Network topology and subgap resonances observed by Fourier transform scanning tunnelling microscopy of cuprate high-temperature superconductors

Fourier transform scanning tunnelling microscopy (STM) on Bi₂Sr₂CaCu₂O_{8+ δ} (BSCCO) subgap resonances has deciphered an octet of ‘quasiparticle’ states that are consistent with the Fermi surface and energy gap observed by angle-resolved photoemission spectroscopy (ARPES), but the origin of the high-intensity k-space octets and the sharply defined r-space chequerboard is unexplained. The filamentary ferroelastic nanodomain model that predicted the r-space chequerboard also explains the k-space octets and the origin of the apparent anisotropic surface d-wave gap by using strong electron–phonon interactions outside the CuO₂ planes. The topological model identifies the factors that stabilize high-intensity k-space octets in the presence of a very high level of irregular r-space chequerboard noise.     

Image state linewidth on Ag(100) studied by two-photon photoemission spectroscopy

The linewidth of the first image state on Ag(100) was studied using two-photon photoemission spectroscopy. The width was found to be 35±9 meV in agreement with a recent time-resolved study (Schoenlein et al.: Phys. Rev. Lett.61, 2596 (1988)) but somewhat larger than the theoretical value of 10–20 meV (Echenique et al.: Phys. Rev. B35, 4529 (1987)).     

Photoemission of molecular adsorbates

In photoemission the observed changes in band intensities as a function of light polarization, electron emission direction, and photon energy can be used to deduce information, for example, about geometry, two-dimensional band structures, intermolecular interactions and chemical reactivity of molecular adsorbates. To characterize the ion states of molecular adsorbates in the so-called inner valence electron region we discuss results from autoionization spectroscopy as a method that yields complementary information to photoemission in this spectral region. We use results on CO, N₂, and CO₂ adsorbates to discuss the various aspects of photoemission of molecular adsorbates.     

Investigation of lateral interactions in chemisorption

It is believed that lateral interactions play an important role in chemisorption site selection. With the advent of angular resolved and polarization dependent photoemission spectroscopies, one is able to compare computed band structure with angular resolved photoemission data. This advance coupled with theoretical calculations provides an excellent opportunity for investigating direct (adsorbate-adsorbate) and indirect (through-substrate) lateral interactions at surfaces. We have modelled the chemisorption of oxygen on Al(111) using direct lateral interactions only (isolated monolayer). The calculated results provide a good framework for interpreting the data. For O₂ monolayer the direct lateral interactions are also appreciable and lead to significant dispersion in the calculated energy bands. We have also computed the electronic structure for two dimensional isolated layers of CO in ordered configurations observed on a Pd(100) surface. The shape and width of the 4σ-derived band is satisfactorily accounted for by this direct lateral interaction only model. Having demonstrated that direct lateral interactions lead to observable band dispersions, it is suggested that they might influence normal vibrational frequencies and ought to be included.     

Coulomb correlations do not fill the e<Superscript>′</Superscript><Subscript>g</Subscript> hole pockets in Na<Subscript>0.3</Subscript>CoO<Subscript>2</Subscript>

There exists presently considerable debate over the question whether local Coulomb interactions can explain the absence of the small e^′ _g Fermi surface hole pockets in photoemission studies of Na_0.3CoO₂. By comparing dynamical mean field results for different single particle Hamiltonians and exact diagonalization as well as quantum Monte Carlo treatments, we show that, for realistic values of the Coulomb energy U and Hund exchange J, the e^′ _g pockets can be slightly enhanced or reduced compared to band structure predictions, but they do not disappear.     

Magnetic circular dichroism in lanthanide 4 d-4 f giant resonant photoemission: terbium

Magnetic circular dichroism in lanthanide 4 f photoemission (PE) multiplets was studied across the 4 d-4 f excitation threshold for the example of Tb metal. The combined experimental and theoretical analysis demonstrates that resonant enhancement of the 4 f PE signal and large magnetic contrast in the PE intensity are obtained simultaneously, when the excitation energy is tuned to the absorption-edge maximum for parallel orientation of magnetization and circular-polarization light helicity. Received 23 February 1999     

A resonant photoemission model for nickel metal

We present a model for the resonance near the 3p threshold in the photoemission associated with a static 3d pair-hole bound state in paramagnetic nickel. We show that, following Auger processes, the strong intra-atomic screening of the pair hole by the 4s?4p conduction band electrons is essential to explain experimental photoemission data not only in copper but also in nickel. From the energy relaxation of the conduction band states we estimate the bare Coulomb interaction between two d-holes in nickel.     

Electronic structure and metal-insulator transition in SrTi 1 - x Ru xO 3

We studied the changes in the electronic structure of SrTi_1-xRu_xO₃ across the metal-insulator transition. The parent compound, SrTiO₃, is a well known diamagnetic insulator; whereas the doped compound, SrTi_1-xRu_xO₃, becomes a ferromagnetic metal above x _C = 0.35. The techniques used in the study were photoemission (PES) and O 1 s X-ray absorption (XAS) spectroscopy. The experimental spectra were analyzed in terms of band structure and Hubbard model calculations. The PES and XAS spectra of SrTi_1-xRu_xO₃ show the Ru 4 d bands growing in the band gap of SrTiO₃ . The analysis in terms of the Hubbard model indicates that the Ti 3 d and Ru 4 d bands are mostly decoupled. This suggests that the metal-insulator transition is a percolation transition like that of metals embedded in a rare gas matrix. Electron correlation effects are present in this system, but they do not seem to play a major role in the transition. Received 10 September 2001     

Photoemission and x-ray studies of metal hydrides and hydride formation at metal/hydride interfaces

Synchrotron radiation photoemission was used to investigate the concentration dependent intensity of hydrogen-induced states for a series of niobium hydrides. These intensities were found to be linearly related to the fraction of β phase hydride present in the sample, as deduced from the bulk phase diagram, and we conclude that fracturing did not produce a self-sustaining surface hydride layer. Photoemission studies of V overlayers on NbH_0.799 showed strong hydrogen outdiffusion and the formation of the interface hydride phase which is more stable than VH prepared in bulk form. X-ray diffraction studies of thick V-H overlayers subsequently showed the absence of long-range order. These results demonstrate that interfacial diffusion of hydrogen from NbH_0.799 into the V overlayer produced a stable hydride with electronic properties analogous to those of crystalline VH but with dominant local interactions.     

Underestimating the width of the bandgap in the electronic spectra of La2CuO4

It is shown that the width of the bandgap determined by the simultaneous analysis of the experimental photoelectron and inverse photoemission spectra of the surface of La₂CuO₄ using a common energy scale is underestimated by 1 eV. The electronic and satellite structures of the spectra of La₂CuO₄ are calculated on the basis of the multiband p-d model and the sudden perturbation approximation. It is shown that shakedown processes shift the one-electron contour of the final two-hole configuration of the photoelectron spectrum 1 eV down the energy scale and shift the contour of the final d ¹⁰ configuration of the inverse photoemission spectrum downward by 2 eV; these shifts cause the energy splitting between the filled and empty bands to be underestimated. Fiz. Tverd. Tela (St. Petersburg) 39, 449–451 (March 1997)     

Spectra of low-energy electrons excited by soft x rays

A study is made of the x-ray photoemission spectra of low-energy electrons (E _kin=0–30 eV) and its dependence on the way in which the potentials are applied to the entrance system of the analyzer. It is shown that under certain experimental conditions the spectrum consists solely of electrons emitted from the sample. The x-ray photoemission spectrum of pure silver is found to have a fine structure, which disappears when a surface layer of carbon or oxygen is deposited on the surface. The spectrum then takes on the “cascade” form. A possible interpretation of the fine structure is suggested in the framework of band theory. Zh. Tekh. Fiz. 67, 73–77 (March 1997)     

Kinematic mechanism of the modulation of the spectral intensity in the spatially uniform system of Hubbard fermions

Using the exact representation of the Green’s function constructed in terms of the Hubbard operators, it has been shown that the kinematic interaction that induces the spin-fluctuation processes in the spatially uniform system of Hubbard fermions leads to significant variations in the spectral intensity A(k, ω) in the Brillouin zone. As a result, the modulation of A(k, ω) appears in the Fermi contour. The sign of the hopping integral within the first coordination sphere is determined by the contour section, where A(k, ω) decreases according to the angle-resolved photoemission spectroscopy data.     

Electron-emission activity of defects in surface layers of crystalline and vitreous silica

This paper deals with the photoemission activity of radiation defects in crystalline and vitreous silica. Spectral and kinetic parameters and the concentration of defects in the surface layer and the bulk of the material were determined. Some features of the distribution of radiation-induced defects along the depth of samples having different degrees of order were analyzed. Regularities of relaxation of excited states of emission centers suggested that thermally activated processes were involved in the release of electrons.     

Occurrence of a Mott-like gap in single-particle spectra of electron systems possessing flat bands

An unconventional type of the Mott’s insulators where the gap in the spectrum of single-particle excitations is associated with repulsive effective interactions between quasiparticles is shown to exist in strongly correlated electron systems of solids that possess flat bands. The occurrence of this gap is demonstrated to be the consequence of violation of particle–hole symmetry, inherent in such systems. The results obtained are applied to elucidate the Fermi arc structure observed at temperatures up to 100 K in angle-resolved photoemission spectra of the compound Sr₂IrO₄, not showing superconductivity down to low T.     

Surface chemistry: from vibrational spectroscopy to photoemission spectromicroscopy

Vibrational spectroscopy and core-level photoelectron spectroscopy are two of the most powerful tools for surface chemical analysis. We have used both techniques in conjunction with others to study hydrocarbon radical interactions with metal and semiconductor surfaces as well as thin film deposition processes. These include the investigations of CH₂ and CH₃ reactions on Cu by high resolution electron energy loss spectroscopy (HREELS), and GaN formation from Ga(CH₃)₃ and NH₃ on SiC by SR-PES. The vibrational analysis has revealed rich surface chemistry of the radicals while the photoemission study has led us to develop new approach for probing ultrafine structures. The effort involves the construction of a scanning photoemission spectromicroscope (SPEM) coupled to a synchrotron radiation light source. Recent results are presented in this brief report.     

Mass-selected Ag<Subscript>3</Subscript> clusters soft-landed onto MgO/Mo(100): femtosecond photoemission and first-principles simulations

The electronic structure of supported mass-selected Ag₃ clusters is analyzed by joint femtosecond photoemission spectroscopy and ab initio theoretical investigations. A wide band gap insulating magnesia ultra-thin film on Mo(100) has been chosen as substrate in order to minimize the electronic interaction between metal clusters and support. After magnesia ultra-thin film preparation no photoemission from the molybdenum substrate is observed anymore, instead very weak two photon photoemission is detected possibly originating from surface or subsurface oxide defect states. Soft-landing deposition of 2 of atomic monolayer equivalents of Ag₃ clusters results in the disappearance also of the MgO two photon photoemission signal, while a strong single photon photoemission signal is detected from states located directly below the Fermi level. The theoretical study of structural, electronic and optical properties of Ag₃ at two model sites of MgO (100), the stoichiometric MgO(100) and an F_S-center defect, based on the DFT method and the embedded cluster model provides insight into the interactions between the cluster and the support which are responsible for the characteristic spectroscopic features.     

Anisotropic scattering rates and antiferromagnetic precursor effects in the t-t'-U Hubbard model

We have investigated the evolution of the electronic properties of the t-t'-U Hubbard model with hole doping and temperature. Due to the shape of the Fermi surface, scattering from short wavelength spin fluctuations leads to strongly anisotropic quasi-particle scattering rates at low temperatures near half-filling. As a consequence, significant variations with momenta near the Fermi surface emerge for the spectral functions and the corresponding ARPES signals. At low doping the inverse lifetime of quasiparticles on the Fermi surface is of order varying linearly in temperature from energies of order t down to a very low energy scale set by the spin fluctuation frequency while at intermediate doping a sub-linear T-dependence is observed. This behavior is possibly relevant for the interpretation of photoemission spectra in cuprate superconductors at different hole doping levels. Received 31 July 2000     

Van Hove singularity as a possible origin of the bandwidth renormalization in layered superconductors

We use angle-resolved photoemission spectroscopy to study bandwidth renormalization in layered superconductor 2H-NbSe₂. Renormalization of the conduction band in comparison with the very similar 2H-TaSe₂ is of the factor of two. We discuss the possibility that the Van Hove singularity could be responsible for this effect not only in dichalcogenides but also in pnictides.     

Quenching of Pyrene Fluorescence by Calix[4]arene and Calix[4]resorcinarenes

Interactions involving calixarene and its derivatives are of major importance due to their widespread applications as unique hosts. Fluorescence from a common probe pyrene is used to study interactions involving calix[4]resorcinarene [1a] and its tetra-morpholine derivative [1b] in 1 M aqueous NaOH. These compounds efficiently quench the pyrene fluorescence. A comparison with the fluorescence quenching behavior of N-methylmorpholine clearly indicates the presence of long-range interactions involving 1a and 1b; the interactions are specific to the calixarene molecular framework. This is not the case for a tetra-nitro-substituted calix[4]arene [2b], an electron/charge acceptor quencher, as p-nitrophenol also shows similar interactions with pyrene. Effectiveness of cesium as the quencher of pyrene fluorescence is reduced in the presence of electron/charge donating 1b; fluorescence enhancement is observed upon addition of cesium as the concentration of 1b is increased in the solution. The role of calixarene framework in interactions involving such compounds is established.     

External photoelectric effect and high-efficiency photocathodes in the soft-x-ray region of the spectrum

Two approaches to calculating the “true” x-ray photoemission from high-efficiency photocathodes are analyzed in the photon energy range 1–10 keV. Analytic expressions are derived for the quantum yield of true x-ray photoelectrons. The calculated values of quantum yield are compared with experimentally measured values. Fiz. Tverd. Tela (St. Petersburg) 39, 1665–1671 (September 1997)