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Electronic structure of the monolayer strontium ruthenate Sr 2 RuO 4 was investigated by high-resolution angle-resolved photoemission spectroscopy. We present photon-energy ( h ν ) dependence of the electronic structure near the Fermi level along the Γ M line. The h ν dependence has shown a strong spectral weight modulation of the Ru 4 d xy and 4 d zx bands.

The electronic structure of the reduced SrTiO 3 (111) surface prepared by annealing in ultrahigh vacuum has been studied using photoemission spectroscopy. In the photoemission spectrum of the reduced surface after annealing at 800°C, a metallic state with a sharp Fermi cut-off is observed in the band gap region. For the reduced surface prepared by annealing at 1200°C, on the other hand, the metallic state becomes drastically weak and a localized state centered at ∼1.1 eV below the Fermi level is observed in the band gap region. Based on the low energy electron diffraction results, we will discuss the relationship between the spectral feature of these band gap states and the surface geometric structure.

The effect of doping electrons on the electronic structure of single-crystalline LaxSr1-xTiO3samples (x = 0.001, 0.01, 0.05) has been studied for the fractured surface by photoemission spectroscopy. Two peaks in the band gap region of SrTiO3(in-gap peaks) are induced by La substitution for Sr: a peak with a sharp Fermi cut-off (metallic peak) and a broad peak centered at 1.5 eV below the Fermi level (1.5 eV peak). The spectral intensity of the metallic peak and the 1.5 eV peak increase with the carrier concentration, but the spectral width of the metallic peak does not depend upon it. The behaviors of doping electrons on the electronic structure near the Fermi level of LaxSr1-xTiO3, such as no change in the spectral width of the metallic peak and the appearance of the 1.5 eV peak, cannot be simply explained by the band theory.

Quasiparticle spectra of the single-crystalline Ca 1− x Sr x VO 3 which are the perovskite-type 3d 1 vanadium oxides are reported. The spectra have been measured around 14 K by high-resolution ultraviolet photoemission spectroscopy. Systematic and prominent spectral weight redistribution due to a large electron correlation has been observed from a renormalized 3d band at the Fermi energy to higher binding energy incoherent features in going from SrVO 3 to CaVO 3 .

We have investigated the electronic structure of an electron-doped double perovskite series Sr 2− x La x FeMoO 6 by photoemission spectroscopy and LDA + U band-structure calculations. Characteristic double-peak structures observed at ∼0.2 eV (Feature A) and ∼1.3 eV (Feature B) have been identified to be a Fe–Mo t 2g↓ band and a Fe e g↑ band, respectively. Upon La substitution, Feature A moves away from the Fermi level by ∼50 meV, reflecting electron-doping. At the same time, the intensity enhancement of both A and B has been observed, which can be understood in terms of the strong Hund's rule energy stabilization in 3d 5 configuration.

Ultrahigh resolution angle-resolved photoemission spectroscopy with low-energy photons is used to study the detailed momentum dependence of the well-known nodal "kink" dispersion anomaly of Bi2Sr2CaCu2O8+{\delta}. We find that the kink's location transitions smoothly from a maximum binding energy of about 65 meV at the node of the d-wave superconducting gap to 55 meV roughly one-third of the way to the antinode. Meanwhile, the self-energy spectrum corresponding to the kink dramatically sharpens and intensifies beyond a critical point in momentum space. We discuss the possible bosonic spectrum in energy and momentum space that can couple to the k-space dispersion of the electronic kinks., Comment: 13 pages, 3 figures. Updates to text and Fig. 2 matching final published version

Clarifying the coupling between electrons and bosonic excitations (phonons or magnetic fluctuations) that mediate the formation of Cooper pairs is pivotal to understand superconductivity. Such coupling effects are contained in the electron self-energy, which is experimentally accessible via angle-resolved photoemission spectroscopy (ARPES). However, in unconventional superconductors, identifying the nature of the electron-boson coupling remains elusive partly because of the significant band renormalization due to electron correlation. Until now, to quantify the electron-boson coupling, the self-energy is most often determined by assuming a phenomenological ‘bare’ band. Here, we demonstrate that the conventional procedure underestimates the electron-boson coupling depending on the electron-electron coupling, even if the self-energy appears to be self-consistent via the Kramers-Kronig relation. Our refined method explains well the electron-boson and electron-electron coupling strength in ruthenate superconductor Sr2RuO4, calling for a critical revision of the bosonic coupling strength from ARPES self-energy in strongly correlated electron systems.

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.

Recently discovered layered-perovskite superconductor Sr 2 RuO 4 shows significant spectral weight redistribution in its low energy photoemission spectrum. This suggests the existence of strong electron correlation in the system. Renormalized mass is estimated by the phenomenological analysis of the spectrum. The importance of a momentum-dependent part of the self-energy is discussed.

The electronic structure of Sr 1− x La x TiO 3 has been studied using the ultraviolet photoemission spectroscopy. Two kinds of doping states in the bulk band gap region of SrTiO 3 are induced by La substitution for Sr; a metallic state with a sharp Fermi cut-off and a broad state centered at ∼1.5eV below the Fermi energy. In proportion to the La concentration, the intensity of the metallic state increases. This behavior is simply explained by the band theory.

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.

An ultrahigh vacuum three-axis cryogenic sample manipulator suitable for angle-resolved photoelectron spectroscopy experiments was developed. The sample manipulator is constructed by combining three modules with translation, polar rotation, and azimuthal-tilt rotation capabilities. Polar rotation and the azimuthal-tilt rotation are performed using a differentially pumped rotary stage and a sample goniometer, respectively. Continuous 360° rotation around the polar axis is possible. The sample goniometer is capable of azimuthal rotation of up to ±180° and tilt rotation from +95° to −5°, measured from the plane normal to the polar axis. Nonmagnetic materials are used near the sample holder of the goniometer. The sample holder can be cooled using a continuous-flow cryostat. To serve as a radiation shield, the lower portion of the goniometer surrounding the sample holder is cooled separately by another cell filled with liquid nitrogen. With liquid nitrogen or liquid helium for the cryostat, the sample holder can be cooled down to 90 or 12.5 K, respectively.

We have characterized the electronic structure of FeSe1−xTex for various x values using soft x-ray photoemission spectroscopy (SXPES), high-resolution photoemission spectroscopy (HRPES) and inverse photoemission spectroscopy (IPES). The SXPES valence band spectral shape shows that the 2 eV feature in FeSe, which was ascribed to the lower Hubbard band in previous theoretical studies, becomes less prominent with increasing x. HRPES exhibits systematic x dependence of the structure near the Fermi level (EF): its splitting near EF and filling of the pseudogap in FeSe. IPES shows two features, near EF and approximately 6 eV above EF; the former may be related to the Fe 3d states hybridized with chalcogenide p states, while the latter may consist of plane-wave-like and Se d components. In the incident electron energy dependence of IPES, the density of states near EF for FeSe and FeTe has the Fano lineshape characteristic of resonant behavior. These compounds exhibit different resonance profiles, which may reflect the differences in their electronic structures. By combining the PES and IPES data the on-site Coulomb energy was estimated at 3.5 eV for FeSe.

The precision and rigidity of compact ultrahigh vacuum (UHV) rotary feedthroughs were substantially improved by preparing and installing an optimal crossed roller bearing with mounting holes. Since there are mounting holes on both the outer and inner races, the bearing can be mounted directly to rotary and stationary stages without any fixing plates and housing. As a result, it is possible to increase the thickness of the bearing or the size of the rolling elements in the bearing without increasing the distance between the rotating and fixing International Conflat flanges of the UHV rotary feedthrough. Larger rolling elements enhance the rigidity of the UHV rotary feedthrough. Moreover, owing to the structure having integrated inner and outer races and mounting holes, the performance is almost entirely unaffected by the installation of the bearing, allowing for a precise optical encoder to be installed in the compact UHV rotary feedthrough. Using position feedback via a worm gear system driven by a stepper motor and a precise rotary encoder, the actual angle of the compact UHV rotary feedthrough can be controlled with extremely high precision.

Anisotropic reflectivity spectra of YAlO 3 single crystals at 297 K measured with the light polarized parallel to each of the a , b and c axes are presented in the range from ∼6.5 eV to ∼120 eV. A comparison of the reflectivity spectra of YAlO 3 with those of Y 2 O 3 , Y 3 Al 5 O 12 and α-Al 2 O 3 single crystals reaffirms that the peaks at ∼35 eV are mainly associated with the Y 3+ 4 p 6 and the features beyond ∼76 eV with the Al 3+ L 2,3 . Anisotropic reflectivity spectra at 10 K are shown in the region from ∼7 eV to 15 eV. Effective optical dielectric constants calculated by the sum rule including the imaginary part of the dielectric constant up to ∼120 eV are seen to saturate at the values corresponding to 98.0% ∼ 99.7% of the literature values of the optical dielectric constant.

CO chemisorption on Fe(100) at 80 K was reinvestigated by angle-resolved ultraviolet photoemission spectroscopy with synchrotron radiation. At 0.25 L, the CO 5σ peak and the CO 4σ peak are observed at 7.5 and 10.5 eV below the Fermi level, and they shift to 8.3 and 10.7 eV respectively, as the coverage increases up to 3.0 L. In contrast to previous observations for CO on Fe(100) [Phys. Rev. B 27 (1983) 3338] where the 1π − 5σ separation was considerably larger (~1.5 eV), the 1π and 5σ peaks are not observed separately as usually. Their separation is found to be ~0.5 eV. The lower binding-energy peak previously assigned to CO 1π emission is due to oxygen contamination. The present data also indicate that the orientation of the CO axis undergoes a two-stage transition depending on the amount of CO exposure: inclined (⩽ 0.5 L) → upright (~2 L) → inclined (4 L).

An in-gap state formed in the band gap of TiO2 upon surface oxygen-vacancy doping has been studied by angle-resolved photoemission spectroscopy. The in-gap state with a formal d1 configuration exhibits a small (1×1) k dispersion, though it never crosses the Fermi level for all the oxygen-vacancy concentration range. We propose that a polaron interaction is responsible for the localization of doped electrons in the in-gap state.

We report an angle-resolved photoemission study of BaFe2As2, a parent compound of iron-based superconductors. Low-energy tunable excitation photons have allowed the first observation of a saddle-point singularity at the Z point, as well as the Gamma point. With antiferromagnetic ordering, both of these two van Hove singularities come down below the Fermi energy, leading to a topological change in the innermost Fermi surface around the kz axis from cylindrical to tear-shaped, as expected from first-principles calculation. These singularities may provide an additional instability for the Fermi surface of the superconductors derived from BaFe2As2., Comment: 14 pages, 4 figures, 1 table

The spectra of optical constants of α-Al 2 O 3 single crystals at 297 K and 10 K are presented over the range of photon energies from 6 eV to 120 eV for the electric fields of incident light parallel to optic axis and normal to it. The optical constants include the complex index of refraction, conductivity, complex dielectric constant, loss functions and effective number of electrons per unit cell. The spectra in E ≥ 78 eV are interpreted in terms of the interband transitions from the O 2p and 2s bands to the Al conduction bands and the free electron-like absorption of photons by the O 2p band electrons. The volume energy loss function has a prominent peak at E ∼26.4 eV for the plasma oscillation dominantly of the 2p electrons. The spectral features in E ≤78 eV resulting from electronic transitions from the Al L 2,3 levels are presented with the L 2,3 exciton doublet, e.g. , at 78.72 eV and 79.15 eV for e 2// .

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)

We have observed changes in the electronic structure of a W(110) surface caused by the hydrogen-induced reconstruction using angle-resolved photoemission spectroscopy. For the unreconstructed H/W(110) surface, a number of hydrogen-induced states are observed, and some of them are found to be in the projected bulk band gap. As W(110) undergoes the surface reconstruction induced by further hydrogen adsorption, one of those states is rapidly quenched, while others simply shift downward in energy by a fraction of an electron volt. To understand the surface reconstruction mechanism for this system, the surface electronic states before and after the surface reconstruction are calculated using the “simple” tight-binding method. The driving mechanism for the hydrogen-induced surface reconstruction is briefly discussed.

The spectra of absorption constant in the intrinsic absorption tail region of α-Al 2 O 3 single crystals at 297 K, 78 K and 10 K are presented for the electric fields of incident light parallel to the optic axis of α-Al 2 O 3 and normal to it. The absorption tail spectra obey the Urbach rule for each polarization, indicating that absorptions in the tail region result from the direct interband transitions. The optical anisotropy characterized by the Urbach rule is interpreted in terms of the bond length O 2- –Al 3+ participating in the absorption processes. The spectra of reflectivity are presented in the range of photon energies from 4.6 eV to 120 eV for electric fields parallel to and normal to it. Spectral positions of main maxima in the reflectivity spectra for each polarization are determined at 297 K and 10 K; the spin-orbit doublet of the Al 3+ L 2,3 is revealed at 78.67 eV and 79.12 eV for the first time in the case of α-Al 2 O 3 .

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.

Off-normal incidence piezoreflectivities of SrF 2 and BaF 2 in the 4 p and 5 p core exciton regions have been measured. The cubic, tetragonal and trigonal deformation potentials are determined. The results are interpreted by the localized excitation model; signs and magnitudes of the deformation potentials of the core excitons, as well as their energies and intensities. Slater-Condon parameters, and Coulomb and exchange interactions between electrons and holes are reasonably determined.

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.

Two-dimensional dispersion of the H 1s-derived energy bands of the Fe(110)-(2 × 1)H and -(3 × 1)H surfaces was studied by angle-resolved photoemission spectroscopy with synchrotron radiation. An H 1s-derived state, split off from the Fe bulk bands, is found at 7.9 eV (8.2 eV) below the Fermi energy at \ gG for the (2 × 1)H phase (for the (3 × 1)H phase). For the (2 × 1)H phase (for the (3 × 1)H phase), this H split-off state disperses upward by ~ 1.5 eV ( ~ 0.9 eV) with k∥ in the [110] azimuth and by ~ 1.0 eV ( ~ 1.6 eV) in the [001] azimuth. For both phases, the effective mass ( m ∗ m ) near \ gG is found to be ~ 1.7 in the [110] azimuth whereas i considerably small ( ~ 0.4) in the [001] azimuth, being consistent with the anisotropy of the overlayer structures. An avoided band crossing of the H 1s-derived band and the Fe 4s band is observed.

Photoemission spectroscopy with low-energy tunable photons on oxygen-deficient iron-based oxypnictide superconductors NdFeAsO0.85 (Tc=52K) reveals a distinct photon-energy dependence of the electronic structure near the Fermi level (EF). A clear shift of the leading-edge can be observed in the superconducting states with 9.5 eV photons, while a clear Fermi cutoff with little leading-edge shift can be observed with 6.0 eV photons. The results are indicative of the superconducting gap opening not on the hole-like ones around Gamma (0,0) point but on the electron-like sheets around M(pi,pi) point., 8 pages, 3 figures

Angle-resolved photoemission spectroscopy with low-energy tunable photons along the nodal direction of oxygen isotope substituted Bi2Sr2CaCu2O8+delta reveals a distinct oxygen isotope shift near the electron-boson coupling "kink" in the electronic dispersion. The magnitude (a few meV) and direction of the kink shift are as expected due to the measured isotopic shift of phonon frequency, which are also in agreement with theoretical expectations. This demonstrates the participation of the phonons as dominant players, as well as pinpointing the most relevant of the phonon branches., 5 pages, 3 figures

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.

The ultraviolet photoemission spectrum of a layered-perovskite superconductor Sr 2 RuO 4 around the Fermi level is considerably different from the density of states obtained by a band calculation based on the local density approximation. Although the significant spectral weight redistribution is considered to be evidence of the large electron correlation in this system, the effective mass is not so enhanced. A possible scenario to coordinate both these phenomena is discussed.

The possibility that a pairing boson might act as the 'glue' to bind electrons into a Cooper pair in superconductors with a high critical temperature (T(c)) is being actively pursued in condensed-matter physics. Gweon et al. claim that there is a large and unusual oxygen-isotope effect on the electronic structure, indicating that phonons have a special importance in high-temperature superconductors. However, we are unable to detect this unusual oxygen-isotope effect in new data collected under almost identical material and experimental conditions. Our findings point towards a more conventional influence of phonons in these materials.

Formation of electron pairs is essential to superconductivity. For conventional superconductors, tunnelling spectroscopy has established that pairing is mediated by bosonic modes (phonons); a peak in the second derivative of tunnel current d2I/dV2 corresponds to each phonon mode . For high-transition-temperature (high-Tc) superconductivity, however, no boson mediating electron pairing has been identified. One explanation could be that electron pair formation and related electron-boson interactions are heterogeneous at the atomic scale and therefore challenging to characterize. However, with the latest advances in d2I/dV2 spectroscopy using scanning tunnelling microscopy, it has become possible to study bosonic modes directly at the atomic scale . Here we report d2I/dV2 imaging studies of the high-Tc superconductor Bi2Sr2CaCu2O8+d. We find intense disorder of electron-boson interaction energies at the nanometre scale, along with the expected modulations in d2I/dV2 (refs 9,10). Changing the density of holes has minimal effects on both the average mode energies and the modulations, indicating that the bosonic modes are unrelated to electronic or magnetic structure. Instead, the modes appear to be local lattice vibrations, as substitution of 18O for 16O throughout the material reduces the average mode energy by approximately 6 per cent - the expected effect of this isotope substitution on lattice vibration frequencies. Significantly, the mode energies are always spatially anticorrelated with the superconducting pairing-gap energies, suggesting an interplay between these lattice vibration modes and the superconductivity., 29 pages, 8 figures. Appears in Nature 442, 546 (2006)

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.

The surface electronic structure of electron-doped SrTiO3 has been studied using the ultraviolet photoemission spectroscopy (UPS). By Nb substitution for Ti, two kinds of doping peaks are induced in the bulk band gap region of SrTiO3; a metallic peak with a sharp Fermi cut-off and a broad peak centered at ∼ 1.3 eV below the Fermi level (EF). The feature of these doping peaks is essentially identical to that recently reported in the case of the La substitution for Sr. The UPS spectra near EF are shown to be very sensitive to the oxygen exposure. The behavior under the oxygen exposure can be explained by the formation of surface dipole layer by negative adsorbed species.

Optical reflectivity spectra are studied for single crystals of a perovskite-related 3d1 correlated metallic system Ca1−xSrxVO3. The measurement is made at room temperature between 0.03 and 40 eV. In the optical conductivity spectra a Drude-like contribution peaked at ω=0 and other two contributions, which may be associated with Hubbard band and a quasiparticle band at EF, are observed below 5 eV. The infinite-dimension Hubbard model seems to reproduce the observed features in CaVO3. However, the evolution of these features on varying the value of x is not exactly what we expect from the model.

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.

Spectra from photoemission spectroscopy (PES) of a recently discovered layered-perovskite superconductor Sr2RuO4 are reported. The PES spectrum of Sr2RuO4 around the Fermi level (EF) is apparently different from the non-interacting density of states obtained by a band calculation, suggesting the importance of strong electron correlation in this system.

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.

The studies of ultra-violet photoemission spectroscopy and soft X-ray absorption spectroscopy of a perovskite-type 3d 1 Mott-Hubbard system Ca 1−x Sr x VO 3 , which is metallic in the whole composition range (0 < x < 1), are reported. As we control electron-electron interaction of the system by changing the Sr concentration (x), we have observed a systematic change of the spectra, which clearly manifests how the single-particle density of states around Fermi level (E F ) behaves near a Mott transition

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.