High-resolution Photoemission Study Research Articles

High-resolution photoemission study of low- Tc superconductors: Phonon-induced electronic structures in low- Tc superconductors and comparison with the results of high- Tc cuprates

Recent advances in the energy resolution and cryogenic techniques used in photoemission spectroscopy have enabled us to directly observe the superconducting electronic structure of superconductors with a relatively lower transition temperature (low- T c). In this paper, we report photoemission results on low- T c superconductors measured with an ultrahigh-resolution low-temperature photoemission spectrometer. The result of Nb 3Al exhibits a superconducting gap and a phonon-induced peak–dip–hump structure characteristic of the strong-coupling superconducting spectral function. From measurements of Ba 0.67K 0.33BiO 3, we observe isotropic s-wave superconducting gap opening below T c as well as depletion of intensity near the Fermi level above T c, which can be attributed to a phonon-induced pseudogap. We will compare these photoemission results with those of high- T c superconductors.

Photoemission study of Sn on vicinal Si( [formula omitted])2×1 surface

We present high resolution photoemission studies of Sn on vicinal Si(1 0 0)2×1 surface in submonolayer regime. Deposition of Sn on clean Si(1 0 0) surface produces an abrupt epitaxial interface having a large variety of surface reconstructions. At room temperature at low coverages Sn dimer rows are formed. After annealing above 500 °C following superstructures are known to exist with increasing Sn coverage: c(4×4), 2×6, c(4×8), and 1×5. The Sn 4d core level spectra are now resolved in details displaying new components related to the different adsorption sites on the Si(1 0 0) surface. The spectra of the annealed phase reconstructions show components originating from dimer bonding, from bonding to the second layer, from Sn atoms in mixed Sn–Si dimers where Sn atoms have replaced one of the Si dimer pair atoms, and from adsorption onto defect sites. The photoemission results are in good agreement with the surface geometric structures.

Electronic structure in low dimensional and correlated transition metal oxides: high resolution photoemission and X-ray emission studies

This paper reports on recent studies of the electronic structure of low dimensional and correlated transition metal oxides. The primary probes used in these studies are angle resolved photoemission spectroscopy, and soft X-ray emission spectroscopy. Topics discussed include non-Fermi liquid behavior in low dimensional conductors, temperature dependent Fermi surface nesting vectors, hidden Fermi surface nesting, band hybridization, many body effects, and low energy excitations. The results of studies on the quasi-low dimensional oxides K0.3MoO3, Li0.9Mo6O17, Na0.9Mo6O17, the narrow gap insulator CdO, the Mott–Hubbard oxide Cr-doped V2O3, and the spin-Peierls oxide CuGeO3 are reported. The paper concludes with a discussion of the future direction of such studies.

High-resolution photoemission study of the discommensurate(5.55×5.55)Cu/Si(111) surface layer

High-resolution photoemission study of the discommensurate<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mo>(</mml:mo><mml:mn>5.55</mml:mn><mml:mn /><mml:mo>×</mml:mo><mml:mn>5</mml:mn><mml:mo>.</mml:mo><mml:mn>5</mml:mn><mml:mn>5</mml:mn><mml:mo>)</mml:mo></mml:math>Cu/Si(111) surface layer

Bulk-sensitive high-energy and high-resolution photoemission study of strongly correlated Ce compounds

Bulk-sensitive Ce 3d–4f resonance photoemission with an unprecedentedly high resolution has revealed clear differences of bulk 4f spectral weights near the Fermi level of very strongly hybridized CeRu2, moderately hybridized CeRu2Si2 and weakly hybridized CeB6 and CeRu2Ge2, reflecting their Kondo temperatures. The bulk spectra of CeRu2 are explained by a band-structure calculation (itinerant model) whereas the other spectra are well reproduced by calculations based on the single impurity Anderson model (model from a localized limit).

High-resolution photoemission study of long-lived d-holes in Ag

We present a high-resolution study of photoemission line widths observed at Ag(1 1 0). A careful data analysis yields k-resolved hole lifetimes τ h of d-holes at the X-point of the bulk band structure. At the upper d-band edge we derive τ h⩾22 fs, i.e. more than one order of magnitude larger than predicted for a free-electron gas. In agreement with recent ab initio calculations for d-hole dynamics in Ag we interpret this lifetime enhancement by a small scattering cross-section of d-holes with sp-states below the Fermi level E F. With increasing distance to E F the d-hole lifetimes get rapidly shorter because of the increasing density of d-states and corresponding contributions of intra-d-band scattering processes.

High resolution photoemission studies of sulfur interaction with model catalytic surfaces

Synchrotron-based photoelectron spectroscopy was used to study sulfur reaction with two model catalytic surfaces: Cu/Ru(0001) and TiO 2 (110). Sulfidation of a strained copper film supported on a Ru substrate proceeds only after chemisorption of at least 0.2 monolayers of sulfur. The induction period was probed by STM to reveal significant restructuring of the metal surface and self-organization of sulfur into several distinct nano-patterns. Large chemical shifts of the S 2p core levels allow a definitive assignment of surface species (atomic vs. sulfidic sulfur). Sulfur reacts with a TiO 2 (110) surface readily from the very early stages of adsorption. We identified at least three types of sulfur species, namely S bonded to Ti rows, vacancies in the bridging oxygen rows, and S associated with the SO x groups.

Performance of the high-resolution high-flux monochromator for bending magnet beamline BL-1C at the Photon Factory

A grazing incidence varied line spacing plane grating monochromator (VLS-PGM) has been designed and installed at the Photon Factory bending magnet beamline, BL-1C. The monochromator is designed to cover 20 to 250 eV with a high resolving power as well as a high photon flux and intended for a high-resolution angle-resolved photoemission study of quantum nano-structures. The resolving power of the beamline is observed to be more than 16,000 at around 65 eV and exceeds 10,000 at all the covered energy ranges of the monochromator with photon flux over 10 9 photons/s which shows that this monochromator reaches the expected design goals.

High resolution photoemission study on SnO 2 gas sensors

Pd-, Pt-doped and undoped SnO 2 gas sensors were studied by synchrotron radiation photoelectron spectroscopy (SRPES) and laboratory source UPS (ultraviolet photoelectron spectroscopy). Valence band spectra were recorded for photon energies of 60–120 eV (SRPES) and for 40.81 eV (UPS). The spectra were analyzed by factor analysis and generation of difference spectra. A comparison of the samples shows that the doping material of these sensors is not introducing additional states to the surface electronic structure within the limit of detection. This leads to the conclusion that the surfaces of the grains are not enriched with the metal additives.

Recent high resolution photoemission studies of electronic structure in quasi-one-dimensional conductors

Abstract The electronic structure near the Fermi level of K 0.3 MoO 3 and Li 0.9 Mo 6 O 17 , two prototypical quasi-one-dimensional (1D) conductors, has been studied using very high resolution angle-resolved photoemission spectroscopy. Our results differ from earlier lower resolution studies. For Li 0.9 Mo 6 O 17 we clearly observe a quasi-1D state dispersing through the Fermi level ( E F ) and the opening up of a gap at E F as the sample is cooled through a bulk metal to non-metal transition at 24 K. For K 0.3 MoO 3 we clearly observe two quasi-1D bands cross E F , and measured a temperature dependence to the Fermi surface nesting vector that correlates well with neutron-scattering measurements of the charge density wave vector.

High-resolution photoemission study of strongly correlated f electron systems

Abstract The importance and powerfulness of high-resolution photoemission spectroscopy in studying the strongly correlated f electron systems are demonstrated by presenting recent results on some Ce and U compounds. Using angle-resolved photoemission spectroscopy (ARPES), we observed a clear change in the Fermi-surface volume accompanied by the magnetic phase transition in CeSb, showing an important role of temperature-dependent p–f mixing in dominating the electronic and magnetic properties. It was also found by ARPES that USb, which is isostructural to CeSb but has 5f electrons instead of 4f electrons in the outermost shell, has a Fermi-surface topology totally different from that of CeSb, indicating the different nature between 4f and 5f electrons. The ARPES result has elucidated the dual (itinerant and localized) character of 5f electrons in USb. We found a temperature-dependent energy pseudogap at E F in CeRhSb and CeRhAs at low temperature, which is ascribed to the ‘Kondo-insulator gap’ formed through the hybridization between f and conduction electrons.

High-resolution angle-resolved photoemission study of UN and USb; Dual character of 5f electrons

We have performed high-resolution angle-resolved photoemission spectroscopy on UN and USb to study the electronic structure near the Fermi level ( E F) and the nature of U 5f electrons. We found that the pnictogen (N and Sb) p bands are fully occupied in contrast with Ce monopnictides and shows a good qualitative agreement with the band calculation by treating the U 5f states as bands. On the other hand, we have not observed dispersive U 5f bands near E F predicted by the band calculation, instead we found two non-dispersive bands near E F, which are assigned as the 5f 2-final-state multiplet indicative of localized U 5f states. The intensity of multiplet structure is stronger in USb than in UN. These results indicate the dual character of U 5f electrons and its difference between USb and UN.

High-resolution photoemission study of the valence transition in YbInCu 4

We have performed a photoemission study of YbInCu 4, which undergoes a first-order phase transition at T v ≃40 K between the low-temperature intermediate-valence state and the high-temperature local-moment state. The Yb 4 f photoemission peak (Kondo peak) just below the Fermi level, which probably originates from the subsurface region of the YbInCu 4 samples but tracks the valence transition in the bulk YbInCu 4, has been studied above and below T v . It has been found that the 4 f photoemission peak is shifted from ∼− 35 meV at 75 K to −40 meV at 7 K.By analyzing the 4 f peak position and the Yb valence in the two phases using the Anderson impurity model, we find that the bare 4 f level, which is located close to the Fermi level, is shifted downward and the hybridization strength decreases in going from the high-temperature to low-temperature phases.

Pseudogap in Quasicrystals Studied with Photoemission Spectroscopy

The results of the low-temperature, high energy resolution photoemission studies of a single-grain icosahedral alloy Al70Pd21.5Mn8.5 have been presented. The existence of the theoretically predicted pseudogap has been confirmed. No evidence of the theoretically predicted spikiness of the density of states could be observed. A review of published experimental data on the electronic structure of quasicrystals has also been presented. The photoemission experimental results confirm the existence of a pseudogap in the density of states in icosahedral quasicrystals, but its existence in the decagonal quasicrystals remains still an open question. The predicted DOS spikiness could not be observed in any of the studied quasicrystals.

High-resolution angle-resolved photoemission study of Pb-substituted Bi2201

We have performed a systematic doping-dependent angle-resolved photoemission spectroscopy (ARPES) of Pb-substituted Bi2201 (Bi 1.80Pb 0.38Sr 2.01CuO 6− δ ) with high energy (7–11 meV) and momentum (0.01 Å −1) resolutions. We found that all the samples show a large hole-like Fermi surface centered at (π,π), whose volume changes systematically as a function of doping. The superconducting gap exhibits a d x 2− y 2 -like anisotropy with a typical gap value of 10–15 meV near (π,0). By comparing with Bi 2Sr 2CaCu 2O 8 (Bi2212), we found that the size of superconducting gap is well scaled with the maximum T c ( T c max). Doping-dependence of spectral line shape at (π,0) is very similar between Bi2201 and Bi2212, while its energy scale is different by a factor of 4, corresponding to the ratio of T c max between the two compounds. These indicate that the superconducting and pseudogap properties are characterized by T c max irrespective of the number of CuO 2 layers or the BiO superstructure.

CO adsorption on Pd(1 1 1): a high-resolution core level photoemission and electron energy loss spectroscopy study

By combining high-resolution X-ray photoelectron and electron energy loss spectroscopies a comprehensive analysis of the adsorption of CO on Pd(1 1 1) at 300 K has been performed. The characteristic fingerprints for various CO–Pd(1 1 1) bonding configurations have been identified from the decomposition analysis of the adsorbate C 1s and the substrate Pd 3d 5/2 core-level photoemission spectra obtained after CO adsorption at 120 K. The c(4×2) structure at 0.5 monolayer (ML) and the (2×2)-3CO structure at 0.75 ML formed at low temperature have been used for calibration purposes. The core-level results are consistent with CO adsorbing in a mixture of fcc and hcp threefold hollow sites in the c(4×2) structure and of hollow and on-top sites in the (2×2) structure, as reported in the literature. For CO adsorption at 300 K, a different site occupation is evidenced by the presence of two components in the C 1s and Pd 3d 5/2 core-level and C–O stretching vibration lineshapes. At coverages up to 0.1 ML only fcc threefold hollow sites in a ( 3 × 3 )R30° structure are occupied. The population of these sites continues with increasing the CO coverage up to 0.32 ML, but in addition adsorption of CO in bridge sites takes place. The latter sites become predominant at the 300 K saturation coverage of 0.5 ML, and are considered to form domain boundaries in a poorly ordered ( 3 × 3 )R30° phase with split low energy electron diffraction reflexes. Between 0.32 and 0.5 ML the domain boundary regions grow at the expense of the ( 3 × 3 )R30° domains.

α-Sn pseudomorphic growth on InSb (111) and ( [formula omitted]) surfaces: a high-resolution photoemission study

The Sn–InSb interface and α-Sn ultrathin film formation on the sputter-annealed clean A- and B-type InSb(111) surfaces are studied by high-resolution UV photoelectron spectroscopy. The valence band and In-4d core levels, along with the relieving of the clean surface reconstruction, suggest a model for the interface formation at low coverage. At higher Sn thickness a good-quality α-Sn(111)-(1×1) layer is formed, and the In-4d and Sn-4d core-level analysis shows a slight In interdiffusion present on both substrates.

Depletion of the density of states near the Fermi energy induced by disorder and electron correlation in alloys

We have performed high-resolution photoemission study of substitutionally disordered alloys Cu–Pt, Cu–Ni, and Pd–Pt. The ratios between alloy spectra and pure metal spectra are found to have dips at the Fermi level when the residual resistivity is high and when strong repulsive electron–electron interaction is expected. This is in accordance with Altshuler and Aronov's model which predicts a depletion of the density of states at the Fermi level when both disorder and electron correlation are present.

Interplay between electron-electron interaction and electron-phonon coupling near the Fermi surface of1T−TaS2

We present a detailed high-resolution angle-resolved photoemission study of the electronic band structure of the room-temperature quasicommensurate charge-density-wave phase of $1T\ensuremath{-}{\mathrm{TaS}}_{2}.$ In particular, we show that no crossings of the Fermi level are visible in the complete Brillouin zone, indicating that an electron-electron correlation-induced pseudogap in the Ta $5d$ derived band exists already above the Mott localization-induced transition at 180 K. Moreover, we find that the electronic structure is governed by at least two quasiparticle peaks, which can be assigned to electrons from starlike shells of Ta atoms within the distorted crystal lattice. These peaks show quasilocalized (dispersionless) behavior in parts of the Brillouin zone where the one-particle band is unoccupied and they follow the one-particle dispersion in the occupied part. In order to address the question of possible Fermi-surface (FS) nesting, we scanned the remaining remnant FS and found regions with a considerable decrease of spectral weight. However, we find no clear evidence for FS nesting.

High resolution photoemission study of C 60 on Si(111) as a precursor of SiC growth

We characterized the room temperature (RT) growth of C 60 on the Si(111) (7×7) surface and the SiC formation upon annealing using high resolution photoelectron spectroscopy techniques. Si 2p core level spectra for RT C 60 deposition unambiguously show the strong attenuation of the rest-atoms components and the growth of at least one new component at 1 ML coverage. This new component grows with annealing temperature and at T>1020 K an abrupt change occurs, corresponding to the formation of SiC, as confirmed by the C 1s core level emission.