ $(R=\text{Ce},\text{Pr})$ have been investigated by employing angle-resolved photoemission spectroscopy (ARPES) and the first-principles band structure method. The $R$ $4f$ hybridization peak $(4{f}^{n}{c}^{m\ensuremath{-}1})$ in the $R$ $4f$ PES spectrum is located deeper in ${\mathrm{PrTe}}_{2}$ than in ${\mathrm{CeTe}}_{2}$ and $R$ $4f$ spectral intensity near ${E}_{F}$ is much weaker in ${\mathrm{PrTe}}_{2}$ than in ${\mathrm{CeTe}}_{2}$, implying the importance of the hybridization between Ce $4f$ and Te(1) $5p$ electrons. For both ${\mathrm{CeTe}}_{2}$ and ${\mathrm{PrTe}}_{2}$, the metallic states crossing the Fermi level $({E}_{F})$ are observed below the CDW transition temperature, indicating the existence of the partially ungapped Fermi surfaces (FSs). The zigzag features having the fourfold rotational symmetry are observed near the $X$ point in the FS of ${\mathrm{CeTe}}_{2}$, but not in the FS of ${\mathrm{PrTe}}_{2}$. The tight-binding model calculations show that the zigzag FS features in ${\mathrm{CeTe}}_{2}$ can be described as the CDW-induced FS reconstruction due to the $4\ifmmode\times\else\texttimes\fi{}4$ CDW supercell structure. The effect of the linear dichroism is observed in ARPES, suggesting that the ${E}_{F}$-crossing states have mainly the in-plane orbital character. The photon-energy maps for the near-${E}_{F}$ states exhibit the straight vertical dispersions for both ${\mathrm{CeTe}}_{2}$ and ${\mathrm{PrTe}}_{2}$, demonstrating the dominant two-dimensional character in $R{\mathrm{Te}}_{2}$ $(R=\text{Ce},\text{Pr})$.

We have investigated the surface states of a potential mixed-valent topological insulator ${\mathrm{SmB}}_{6}$ based on first-principles density functional theory slab band structure analysis. We have found that metallic surface states are formed in the bulk band gap region, providing evidence for the topological insulating nature of ${\mathrm{SmB}}_{6}$. The obtained surface in-gap states are quite different from those in existing reports in that they are formed differently depending on the Sm or ${\mathrm{B}}_{6}$ surface termination, and are composed of mainly the Sm $4f$ state indicating the essentiality of including $f$ electrons in describing the surface states. We have obtained the spin chiral structures of the Fermi surfaces, which are also in accordance with the topological insulating nature of ${\mathrm{SmB}}_{6}$.

The electronic structures of the AgTO2 delafossites ( T 5 Fe, Co, Ni! have been investigated using photoemission spectroscopy. T 2p spectra exhibit satellite structures around 6 eV below main peaks, reflecting the strong on-site 3 d Coulomb interaction. T 2p spectra of AgTO2 (T5Fe, Co, Ni! are similar to those of aFe2O3, CoO, and NiO, respectively, suggesting the strong covalent bonding in AgTO2. The valence-band spectrum of Ag0.75CoO2 shows negligible spectral weight near EF , consistent with its semiconducting behavior. The extracted Ag 4d and Co 3d partial spectral weight ~PSW! distributions of Ag0.75CoO2 reveal a small overlap between these two states, while the O 2 p PSW shows a significant overlap with the Co 3d PSW. Such trends in the PSWs are consistent with those in the calculated partial densities of states of AgCoO 2.

A resonant photoemission study has been performed for Pr{sub 1{minus}x}Sr{sub x}MnO{sub 3}thinsp(x=1/3,1/2) and the partial spectral weight (PSW) distributions of both Pr 4f and Mn 3d electrons have been determined. With varying temperature ({ital T}) and {ital x}, metal-insulator transitions are observed in the photoemission data. The Pr ions in Pr{sub 1{minus}x}Sr{sub x}MnO{sub 3} are trivalent, and Pr 4f PSW exhibits a dominant hybridization peak indicating large Pr 4f{minus}Othinsp2p mixing. The Mn 3d PSW at low {ital T} shows a charge gap for x=1/2, and a reduced spectral intensity near E{sub F} for x=1/3, suggesting the localized nature due to the lattice polaron formation. thinsp {copyright} {ital 1999} {ital The American Physical Society}

Photoemission study has been performed for a high-quality single crystal of ${\mathrm{CeRu}}_{2},$ including resonant photoemission near the Ce $4\stackrel{\ensuremath{\rightarrow}}{d}4f$ absorption edge and high-resolution photoemission at a low-photon energy. Resonant photoemission study provides evidence for the large hybridization between Ce $4f$ and conduction electrons, indicating mixed-valence in ${\mathrm{CeRu}}_{2}.$ It is found that the line shape of the high-resolution spectrum near the Fermi level depends on the experimental conditions and the sample surface treatment. Comparison of photoemission data to the calculated density of states reveals that band calculations provide a reasonably good description of the electronic structure of ${\mathrm{CeRu}}_{2}.$

The electronic structure of single crystalline CeNiSn has been investigated using photoemission spectroscopy. The extracted Ce 4f spectrum exhibits three peak structures; the typical Fermi level and 2 eV peaks, and another between them, {approximately}0.9 eV below the Fermi level E{sub F}. The near-E{sub F} peak reflects a substantial Ce 4f-Sn sp hybridization, whereas the 0.9 eV peak arises from the Ce 4f-Ni 3d and Ce 5d-Ni 3d hybridization. A Ni 3d satellite feature is observed about 6 eV below the Ni 3d main band, indicating a strong Ni 3d Coulomb correlation in CeNiSn. The high-resolution photoemission study indicates a finite metallic density of states at E{sub F}, implying a semimetallic ground state. The electronic states near E{sub F} are found to have mixed character from the Sn sp, Ce 5d, Ce 4f, and Ni 3d electrons. Constant-initial-state and constant-final-state yield spectra across the 4d{r_arrow}4f threshold indicate the Ce valence to be close to 3+. {copyright} {ital 1998} {ital The American Physical Society}

Atomic and electronic structures of a Pd(111) single crystal and a polycrystalline Pd film have been investigated by using low-energy electron diffraction (LEED) and photoemission spectroscopy (PES). LEED patterns of a clean Pd(111) show a threefold symmetry, suggesting no surface reconstruction. Band structures of Pd(111) for ${k}_{\ensuremath{\parallel}}$ along the $\overline{\ensuremath{\Gamma}}\ensuremath{-}{\overline{M}}^{\ensuremath{'}},$ $\overline{\ensuremath{\Gamma}}\ensuremath{-}\overline{K},$ and $\overline{\ensuremath{\Gamma}}\ensuremath{-}\overline{M}$ symmetry lines have been determined experimentally from angle-resolved photoemission spectroscopy measurements and compared to those calculated within the local-density-functional approximation (LDA). The valence band PES spectrum of Pd film is also obtained and compared to the calculated $4d$ angular momentum projected local density of states within LDA. Experimental and LDA band structures of Pd(111) exhibit a reasonably good agreement, whereas experimental bandwidths are narrower than the LDA bulk bandwidths by 0.5--1 eV. Such a discrepancy can be improved theoretically by taking into account more localized Pd surface states. In contrast, the calculated Pd $4d$ quasiparticle spectral density by including the on-site Coulomb correlation cannot resolve the disagreement between PES and LDA, suggesting that the Coulomb correlation effect is not substantial in understanding PES spectra of Pd.

The electronic structures of $R$ $4f$ and Fe $3d$ states of $R$Fe${}_{2}$O${}_{4}$ ($R=\text{Er}$, Tm, Lu) have been investigated by employing soft x-ray absorption spectroscopy (XAS) and magnetic circular dichroism at the Fe $2p$ and $R$ $3d$ absorption edges. It is found that the valence states of Fe and $R$ ions are nearly Fe${}^{2.5+}$ and ${R}^{3+}$, and that the net magnetic moments of Fe${}^{2+}$ and Fe${}^{3+}$ ions are antiparallel to each other. Both $R$ $3d$ and O $1s$ XAS spectra show that the localized $R$ $4f$ states do not contribute to the multiferroicity of $R$Fe${}_{2}$O${}_{4}$. On the other hand, the magnetization data for $R$Fe${}_{2}$O${}_{4}$ at low temperature ($T$) reveal the cluster glass behavior for $R=\text{Tm}$ and Lu, but not for $R=\text{Er}$. This work suggests that the $T$-dependent structural differences among $R$Fe${}_{2}$O${}_{4}$, caused by different $R$ ions, are closely related to the magnetoelectricity at low temperature.

Electronic structures of Fe on Cr (Fe/Cr) and Cr/Fe/Cr sandwich films, with an Fe coverage of 1--20 A, have been investigated by using photoemission spectroscopy (PES). The Fe 3[ital d] partial spectral weight in Fe/Cr has been extracted as a function of the Fe overlayer thickness. Experimental results are compared with supercell band-structure calculations for a system with monolayer (ML) Fe on each side of five-layer Cr, Fe(1 ML)/Cr(5 ML)/Fe(1 ML), by using the linearized muffin-tin orbital band method. For a very thin Fe overlayer, very interesting features are observed: (i) a sharp emission just at [ital E][sub [ital F]], and (ii) two other structures at higher binding energies which resemble the Cr 3[ital d] valence bands. The former feature is expected to originate primarily from hybridization between Fe and Cr 3[ital d] electrons at the Fe/Cr interface, and partially from the Fe 3[ital d] [ital surface] states in the Fe overlayer. The latter features also suggest large hybridization between Fe and Cr 3[ital d] states at the Fe/Cr interface, as confirmed by band-structure calculations. The trends observed in PES measurements are qualitatively consistent with those in band-structure calculations for the Fe(1 ML)/Cr(5 ML)/Fe(1 ML) system. This work suggests themore » importance of electronic structures, such as Fermi surface effects, in determining the character of the exchange coupling as well as the magnitude of magnetoresistance in the Fe/Cr superlattice« less

The electronic structure of spinel FeV${}_{2}$O${}_{4}$, which contains two Jahn-Teller active Fe and V ions, has been investigated by employing soft x-ray absorption spectroscopy (XAS), soft x-ray magnetic circular dichroism (XMCD), and nuclear magnetic resonance (NMR). XAS indicates that V ions are trivalent and Fe ions are nearly divalent. The signs of V and Fe $2p$ XMCD spectra are opposite to each other. It is found that the effect of the V $3d$ spin-orbit interaction on the V $2p$ XMCD spectrum is negligible, indicating that the orbital ordering of V ${t}_{2g}$ states occurs from the real orbital states and that the orbital moment of a V${}^{3+}$ ion is mostly quenched. NMR shows that V spins are canted to have a Yafet-Kittel-type triangular spin configuration.

Electronic structures of a charge-density wave (CDW) system CeTe${}_{2\ensuremath{-}x}$Sb${}_{x}$ ($x=0$, 0.05) have been investigated by employing angle-resolved photoemission spectroscopy (ARPES) and the first-principles electronic and phonon band-structure methods. The observed Fermi surface (FS) agrees very well with the calculated FS for the undistorted CeTe${}_{2}$ both in shapes and sizes. The metallic states crossing the Fermi level (${E}_{F}$) are observed in ARPES even in the CDW state. The carriers near ${E}_{F}$ have mainly the Te(1) $5p$ character, with a negligible contribution from Ce $4f$ states. The supercell (shadow) bands and the corresponding very weak FSs are found to arise from band folding due to the interaction of Te(1) layers with Ce-Te(2) layers. We found that the CDW modulation vector is along $\ensuremath{\Gamma}$-$X$ (${\mathbf{Q}}_{\text{CDW}}\ensuremath{\approx}X$), which is not coincident with the most prominent FS nesting vector.

We have investigated the electronic structures of single- and double-layered MoS${}_{2}$, composing of heterojunction structures such as graphene, MoS${}_{2}$, and SiO${}_{2}$ and MoS${}_{2}$ and SiO${}_{2}$, using scanning photoelectron microscopy. Negative shifts of both core levels and valence bands toward the Fermi energy have been observed. In connection with first-principles calculations, we have confirmed that the direct gap of single-layer MoS${}_{2}$ is changed to an indirect gap by stacking additional layers via van der Waals interlayer interactions.

The electronic structure of SmCo 2 is investigated with photoemission spectroscopy (PES). All the PES spectra are consistent in showing that the bulk Sm ions are trivalent, but that the surface has divalent Sm ions and is probably inhomogeneously mixed valent. Both the Sm 4f valence band and Sm 3d core level PES spectra are found to be substantially broader than those of Sm metal. The observed larger linewidths for SmCo 2 , as compared to those of pure Sm metal, suggest a larger lifetime broadening, probably due to an increased number of valence-band electrons, and a larger 4f hybridization, mainly to Co 3d states. Weak satellite structures are observed in the Co 3d valence band and Co 2p core level PES spectra, indicating Co 3d correlation effects

Electronic and magnetic structures of Nd 2 Fe 14 B are investigated by performing self-consistent local-density-approximation band-structure calculations and photoemission-spectroscopy (PES) measurements. Employing the linear muffin-tin-orbital band method, we have obtained electronic band structures for both paramagnetic and ferromagnetic phases of Nd 2 Fe 14 B, and compared the calculated density of states (DOS) with the experimental data obtained from PES measurements. It is found that B atoms contribute to the stabilization of the structure and substantially reduce the magnetic moment of neighboring Fe atoms through a hybridization interaction

The electronic structures of ${\text{SrMn}}_{1\ensuremath{-}x}{\text{Fe}}_{x}{\text{O}}_{3}$ $(0\ensuremath{\le}x\ensuremath{\le}1)$ have been investigated by using photoemission spectroscopy (PES), soft x-ray absorption spectroscopy, and soft x-ray magnetic circular dichroism (XMCD). Both Mn and Fe ions are found to be nearly tetravalent for the whole range of $x$ in ${\text{SrMn}}_{1\ensuremath{-}x}{\text{Fe}}_{x}{\text{O}}_{3}$. Valence-band PES measurements show the broader $\text{Fe}\text{ }3d$ $({t}_{2g}^{3}{e}_{g}^{1})$ bands than the $\text{Mn}\text{ }3d$ $({t}_{2g}^{3})$ bands and the finite metallic intensity near ${E}_{F}$ in the $\text{Fe}\text{ }3d$ PES of ${\text{SrFeO}}_{3}$ in agreement with the metallic ground state of ${\text{SrFeO}}_{3}$. The $\text{Fe}\text{ }2p$ XMCD spectra exhibit the enhanced XMCD effect for $0.6\ensuremath{\le}x\ensuremath{\le}0.8$ in ${\text{SrMn}}_{1\ensuremath{-}x}{\text{Fe}}_{x}{\text{O}}_{3}$, which is consistent with the spin-glass behavior in the intermediate substitution, while the $\text{Mn}\text{ }2p$ states exhibit the negligible XMCD effect.

The valence and spin states of $\mathrm{Mn}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ spinel oxide have been investigated by employing soft x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). The measured $\mathrm{Mn}\phantom{\rule{0.2em}{0ex}}2p$ and $\mathrm{Fe}\phantom{\rule{0.2em}{0ex}}2p$ XAS spectra indicate that Mn and Fe ions are nearly divalent $({\mathrm{Mn}}^{2+})$ and trivalent $({\mathrm{Fe}}^{3+})$, respectively. Our XAS and XMCD spectra for $\mathrm{Mn}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ do not show clear evidence of mixed-valent states but provide evidence for the inversion of both Mn and Fe ions, with ${T}_{d}∕{O}_{h}\ensuremath{\sim}0.8∕0.2$ for Mn ions and ${T}_{d}∕{O}_{h}\ensuremath{\sim}0.1∕0.9$ for Fe ions, respectively. Based on our data, $\mathrm{Mn}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ can be described either by the single-valence states of ${({\mathrm{Mn}}_{0.8}^{2+}{\mathrm{Fe}}_{0.2}^{3+})}_{A}{[{\mathrm{Fe}}_{1.8}^{3+}{\mathrm{Mn}}_{0.2}^{2+}]}_{B}{\mathrm{O}}_{4}$ or by the mixed-valence states of ${({\mathrm{Mn}}_{0.8}^{2+}{\mathrm{Fe}}_{0.2}^{3+})}_{A}{[{\mathrm{Fe}}_{1.6}^{3+}{\mathrm{Fe}}_{0.2}^{2+}{\mathrm{Mn}}_{0.2}^{3+}]}_{B}{\mathrm{O}}_{4}$.

We have performed resonance photoemission, angle-resolved photoemission, and core-level photoemission studies of single-crystalline CoO. On the one hand, strong correlation effects among the {ital d} electrons are observed, as signaled by a strong reduction of Co 3{ital d} bandwidths and satellites in both the valence band and the cation core levels. On the other hand, the oxygen states are found to be very bandlike, as indicated by strong dispersions of oxygen states in the valence band and the lack of oxygen satellites. We give estimations of 30 and 1.5 for {ital U}/{ital W} (Coulomb interaction divided by bandwidth) ratio of Co 3{ital d} bands and O 2{ital p} bands, respectively. By comparing the experimental and theoretical {ital E} versus {bold k} relation, we show that the density-functional band calculation works well for the oxygen bands but not for the Co bands. We argue that CoO is not a band insulator, but a charge-transfer insulator. We have also observed the effects of local magnetic order on the electronic structure. Finally, we suggest a guideline on calculating the band structure of CoO: introducing a mechanism that reduces the Co 3{ital d} bandwidth by 25% while still retaining the other essential features ofmore » the band calculation.« less

Analyse theorique des donnees de spectroscopie electronique des supraconducteurs a fermion lourd, selon le modele de l'impurete de Anderson. Les spectres sont generalement en accord avec une description de liquide de Fermi de Kondo de ce materiau

We present calculations of resonance photoemission spectra for Nd{sub 2}CuO{sub 4}. The calculations use a model Hamiltonian for which all parameters for the valence electrons are obtained from {ital ab initio} calculations or atomic data. Most features of the calculated sp agree well with experiment where comparison is possible. A substantial dependence on the polarization of the light is predicted for the occurrence of resonance behavior, and for the off-resonance intensity of the local singlet and one of the satellites.

We have studied the electronic structure of the heavy-electron compounds Yb{ital X}Cu{sub 4}, with {ital X} = Ag,Au,Pd, using x-ray photoemission and bremsstrahlung isochromat spectroscopies. Consistent with other evidence of small Kondo temperatures, we find that Yb in this system is nearly trivalent and that Kondo features in the Yb 4{ital f} BIS spectra are either absent or weak.

Electronic structures of delafossite oxides $\mathrm{Ag}{\mathrm{Ni}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{O}}_{2}$ and the frustrated Jahn-Teller (JT) system $A\mathrm{Ni}{\mathrm{O}}_{2}$ $(A=\mathrm{Li},\mathrm{Na})$ have been investigated by employing soft x-ray absorption spectroscopy and photoemission spectroscopy (PES). It is found that Ni ions are in the ${\mathrm{Ni}}^{2+}\text{\ensuremath{-}}{\mathrm{Ni}}^{3+}$ mixed-valent states and that the low-spin (LS) ${\mathrm{Ni}}^{3+}$ component increases from $\mathrm{Li}\mathrm{Ni}{\mathrm{O}}_{2}$ to $\mathrm{Ag}\mathrm{Ni}{\mathrm{O}}_{2}$ and $\mathrm{Na}\mathrm{Ni}{\mathrm{O}}_{2}$, in agreement with the presence of the JT transition in $\mathrm{Na}\mathrm{Ni}{\mathrm{O}}_{2}$ and the absence of the JT transition in $\mathrm{Li}\mathrm{Ni}{\mathrm{O}}_{2}$ and $\mathrm{Ag}\mathrm{Ni}{\mathrm{O}}_{2}$. In $\mathrm{Ag}{\mathrm{Ni}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{O}}_{2}$, the ${\mathrm{Ni}}^{3+}$ component increases with $x$, while Co ions are in the LS ${\mathrm{Co}}^{3+}$ states for all $x$, which is consistent with the metallic nature for low values of $x$. A good agreement is found between the measured PES spectra and the calculated local spin density approximation (LSDA) electronic structures of $\mathrm{Ag}\mathrm{Ni}{\mathrm{O}}_{2}$ and $\mathrm{Ag}\mathrm{Co}{\mathrm{O}}_{2}$, but the pseudogap feature in PES of $\mathrm{Ag}\mathrm{Ni}{\mathrm{O}}_{2}$ is not described by the LSDA.

The electronic structure of an $f$-electron charge-density-wave (CDW) system, $\mathrm{Ce}{\mathrm{Te}}_{2}$, has been investigated using high-resolution angle-resolved photoemission spectroscopy (ARPES). Two gaps of $\ensuremath{\sim}100$ and $\ensuremath{\sim}500\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ have been observed both along $\ensuremath{\Gamma}X$ and $\ensuremath{\Gamma}Y$, implying the $2\ifmmode\times\else\texttimes\fi{}2$ lattice deformation in the Te(1) sheets with the CDW gap of $\ensuremath{\approx}100\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. By employing the fine-structure (FS) resonance, the experimental Ce $4f$ ARPES spectra have been measured. The FS resonance ARPES shows a typical two-peak structure with the peaks at $\ensuremath{\sim}\ensuremath{-}4$ and $\ensuremath{\sim}\ensuremath{-}1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, and the weight shift in the latter peak in contrast to the traditional giant resonance. The Ce $4f$ states have the negligibly small intensity near ${E}_{\mathrm{F}}$, reflecting the minor contribution from Ce $4f$ electrons to the metallic ground state of $\mathrm{Ce}{\mathrm{Te}}_{2}$. This study reveals that the carriers near ${E}_{\mathrm{F}}$ should have mainly the Te(1) $5p$ and Ce $5d$ character, but that only the Ce $5d$ bands cross ${E}_{\mathrm{F}}$.

The electronic structure of ${\mathrm{La}}_{0.7}{\mathrm{Ce}}_{0.3}{\mathrm{MnO}}_{3}$ (LCeMO) thin films has been investigated using photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). The Ce $3d$ core-level PES and XAS spectra of LCeMO are very similar to those of ${\mathrm{CeO}}_{2},$ indicating that Ce ions are close to being tetravalent. A very weak $4f$ resonance is observed around the Ce $4\stackrel{\ensuremath{\rightarrow}}{d}4f$ absorption edge, suggesting that the localized Ce $4f$ states are almost empty in the ground state. The Mn $2p$ XAS spectrum reveals the existence of the ${\mathrm{Mn}}^{2+}$ multiplet feature, confirming the ${\mathrm{Mn}}^{2+}\ensuremath{-}{\mathrm{Mn}}^{3+}$ mixed-valent states of Mn ions in LCeMO. The measured Mn $3d$ PES/XAS spectra for LCeMO agree reasonably well with the Mn $3d$ partial density of states calculated using the $\mathrm{LSDA}+U$ (LSDA, local spin-density approximation) method. The $\mathrm{LSDA}+U$ calculation predicts a half-metallic ground state for LCeMO. This study confirms that the LCeMO films are indeed electron doped.

Electronic structures of very dilute Cr- or Ru-doped ${\mathrm{Nd}}_{1/2}{A}_{1/2}{\mathrm{MnO}}_{3}$ (NAMO) $(A=\mathrm{Ca},\mathrm{Sr})$ manganites have been investigated using the Mn and Cr $2\stackrel{\ensuremath{\rightarrow}}{p}3d$ resonant photoemission spectroscopy (PES). All the Cr- and Ru-doped NAMO systems exhibit the clear metallic Fermi edges in the Mn ${e}_{g}$ spectra near ${E}_{F},$ consistent with their metallic ground states. The Cr $3d$ states with ${t}_{2g}^{3}$ configuration are at $\ensuremath{\sim}1.3\mathrm{eV}$ below ${E}_{F},$ and the Cr ${e}_{g}$ states do not participate in the formation of the band near ${E}_{F}.$ Cr- and Ru-induced ferromagnetism and insulator-to-metal transitions can be understood based on their measured electronic structures.

The electronic structures of ${\mathrm{Sr}}_{2}{\mathrm{FeMoO}}_{6}$ (SFMO) and ${\mathrm{Ba}}_{2}{\mathrm{FeMoO}}_{6}$ (BFMO) double perovskites have been investigated using Fe $2\stackrel{\ensuremath{\rightarrow}}{p}3d$ resonant photoemission spectroscopy (PES) and the Cooper minimum in the Mo $4d$ photoionization cross section. The states close to the Fermi level are found to have a strongly mixed Mo\char21{}Fe ${t}_{2g}$ character, indicating that the Fe valence is far from pure $3+.$ The Fe ${2p}_{3/2}$ x-ray absorption spectroscopy spectra reveal the mixed-valent ${\mathrm{Fe}}^{3+}\char21{}{\mathrm{Fe}}^{2+}$ configurations, and a larger ${\mathrm{Fe}}^{2+}$ component for BFMO than for SFMO, suggesting an operative double exchange interaction. The valence-band PES spectra reveal good agreement with the local-spin-density-approximation $(\mathrm{LSDA})+U$ calculation.

The electronic structures of Mo pyrochlore systems of ${R}_{2}{\mathrm{Mo}}_{2}{\mathrm{O}}_{7}$ $(R=\mathrm{Nd},$ Sm) have been investigated using photoemission spectroscopy (PES). The electronic states near ${E}_{\mathrm{F}}$ have mainly Mo $4d$ character. The R $4f$ states are located well below the Fermi level and the valence states of bulk Nd and Sm ions are very close to $3+.$ The local spin density approximation (LSDA) and $\mathrm{LSDA}+U+\mathrm{s}\mathrm{p}\mathrm{i}\mathrm{n}\ensuremath{-}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{i}\mathrm{t}$ (SO) band calculations incorporating the effects of the on-site Coulomb correlation U and the SO interaction of Nd $4f$ electrons show the large hybridization between the Mo $4d$ and O $2p$ orbitals and the magnetic moment of $\ensuremath{\sim}1.0{\ensuremath{\mu}}_{B} \mathrm{per}$ Mo atom. The qualitative features of the measured valence-band PES spectra of ${\mathrm{Nd}}_{2}{\mathrm{Mo}}_{2}{\mathrm{O}}_{7}$ agree well with the $\mathrm{LSDA}+U+\mathrm{SO}$ band structure calculation. The origin of the stable ferromagnetic phase in ${\mathrm{Nd}}_{2}{\mathrm{Mo}}_{2}{\mathrm{O}}_{7}$ has been discussed based on the nearly half-metallic electronic structure.

The electronic structure of UNiSn has been investigated using photoemission spectroscopy (PES). The U 5f partial spectral weight (PSW) exhibits a broad peak centered at approximate to0.3 eV below E-F. The Ni 3d PSW shows the main peak well below E-F and a very low density of states (DOS) at E-F. The h nu dependence of the valence-band spectrum reveals a dominant U 5f electron character for the states near the Fermi level E-F, with a small contribution from the U 6d, Ni 3d, and Sn sp states. Comparison of the measured PES spectra to the LSDA+ U band structure calculation indicates the importance of the on-site Coulomb interaction between U 5f electrons in UNiSn. The high-resolution photoemission spectrum of UNiSn is described well by a V-shaped metallic DOS near E-F, suggesting a finite but reduced DOS at E-F. A possible origin for the reduced DOS at E-F might be the hybridization of the U 5f states to the Ni 3d states that have a very low DOS at E-F. T-dependent high-resolution PES for UNiSn reveals a finite DOS at E-F even above T-N.

The electronic structure of ${\mathrm{Ba}}_{2}{\mathrm{FeMoO}}_{6}$ (BFMO) has been investigated by using photoemission spectroscopy (PES). By varying $h\ensuremath{\nu}$ across the Mo $4d$ Cooper minimum, it is found that the states close to the Fermi level ${E}_{F}$ are predominantly of the Mo ${t}_{2g}\ensuremath{\downarrow}$ and Fe ${t}_{2g}\ensuremath{\downarrow}$ character. The measured PES spectrum is compared to the calculated electronic structure obtained in the local spin-density approximation (LSDA) and $\mathrm{LSDA}+U$ methods. The $\mathrm{LSDA}+U$ calculation yields better agreement with experiment in the peak positions than does the LSDA calculation. The present study supports the double exchange mechanism for the half-metallic ferrimagnetism in BFMO.

Electronic structures of the giant magnetoresistive ${\mathrm{Fe}}_{0.5}{\mathrm{Cu}}_{0.5}{\mathrm{Cr}}_{2}{\mathrm{S}}_{4}$ (FCCS) spinel compound have been investigated using photoemission spectroscopy (PES). Resonant PES measurements around the Cu, Fe, Cr $3\stackrel{\ensuremath{\rightarrow}}{p}3d$ absorption edges exhibit negligible resonant interference behavior for the Cu $3d$ valence electrons, indicating the monovalent valence state of the Cu ion in FCCS. The top of the valence band is found to be predominantly of the Cr $3d$ and the nearly filled Cu $3d$ electron character, whereas the Fe $3d$ electron character is distributed over the whole valence band. The measured valence-band PES spectra of FCCS yield better agreement with the $\mathrm{LSDA}+U$ calculation than with the local spin-density approximation (LSDA) calculation, suggesting the importance of the large Coulomb interactions U between d electrons. On the other hand, the low spectral intensity near ${E}_{F}$ in the measured valence-band spectrum suggests an extra localization in FCCS, not explained by the large U alone.

We describe valence-band and core-level photoemission data for copper oxide superconductors using the Anderson Hamiltonian applied to an impurity-cluster configuration-interaction model. We obtain experimental values of the parameters of the model, the copper\ensuremath{\rightleftarrows}oxygen chargetransfer energy $\ensuremath{\Delta}\ensuremath{\sim}0.4$ eV, the $d\ensuremath{-}d$ Coulomb interaction $U\ensuremath{\sim}6$ eV, and the ligand-$d$ hybridization $T\ensuremath{\sim}2.4$ eV. Using these parameters, we evaluate the linear Cu-O-Cu superexchange interaction $J$ and find it is dominated by the charge-transfer fluctuations. The magnitude obtained for $J$ is much larger than typical N\'eel temperatures of these materials, and is somewhat larger than that estimated from applying the resonating-valence-bond picture to ${\mathrm{La}}_{2}$Cu${\mathrm{O}}_{4}$. We point out that for $\ensuremath{\Delta}\ensuremath{\ll}U$ and $T\ensuremath{\gg}\ensuremath{\Delta}$ the charge-transfer degrees of freedom, and the lattice aspects of the Anderson lattice Hamiltonian, should not be neglected in constructing models for the high-${T}_{c}$ superconductivity. We also emphasize our resonant-photoemission result that the very small density of states at or near the Fermi level in all these materials has a substantial contribution from Cu $3d$ states, suggesting their importance for the superconductivity. We report other details of the resonant-photoemission data involving La and Ba states in the materials containing these elements.

The $5f$-electron photoemission peak 1 eV below the Fermi energy ${E}_{F}$ in U${\mathrm{Pd}}_{3}$ is generally assigned as a $5{f}^{2}\ensuremath{\rightarrow}5{f}^{1}$ transition, consistent with other evidence of a stable ${\mathrm{U}}^{4+}$ valence state. We report resonant photoemission spectra for the system ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{U}}_{x}{\mathrm{Pd}}_{3}$ which show that the peak narrows and shifts to the Fermi level as $x$ decreases from 1. We hypothesize that the shift arises from a decrease in the Fermi energy as ${\mathrm{Y}}^{3+}$ replaces ${\mathrm{U}}^{4+}$, and therefore expect interesting changes in transport properties as ${E}_{F}$ is tuned in this way.

Photoemission measurements in the constant-final-state (absorption) mode were performed on three different classes of high-temperature superconductors ${\mathrm{Bi}}_{2.0}$${\mathrm{Sr}}_{1.8}$${\mathrm{Ca}}_{0.8}$${\mathrm{La}}_{0.3}$${\mathrm{Cu}}_{2.1}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$, ${\mathrm{BaBiO}}_{3}$, and ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4}$ using synchrotron radiation from 20 to 200 eV. Absorption signals from all elements but Ce are identified. The results firmly show that the Bi 6s electrons are more delocalized in ${\mathrm{BaBiO}}_{3}$ than in ${\mathrm{Bi}}_{2.0}$${\mathrm{Sr}}_{1.8}$${\mathrm{Ca}}_{0.8}$${\mathrm{La}}_{0.3}$${\mathrm{Cu}}_{2.1}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$, in agreement with the results of band-structure calculations. Differences in the absorption signals due to O and Bi excitations between ${\mathrm{BaBiO}}_{3}$ and ${\mathrm{Bi}}_{2.0}$${\mathrm{Sr}}_{1.8}$${\mathrm{Ca}}_{0.8}$${\mathrm{La}}_{0.3}$${\mathrm{Cu}}_{2.1}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ are discussed. Delayed absorption onsets attributed to giant resonances (Ba 4d\ensuremath{\rightarrow}4f, La 4d\ensuremath{\rightarrow}4f, and Nd 4d\ensuremath{\rightarrow}4f transitions) are also reported.

Anderson Hamiltonian parameters for mixed-valence ${\mathrm{YbAl}}_{3}$ were determined from the magnetic susceptibility, photoelectron-spectroscopy, and bremsstrahlung-isochromat-spectroscopy data. The 4${f}^{13}$\ensuremath{\rightarrow}4${f}^{14}$ electron affinity energy is \ensuremath{\sim}0.5 eV and the hybridization between Yb 4f and Al conduction electrons is \ensuremath{\sim}0.05 eV, both of which are larger than conventional estimates. This hybridization and the resulting Kondo resonance are considered to play important roles for the anomalous properties of valence-fluctuating Yb compounds, as in the case of Ce compounds.