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Quantum mechanics tells us that the hopping integral between local orbitals makes the energy band dispersive. In a lattice with geometric frustration, however, dispersionless flat bands may appear due to quantum interference. Several models possessing flat bands have been proposed theoretically, and many attracting magnetic and electronic properties are predicted. However, despite many attempts to realize these models experimentally, compounds that are appropriately described by this model have not been found so far. Here we show that pyrochlore oxides ${\mathrm{Sn}}_{2}{\mathrm{Nb}}_{2}{\mathrm{O}}_{7}$ and ${\mathrm{Sn}}_{2}{\mathrm{Ta}}_{2}{\mathrm{O}}_{7}$ are such examples, by performing first-principles band calculation and several tight-binding analyses. Moreover, spin-polarized band calculation shows that the hole-doped systems ${\mathrm{Sn}}_{2}{\mathrm{Nb}}_{2}{\mathrm{O}}_{6}\mathrm{N}$ and ${\mathrm{Sn}}_{2}{\mathrm{Ta}}_{2}{\mathrm{O}}_{6}\mathrm{N}$ have complete spin polarization, and their magnetic moments are mostly carried by Sn-$s$ and N-$p$ orbitals, which are usually nonmagnetic. These compounds are not only candidates for ferromagnets without a magnetic element, but also will provide an experimental platform for a flat-band model which shows a wide range of physical properties.

By controlling lattice defects through the annealing condition during crystal growth, a wide-gap and $p$-type semiconductor ${\mathrm{Sn}}_{2}({\mathrm{Nb}}_{2\text{\ensuremath{-}}x}{\mathrm{Ta}}_{x}){\mathrm{O}}_{7}$ $(x=$ $0--2)$ was developed. The band gap is variable in the range from 2.4 eV for ${\mathrm{Sn}}_{2}{\mathrm{Nb}}_{2}{\mathrm{O}}_{7}$ $(x=0)$ to 3.0 eV for ${\mathrm{Sn}}_{2}{\mathrm{Ta}}_{2}{\mathrm{O}}_{7}$ $(x=2)$, respectively. The carrier density and the hole mobility were estimated to be $0.20--1.4$ $(\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3})$ and $0.28--1.9$ $(\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}1}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1})$, respectively. From the mutual relation between the defect content and the electrical properties, we concluded that the hole carriers in the $p$-type semiconducting ${\mathrm{Sn}}_{2}({\mathrm{Nb}}_{2\text{\ensuremath{-}}x}{\mathrm{Ta}}_{x}){\mathrm{O}}_{7}$ are generated by ${\mathrm{Sn}}^{4+}$-on-${\mathrm{Nb}}^{5+}/{\mathrm{Ta}}^{5+}$ substitutional defects in (Nb,${\mathrm{Ta})}_{2}{\mathrm{O}}_{6}$ octahedral chains.

We have prepared oxygen isotope exchanged crystals of impurity-free ${\text{YBa}}_{2}$${\text{Cu}}_{3}$${\text{O}}_{y}$ with various oxygen contents, and examined pure doping ($p$) dependence of isotope effect on superconducting transition temperature. With decreasing oxygen contents, the isotope exponent $\ensuremath{\alpha}$ monotonously increases without any anomaly around $p=1/8$. The monotonous increase in $\ensuremath{\alpha}$ indicates that phonons are involved in the mechanism which causes the monotonous ${T}_{c}$ suppression with underdoping.

We present a reexamination of the electronic structure and Fermi surface (FS) topology of Bi-Sr-Ca-Cu-O (BSCCO) as obtained from angle-resolved photoemission experiments. By applying a stricter set of FS crossing criteria as well as by varying the incident photon energy outside the usual range, we have found very different behavior from that previously observed. In particular, we have found a FS that is centered around the $\ensuremath{\Gamma}$ point and contains electronlike portions, and we observe a depletion of spectral weight around $\overline{M}$. The flat bands observed at other photon energies may indicate the presence of two electronic components in the cuprates.

We use the tomographic density of states (TDOS), which is a measure of the density of states for a single slice through the band structure of a solid, to study the temperature evolution of the superconducting gap in the cuprates. The TDOS provides accuracy in determining both the superconducting pair-forming strength $\ensuremath{\Delta}$ and the pair-breaking rate $\ensuremath{\Gamma}$. In both optimally and underdoped Bi${}_{2}$Sr${}_{2}$CaCu${}_{2}$O${}_{8+\ensuremath{\delta}}$, we find the near-nodal $\ensuremath{\Delta}$ smoothly evolves through the superconducting transition temperature---clear evidence for the existence of preformed pairs. Additionally, we find the long-observed ``filling'' of the superconducting gap in the cuprates is due to the strongly temperature-dependent $\ensuremath{\Gamma}$.

In this study, we investigate the electronic structure and electron self-energy of palladium single crystals using polarization-dependent high-resolution angle-resolved photoemission spectroscopy. The observed Fermi surfaces and energy-band dispersions agree with those given by the band-structure calculation. A detailed comparison between the observed and theoretical band dispersions of the ${\ensuremath{\Sigma}}_{1}$ band forming the electronlike Fermi surface indicates an electron-electron coupling parameter of ${\ensuremath{\lambda}}_{ee}\ensuremath{\sim}0.02$. Near the Fermi level, a kink structure in the energy-band dispersion exists at $\ensuremath{\sim}\ensuremath{-}20$ meV, in agreement with the Debye energy. The electron-phonon coupling parameter is estimated to be ${\ensuremath{\lambda}}_{ep}=0.39\ifmmode\pm\else\textpm\fi{}0.05$ at 8 K for the ${\ensuremath{\Sigma}}_{1}$ band, which is consistent with the theoretical values of ${\ensuremath{\lambda}}_{ep}=0.35--0.41$. Furthermore, analyses of the self-energy indicate a possible contribution from the electron-paramagnon interaction in the energy range of $\ensuremath{-}50\ensuremath{\sim}\ensuremath{-}150$ meV. The evaluated electron-paramagnon coupling parameter is ${\ensuremath{\lambda}}_{em}\ensuremath{\sim}0.06$ for the ${\ensuremath{\Sigma}}_{1}$ band. We found that the magnitudes of ${\ensuremath{\lambda}}_{ep}$ and ${\ensuremath{\lambda}}_{em}$ depend on the Fermi surface points. The total effective mass enhancement factor is estimated to be $1+{\ensuremath{\lambda}}_{ep}+{\ensuremath{\lambda}}_{ee}+{\ensuremath{\lambda}}_{em}\ensuremath{\sim}1.5$ for the ${\ensuremath{\Sigma}}_{1}$ band, which is close to the values ${m}^{*}/{m}_{b}\ensuremath{\sim}1.5--1.7$ given by the de Haas--van Alphen and electron specific-heat measurements.

Angle-resolved photoemission spectra of ${\mathrm{SrTiO}}_{3}$ (100) and (110) surfaces have been obtained. The normal-emission data are analyzed using the direct transition model and, as a result, the valence band is determined along the \ensuremath{\Delta} and \ensuremath{\Sigma} lines in the bulk Brillouin zone. The empirical valence-band structure is found to be consistent with the band calculations if and only if proper magnitude of the O 2p crystal-field splitting is adopted. The effect of the O 2p crystal-field splitting on the band structure is discussed. \textcopyright{} 1996 The American Physical Society.

We present an experimental manifestation of how the single-particle density of states around the Fermi level $({E}_{F})$ changes near a Mott transition. In a metallic perovskite-type ${3d}^{1}$ Mott-Hubbard system ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{VO}}_{3}$, as one decreases $x$ the spectral weight of the coherent band near ${E}_{F}$ is gradually transferred to that of the precursor of the lower Hubbard band $\ensuremath{\sim}$2 eV below ${E}_{F}$. The momentum dependence of the quasiparticle self-energy and hence the long-range exchange or correlation is shown to become progressively important as one approaches the Mott transition from the metallic side.

There are two structures in the photoemission spectra of CaVO 3-δ and SrVO 3-δ in the d-band region, One peak is within ∼1 eV of the Fermi level and has a sharp Fermi cutoff. The other is a broad peak centered at ∼1.6 eV. From our photoemission spectra of CaVO 3-δ and SrVO 3-δ both with oxygen vacancies (δ=∼0.1) and without vacancies (6 = 0), it is shown that the relative intensity of the peak near the Fermi level for CaVO 3-δ drastically decreases with decreasing oxygen vacancy concentration (δ)

In order to clarify many-body interactions in ferromagnetic iron, we performed high-resolution angle-resolved photoemission spectroscopy (ARPES) on a Fe(110) single crystal. We quantitatively analyzed the ARPES line shapes for a majority-spin band crossing the Fermi level $({E}_{F})$. The observed group velocity was reduced with respect to the band-structure calculation by a factor of 1/2.7, giving a coupling parameter due to the electron-electron interaction of ${\ensuremath{\lambda}}_{ee}=1.7\ifmmode\pm\else\textpm\fi{}0.1$. This suggests a strong electron correlation in $\text{Fe}\text{ }3d$, which is consistent with recent high-resolution ARPES results. The real and imaginary parts of the self-energy have been experimentally evaluated near ${E}_{F}$. The coupling parameter of the electron-phonon interaction was estimated to be ${\ensuremath{\lambda}}_{ep}=0.16\ifmmode\pm\else\textpm\fi{}0.02$, which is 1/10 of ${\ensuremath{\lambda}}_{ee}$. The effective-mass enhancement with respect to the band mass ${m}^{\ensuremath{\ast}}/{m}_{b}\ensuremath{\sim}1+{\ensuremath{\lambda}}_{ee}+{\ensuremath{\lambda}}_{ep}\ensuremath{\sim}3$ was, therefore, mainly caused by electron correlation.

Using polarization- and hν-dependent angle-resolved photoemission spectroscopy, we uncovered the fine details of a quasiparticle's dynamics of a typical multiband superconductor, Sr2RuO4. We found strong hybridization between the in-plane and out-of-plane quasiparticles via the Coulomb and spin-orbit interactions. This effect enhances the quasiparticle mass due to the inflow of out-of-plane quasiparticles into the two-dimensional Fermi surface sheet, where the quasiparticles are further subjected to the multiple electron-boson interactions. We suggest that the spin-triplet p-wave superconductivity of Sr2RuO4 is phonon mediated.

Using low photon energy angle-resolved photoemission, we study the low-energy dispersion along the nodal ($\ensuremath{\pi},\ensuremath{\pi}$) direction in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ as a function of temperature. Less than 10 meV below the Fermi energy, the high-resolution data reveal a novel ``kinklike'' feature in the electron self-energy that is distinct from the larger well-known kink roughly 70 meV below ${E}_{F}$. This new kink is strongest below the superconducting critical temperature and weakens substantially at higher temperatures. A corollary of this finding is that the Fermi velocity ${v}_{F}$, as measured in this low-energy range, varies rapidly with temperature---increasing by almost 30% from 70 to 110 K. The behavior of ${v}_{F}(T)$ appears to shift as a function of doping, suggesting a departure from simple ``universality'' in the nodal Fermi velocity of cuprates.

In order to investigate the change in the electronic structure with Mn doping of ${\text{SrRu}}_{1\ensuremath{-}x}{\text{Mn}}_{x}{\text{O}}_{3}$ (SRMO), we have performed soft x-ray synchrotron photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). In the soft x-ray PES spectra, the sharp peak structure near the Fermi level $({E}_{\text{F}})$, which was clearly observed in ${\text{SrRuO}}_{3}$, was suppressed accompanied with Mn doping in SRMO. In the $\text{Mn}\text{ }2p$ XAS and $\text{Mn}\text{ }2p\text{\ensuremath{-}}3d$ resonant PES spectra, we have confirmed the existence of ${\text{Mn}}^{3+}$ ions with the electron doping to $\text{Mn}\text{ }3d\text{ }{e}_{g}$ states and $\text{Mn}\text{ }3d$ partial density of state never have finite intensity at ${E}_{\text{F}}$ in SRMO. These results suggest that the metal-insulator transition of SRMO with Mn doping is originated from the charge transfer from the itinerant $\text{Ru}\text{ }4d\text{ }{t}_{2g}$ bands to the localized $\text{Mn}\text{ }3d\text{ }{e}_{g}$ orbitals.

The disorder dependence of the oxygen isotope effect in optimally doped ${\text{Bi}}_{2}{\text{Sr}}_{1.6}{\text{Ln}}_{0.4}{\text{CuO}}_{6+\ensuremath{\delta}}$ superconductors which is introduced into cation sites in the planes adjacent to ${\text{CuO}}_{2}$ planes is studied. The degree of out-of-plane disorder is controlled by substituting Ln ions with different ionic radii from La to Gd. This type of disorder does not explicitly alter the nodal electronic structure or the underlying Fermi surface but markedly enhances the oxygen isotope exponent at the superconducting transition temperature $({\ensuremath{\alpha}}_{{T}_{c}})$. The enhancement in ${\ensuremath{\alpha}}_{{T}_{c}}$ is well explained by the pair-breaking model, suggesting the growth of a pseudogap state around the antinodal region due to the out-of-plane disorder.

We investigate the electronic dispersion of high-Tc superconductor on the basis of the two-dimensional three-band Hubbard model with the electron-phonon interaction together with the strong electron-electron interaction. In our model, it is shown across the hole-doped region of high-Tc superconductor that the electron-phonon interaction makes a dispersion kink, observed along the nodal direction, and that the small isotope effect appears on the electronic dispersion.

The electronic structure of ${\mathrm{Nd}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$(001) single-crystal thin films, which show (1\ifmmode\times\else\texttimes\fi{}1) low-energy electron diffraction patterns, has been studied by angle-resolved ultraviolet photoemission spectroscopy with synchrotron radiation. The data show a well-defined but resolution-limited Fermi edge with a maximum intensity of more than 6% as compared to the main peak intensity. The main peaks exhibit ill-defined dispersion of less than 0.4 eV. All in all, the band calculation can describe the experimental spectra to a fair degree. We find a dispersive bandlike Fermi-liquid state of dominant Cu 3d character (almost no O 2p and Cu 4s character) just below the Fermi level. Its dispersion of 0.2 eV is 8--10 times smaller than that expected from the band calculation. These facts indicate that there is a possibility of Kondo-type renormalized heavy-electron band formation at the Fermi level, caused by spin fluctuations.

A new low photon energy regime of angle-resolved photoemission spectroscopy is accessed with lasers and used to study the high ${T}_{c}$ superconductor ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$. The low energy increases bulk sensitivity, reduces background, and improves resolution. With this we observe spectral peaks which are sharp on the scale of their binding energy---the clearest evidence yet for quasiparticles in the normal state. Crucial aspects of the data such as the dispersion, superconducting gaps, and the bosonic coupling kink are found to be robust to a possible breakdown of the sudden approximation.

We present detailed energy dispersions near the Fermi level on the monolayer perovskite ruthenate ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$, determined by high-resolution angle-resolved photoemission spectroscopy. An orbital selectivity of the kink in the dispersion of ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ has been found: A kink for the Ru $4{d}_{xy}$ orbital is clearly observed, but not for the Ru $4{d}_{yz}$ and $4{d}_{zx}$ ones. The result provides insight into the origin of the kink.

We have investigated the electronic structure of electron-doped ${\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{La}}_{x}\mathrm{Fe}\mathrm{Mo}{\mathrm{O}}_{6}$ ($x=0.0$ and 0.2) by photoemission spectroscopy and band-structure calculations within the local density $\text{approximation}+U$ scheme. A characteristic double-peak feature near the Fermi level $({E}_{\mathrm{F}})$ has been observed in the valence-band photoemission spectra of both $x=0.0$ and 0.2 samples. A photon-energy dependence of the spectra in the $\mathrm{Mo}\phantom{\rule{0.2em}{0ex}}4d$ Cooper minimum region compared with the band-structure calculations has shown that the first peak crossing ${E}_{\mathrm{F}}$ consists of the $(\mathrm{Fe}+\mathrm{Mo})$ ${t}_{2g\ensuremath{\downarrow}}$ states (feature A) and the second peak well below ${E}_{\mathrm{F}}$ is dominated by the $\mathrm{Fe}\phantom{\rule{0.2em}{0ex}}{e}_{g\ensuremath{\uparrow}}$ states (feature B). Upon La substitution, the feature A moves away from ${E}_{\mathrm{F}}$ by $\ensuremath{\sim}50\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, which is smaller than the prediction of our band theory, $112\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. In addition, an intensity enhancement of both A and B has been observed, although B does not cross ${E}_{\mathrm{F}}$. Those two facts are apparently incompatible with the simple rigid-band shift due to electron doping. We point out that such phenomena can be understood in terms of the strong Hund's rule energy stabilization in the $3{d}^{5}$ configuration at the Fe sites in this compound. From an observed band-narrowing, we have also deduced a mass enhancement of $\ensuremath{\sim}2.5$ with respect to the band theory, in good agreement with a specific heat measurement.

We have carried out extensive high-resolution angle-resolved photoemission (ARPES) experiments on ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ samples, covering the entire doping range from the overdoped to the optimally and underdoped regimes in the normal state. Our focus is on delineating the doping dependence of the bilayer splitting which is associated with the intracell coupling of electrons between the two ${\mathrm{CuO}}_{2}$ planes. We exploit the photon energy of 47 eV, where strong ARPES matrix element effects are found to provide a tremendous enhancement of the antibonding to bonding component of the bilayer split bands near $(\ensuremath{\pi},0),$ in good agreement with the predictions of corresponding first-principles simulations. Our detailed analysis indicates that the size of the bilayer splitting is only weakly dependent on the doping level, implying that electronic excitations continue to maintain some degree of coherence even in the underdoped regime.

We present a detailed angle-resolved photoemission study on the layered transition-metal dichalcogenide 1T-TaS1.2Se0.8 in the commensurate charge-density-wave (CDW) phase. A drastic reduction in the spectral weight along the high symmetry line GammaM, particularly around the point M, is observed when s-polarized light was used. This implies that the initial state must be symmetric with respect to a mirror plane perpendicular to the line GammaK, which is consistent with conventional band calculations in the absence of the CDW. We conclude that there is only a limited amount of modification of the electronic structure of 1T-TaS1.2Se0.8 in the commensurate CDW phase due to the CDW-related potential.

Using high-resolution angle-resolved photoemission spectroscopy on ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}},$ we observe a new mass renormalization or ``kink'' in the E vs $\stackrel{\ensuremath{\rightarrow}}{k}$ dispersion relations localized near $(\ensuremath{\pi},0).$ The resolution of bilayer splitting allowed the first direct measurements of this interaction effect. The kink is clearly stronger than the kink observed along the nodal direction, appears at a lower energy (near 40 meV for overdoped samples), and is only apparent in the superconducting state. The kink energy scale defines a cutoff below which well-defined quasiparticle excitations occur. The most likely origin of this effect is coupling to the magnetic-resonance mode observed in inelastic neutron scattering.

We have investigated the electronic structure of the commensurate charge-density-wave (CDW) phase for $1T\ensuremath{-}{\mathrm{TaSe}}_{2}$ by Ta $4f$ core-level and the angle-resolved photoemission. From the energy shifts of Ta $4f$ core levels due to a CDW formation, it is found that the CDW amplitude in the $1T\ensuremath{-}{\mathrm{TaSe}}_{2}$ is smaller than that in $1T\ensuremath{-}{\mathrm{TaS}}_{2}$ reported previously. The band dispersion along the normal to the two-dimensional layers indicates that $1T\ensuremath{-}{\mathrm{TaSe}}_{2}$ has a three-dimensional (3D) character in the Fermi surface compared with $1T\ensuremath{-}{\mathrm{TaS}}_{2},$ which has a two-dimensional Fermi surface. The 3D Fermi surface of $1T\ensuremath{-}{\mathrm{TaSe}}_{2}$ is thought to originate from the larger interaction between layers due to a large charge transfer between Ta $5d$ and Se $4p$ orbitals.

We have investigated the electronic structure of ${\mathrm{Sr}}_{2}{\mathrm{FeMoO}}_{6}$ by photoemission spectroscopy and band-structure calculations within the local-density $\mathrm{approximation}+U$ $(\mathrm{LDA}+U)$ scheme. In valence-band photoemission spectra, a distinct double-peak feature has been observed near the Fermi level ${(E}_{F}).$ A photon-energy dependence of the spectra and the $\mathrm{LDA}+U$ band-structure calculation have revealed that the first peak crossing ${E}_{F}$ consists of the (Fe+Mo) ${t}_{2g\ensuremath{\downarrow}}$ states and the second peak well below ${E}_{F}$ is dominated by the Fe ${e}_{g\ensuremath{\uparrow}}$ states. This clearly shows that only the down-spin states contribute to the ${E}_{F}$ intensity, thus the half-metallic density of states (DOS) is realized. We point out that the observed half-metallic DOS can be attributed to the strong Hund's rule energy stabilization due to the high-spin ${3d}^{5}$ configuration at the Fe site.

We present high resolution angle resolved photoemission data of the bilayer superconductor ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ (Bi2212) showing a clear doubling of the near ${E}_{F}$ bands. This splitting approaches zero along the $(0,0)\ensuremath{\rightarrow}(\ensuremath{\pi},\ensuremath{\pi})$ nodal line and is not observed in single layer ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Cu}{\mathrm{O}}_{6+\ensuremath{\delta}}$ (Bi2201), indicating that the splitting is due to the long sought after bilayer splitting effect. The splitting has a magnitude of approximately 75 meV near the middle of the zone, extrapolating to about 110 meV near the $(\ensuremath{\pi},0)$ point. The existence of these two bands also helps to clear up the recent controversy concerning the topology of the Fermi surface.

The electronic state of oxygen adsorbed on a Cr(110) surface at 80 K has been studied by angle-resolved photoemission spectroscopy with synchrotron radiation. Dissociative chemisorption occurs up to \ensuremath{\sim}2 L, and then oxidation starts at \ensuremath{\sim}4 L, leading to the formation of a disordered ${\mathrm{Cr}}_{2}$${\mathrm{O}}_{3}$ layer. An emission at \ensuremath{\sim}7.5 eV below the Fermi energy, being attributable to a chemisorbed ${\mathrm{O}}_{2}$ molecule, was observed at above \ensuremath{\sim}20 L. Dispersion effects on the ${\mathrm{O}}_{2}$p--derived levels were found for the Cr(110)p(4\ifmmode\times\else\texttimes\fi{}2)-O and faceted Cr(110)-O surfaces produced by annealing.

The electronic energy-band properties of epitaxially grown YBa/sub 2/CuO/sub 3/O/sub 7-//sub x/(001) single-crystal thin films have been investigated by angle-resolved ultraviolet photoemission spectroscopy (ARUPS) with synchrotron radiation as the light source. The (001) surfaces exhibit sharp (1 x 1) low-energy electron-diffraction patterns. Our ARUPS data show a clear Fermi edge and dispersive nature of the valence bands, suggesting the validity of the band concept for this oxide. We determined energy-band dispersion along the GAMMA-X-M-GAMMA directions in the bulk Brillouin zone. The experimental results are compared with the existing ground-state band calculations.

Angle-resolved photoemission from epitaxially grown $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}(001)$ single-crystal thin films reveals a clear Fermi edge and fine structure near the edge. Some of the valence-band photoemission features show $k$-space dispersion. These new results suggest that the electronic structure of this oxide can be treated within the framework of the one-electron band picture.

The electronic state of the hydrogen-adsorbed Cr(110) surface at 80 K has been investigated using angle-resolved photoemission spectroscopy. We observed a p(2\ifmmode\times\else\texttimes\fi{}2) low-energy electron-diffraction pattern at \ensuremath{\sim}2 L, and a ``streaked'' (1\ifmmode\times\else\texttimes\fi{}1) pattern along the [11\ifmmode\bar\else\textasciimacron\fi{}0] azimuth at above 10 L. The photoemission results show that for the p(2\ifmmode\times\else\texttimes\fi{}2) phase an H 1s--derived state, having a nearly constant binding energy of \ensuremath{\sim}5.5 eV below the Fermi level, is observed in the outer part of the surface Brillouin zone, that is, outside the Cr s-p band projections. [Here 1 langmuir (L)\ensuremath{\equiv}${10}^{\mathrm{\ensuremath{-}}6}$ Torr sec.] On the other hand, for the streaked (1\ifmmode\times\else\texttimes\fi{}1) phase, an H 1s--derived state, split off from the Cr bulk bands, is found at the binding energy of 7.8 eV at \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{} and shows considerable dispersion of \ensuremath{\sim}2.8 eV in the [001] azimuth being consistent with the periodicity of the substrate structure. In addition, a H-induced surface state is found at 2.5 eV at \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{} for both phases. The nature of the Cr-H chemisorption bond is discussed.

Two-dimensional band dispersion was studied by angle-resolved photoemission for a p(2\ifmmode\times\else\texttimes\fi{}2)-Xe overlayer on Cr(110) with a Xe-Xe nearest-neighbor distance of 4.99 A\r{}, which is much larger than earlier reported values. The Xe 5${p}_{1/2}$ and 5${p}_{3/2}$ levels show \ensuremath{\sim}0.2 eV dispersion in good agreement with known theoretical results for the isolated layer, which confirms the theoretical prediction that the bandwidth is exponentially dependent on the interatomic spacing within the layer.