Angle-resolved photoemission study of the MX-chain compound [Ni(chxn)(2)Br]Br(2): spin-charge separation in hybridized d-p chains

We report on the results of angle-resolved photoemission experiments on a quasi-one-dimensional (1D) MX-chain compound [Ni(chxn)2Br]Br2, which shows a gigantic nonlinear optical effect. A "band" having about 500 meV energy dispersion is found in the first half of the Brillouin zone, but disappears at kb/pi approximately 1/2. These spectral features are well reproduced by the d-p chain model with a small charge-transfer energy Delta compared with that of 1D Cu-O compounds. We propose that this smaller Delta is the origin of the absence of clear spin-charge separation in the photoemission spectra and the strong nonlinear optical effect.     

Excitation of 2D plasmons in a Cs/W(110) system

The evolution of surface photoemission spectra was investigated for a Cs/W(110) system with metastable Cs coatings larger than a monolayer. It is demonstrated that 2D plasmons can be detected by threshold photoemission spectroscopy. Three photoemission peaks were observed, whose dependence on the Cs adsorption dose showed a complicated behavior. The peaks may be due to the photoinduced excitation of a plasmon in quasi-2D Cs clusters, a surface Cs plasmon, or an interface Cs-W plasmon.     

The surface electronic structure of Ag8Au4 superlattice

We present the calculations of electronic structure and photoemission spectra for a lattice-matched Ag-Au(111) superlattice. The selfconsistent band structure exhibits a superlattice gap at about 1 eV below the Fermi level. A surface state is found in this gap and its dispersion properties are investigated. Its energy location is varied with location of surface terminating plane within the superlattice period. The calculated normal photoemission spectra explain well available experimental data.     

Mutual experimental and theoretical validation of bulk photoemission spectra of Sr1-xCaxVO3

We report high-resolution high-energy photoemission spectra together with parameter-free LDA + DMFT (local density approximation + dynamical mean-field theory) results for Sr1-xCaxVO3, a prototype 3d(1) system. In contrast to earlier investigations the bulk spectra are found to be insensitive to x. The good agreement between experiment and theory confirms the bulk sensitivity of the high-energy photoemission spectra.     

Application of the Lucy-Richardson deconvolution procedure to high resolution photoemission spectra

Angle-resolved photoemission has developed into one of the leading probes of the electronic structure and associated dynamics of condensed matter systems. As with any experimental technique the ability to resolve features in the spectra is ultimately limited by the resolution of the instrumentation used in the measurement. Previously developed for sharpening astronomical images, the Lucy-Richardson deconvolution technique proves to be a useful tool for improving the photoemission spectra obtained in modern hemispherical electron spectrometers where the photoelectron spectrum is displayed as a 2D image in energy and momentum space.     

Spectral signatures of the Fulde-Ferrell-Larkin-Ovchinnikov order parameter in one-dimensional optical lattices

We address an imbalanced two-component atomic Fermi gas restricted by a one-dimensional (1D) optical lattice and an external harmonic potential, within the mean-field Bogoliubov-de Gennes formalism. We show that characteristic features of the Fulde-Ferrell-Larkin-Ovchinnikov state are visible in the rf spectra and in the momentum-resolved photoemission spectra of the gas. Specially, Andreev states or midgap states can be clearly resolved, which gives a direct experimentally observable signature of the oscillating order parameter.     

Study of oxygen and carbon monoxide adsorption on molybdenum (110) by metastable helium de-excitation spectroscopy

The adsorption of oxygen and carbon monoxide on the (110) face of molybdenum is studied at room temperature by metastable helium atom de-excitation spectroscopy (MDS). On the basis of the interaction mechanism described by H.D. Hagstrum, and which was shown to be valid for the He¹?Mo couple in a previous article and again here, the secondary electron energy spectra are interpretated as relating to the self-convolution product of the electron state density of occupied valence band levels of the solid. The results obtained after deconvolution show the variations in this density of states during oxygen and carbon monoxide adsorption, and they are compared with photoemission spectra (UPS) obtained at the same time as the MDS spectra. They show that MDS is a technique well suited to the study of adsorption phenomena.     

Hydrogen chemisorption on the 100 (2 × 1) surfaces of Si and Ge

This paper combines a theoretical study of the Si(100) surface having a monolayer of atomic hydrogen chemisorbed to it with an experimental study of the analogous Ge(100) and Ge(110) surfaces. In the theoretical work the underlying (100) silicon surface is taken to be reconstructed according to the Schlier-Farnsworth-Levine pairing model with the hydrogen located on the unfilled tetrahedral bonds of this structure. Self-consistent calculations of the electronic potential, charge density, spectrum, and occupied surface density of states are carried out. The force on the hydrogen atoms is then calculated using the Hellman-Feynman theorem. This force is found to be close to zero, confirming that the hydrogen atoms are indeed at the equilibrium position for the chosen silicon geometry. Features in the calculated photoemission spectrum for the Si(100) 2 × 1 : H surface are discussed in terms of related features in the photoemission spectrum of Si(111) : H, but are found not to agree with the previously measured photoemission spectrum of Si(100) 2 × 1 : H. Measured photoemission and ion-neutralization spectra for Ge(100) 2 × 1 : H agree in their major features with what is calculated for Si(100) 2 × 1 : H, however, suggesting that the Ge(100) 2 × 1 : H surface is reconstricted according to the pairing model. Similarly, measured spectra for clean Ge(100) 2 × 1 agree with calculations for the row dimerized Si(100) surface.     

Charge accumulation nanolayer: A 2D electronic channel in Cs/n-InGaN ultrathin interfaces

This paper reports on the formation of a 2D electronic channel, i.e., a charge accumulation layer on the n-InGaN(0001) surface observed under adsorption of submonolayer Cs coatings. It is found that photoemission from the accumulation layer is excited by light in the InGaN transparency region. It is established that the potential well depth and the density of electronic states in the accumulation layer can be controlled by properly varying the Cs coverage. It is shown that the accumulation layer exhibits quantum-well effects. The photoemission matrix element is calculated, and the energy parameters of the accumulation layer are obtained. An oscillatory structure originating from the Fabry-Perot interference is revealed in the spectra of photoemission from the accumulation layer.     

Comparison of calculated and experimental k-resolved inverse photoemission spectra from TiN(100) and TiC(100)

Calculated k-resolved inverse photoemission spectra from TiN(100) and TiC(100) are presented along the 011 and 010 azimuthal directions. Spectra were calculated utilising the same computational code and crystal potentials as used in earlier calculations of angle resolved photoemission spectra. A comparison with earlier published experimental results is made and the calculated spectra are found to reproduce the experimental spectra quite well.     

Thickness dependence of X-ray absorption and photoemission in Fe thin films on Si(0 0 1)

To investigate the initial growth of Fe films on Si(0 0 1) and the Fe/Si interface, Fe films at various thicknesses have been systematically studied by soft X-ray absorption spectroscopy (XAS) and X-ray photoemission spectroscopy (XPS). The Fe L edge XAS spectrum shows a strong thickness dependence with broader line-width for thinner films. Detailed analysis of the Fe absorption signal as a function of the thickness shows that the broad linewidth of Fe L edge XAS spectra is mostly contributed by the first Fe layer at the Fe/Si interface. In contrast to XAS, Fe 2p photoemission spectra for these films are identical. However, valence band photoemission also shows a strong thickness dependence. Comparing the valence band photoemission spectra of the thin Fe/Si(0 0 1) films with that of pure Si and the thickest Fe film, the difference spectra at all thicknesses show almost identical shape indicating the same origin: the Fe/Si interface. Thus, it is mainly the first Fe layer at Fe/Si layer that is reactive with the Si substrate changing its electronic structure.     

Surface electronic structure of the organic superconductor κ-(ET)2Cu(NCS)2 studied via photoemission microscopy

The surface electronic structure of cleaved single crystals of the organic superconductor κ-(ET)₂Cu(NCS)₂ has been studied using photoemission microscopy. Two types of cleaved surfaces were observed, displaying different valence band photoemission spectra and different spectral behavior near the Fermi level, E_F. In particular, spectra from one surface type display relatively broad spectral features in the valence band and finite spectral intensity at E_F, while spectra from the other surface type show well-defined valence band emission features and zero photoemission intensity at E_F. We propose that the spectral differences are due to a very short electron mean free path in this material, and our results are used to explain the differences between previously published photoemission spectra from this superconductor. We also report the results of an investigation of the electronic structure of defects in this material.     

A detailed analysis of the photoemission spectra of basic thioaromatic monolayers on noble metal substrates

Analysis of photoemission spectra of complex thioaromatic self-assembled monolayers (SAMs) requires the knowledge and understanding of such spectra for the basic systems. Keeping this goal in mind, synchrotron-based high-resolution X-ray photoelectron spectroscopy was used to characterize SAMs formed from the simplest thioaromatic compounds, namely thiophenol, 1,1′-biphenyl-4-thiol and 1,1′;4′,1″-terphenyl-4-thiol on evaporated Au(1 1 1) and Ag(1 1 1) substrates. The acquired S 2p and C 1s spectra were analyzed in terms of fine structure and initial and final state effects in the photoemission process. The assignment of the individual spectral features was considered in detail. Conclusions on quality and chemical and structural homogeneity of the investigated SAMs were derived.     

Threshold photoemission spectroscopy in solids

Threshold photoemission spectroscopy (TPES) is used to measure the Fe 2p spectrum of a stainless steel sample. The obtained spectrum is compared with analogous spectra measured by X-ray photoemission and absorption spectroscopies. The results of this comparison suggest that resonant two-electron autoionization processes, rather than direct photoemission from the core level, are the main mechanisms contributing to the signal. Limits and applicability of this experimental approach to investigate bulk electronic properties in solids are discussed.     

Calculated oxygen 1s core-level photoemission spectra from cuprate superconductors

The observed O 1s X-ray photoemission spectra of the cuprate superconductors often exhibit a satellite at a higher binding energy to the main peak. The origin of this satellite is not fully understood. We have done model cluster calculations to investigate the origin of this satellite using the configuration interaction approach. The calculated spectra for the divalent and superconducting cuprates essentially exhibit a single peak. On distorting the cluster in-plane, the peak shifts to higher binding energies. This substantiates a deterioration of the surface leading to the observed satellite structure in the O 1s core-level photoemission spectra of the cuprate superconductors.     

Photoemission study of Ag nanofilm grown on pseudomorphic fcc Fe(1 0 0)

An angle-resolved photoemission study for Ag nanofilm grown on pseudomorphic metastable-fcc-phase Fe(1 0 0) has been done in order to investigate in detail the quantized electronic structures. From the low-energy electron-diffraction and angle-resolved photoemission spectra, it is found that the present Ag nanofilms were grown in the direction of [1 1 1] on pseudomorphic fcc Fe(1 0 0) substrates. The angle-resolved photoemission spectra of Ag nanofilms grown on pseudomorphic fcc Fe(1 0 0) exhibit the features derived from Shockley-type surface state and additional fine-structures derived from the quantized state of Ag sp valence electron. The experimental nanofilm-thickness dependence of binding energies of these quantized states is compared with the theoretical calculation based on the phase accumulation model, taking into account the phase shifts of electron reflection at both interfaces of the Ag nanofilm. From these results, we discuss the quantized electronic structure in Ag nanofilm grown on pseudomorphic fcc Fe(1 0 0).     

Unoccupied band dispersion of Si(1 1 1):√3 × √3-Ag surface studied by time- and angle-resolved two-photon photoemission spectroscopy

Band dispersion and transient population of unoccupied electronic states on Si(1 1 1):√3 × √3-Ag surface have been studied by time-resolved (TR) and angle-resolved (AR) two-photon photoemission (2PPE) spectroscopy. The band dispersions originating from unoccupied electronic states have been identified from the comparison between AR-2PPE spectra and angle-resolved one-photon photoemission spectra with synchrotron radiation. A lifetime of unoccupied surface state has been determined from the TR-2PPE spectra.     

Chalcogen adsorption on Ni(100)

The successive stages of the oxidation of Ni(100) have been investigated by angle-resolved uv photoemission. The adsorption spectra are very similar to that measured previously for the same surface saturated with sulphur. Results are interpreted using recent theoretical calculations. It is found that this interpretation can be extended to the other chalcogens adsorbed on Ni(100). When increasing oxygen exposures, photoemission spectra have shown a continuity of the electronic character in the oxidation process.     

Asymmetric electron-nuclear dynamics in two-color laser fields: laser phase directional control of photofragments in H+2

Exact non-Born-Oppenheimer numerical solutions of the time-dependent Schrodinger equation for the 1D H+2 molecule in an intense, two-color (omega+2omega) laser field have been obtained. Both electron and proton kinetic energy spectra show spatial, correlated, asymmetric distributions. The calculated spectra exhibit the same unusual correlations as in experiments, in which both positively charged nuclear fragments and negatively charged photoelectrons were preferentially emitted in the same direction. The above asymmetries of photoemission of electrons seen in our quantum simulation are interpreted in the framework of a quasistatic tunneling model.     

Time evolution of the electronic structure of 1T-TaS2 through the insulator-metal transition

Femtosecond time-resolved photoemission is used to investigate the time evolution of electronic structure in the Mott insulator 1T-TaS2. A collapse of the electronic gap is observed within 100 femtoseconds after optical excitation. The photoemission spectra and the spectral function calculated by dynamical mean field theory show that this insulator-metal transition is driven solely by hot electrons. A coherently excited lattice displacement results in a periodic shift of the spectra lasting for 20 ps without perturbing the insulating phase. This capability to disentangle electronic and phononic excitations opens new directions to study electron correlation in solids.