H. Wern

Dispersion of the oxygen-induced bands on Cu (110) - an angle-resolved ups study of the system p(2×1)O/Cu(110)

Abstract The two-dimensional electronic energy bands induced by adsorption of atomic oxygen on Cu(110) are measured with angle-resolved photoelectron spectroscopy (ARUPS). Three oxygen-derived bonding bands and two Cu - derived antibonding bands are found below the Fermi energy. The dispersions can be described by assuming nearest-neighbour σ and π CuO bonds along the [001] direction with oxygen in a long bridge site.

On the band structure of silver and platinum from angle-resolved photoelectron spectroscopy (ARUPS) measurements

The general problems of an accurate a priori determination of electronic band structures are discussed. Data for the Γ ΛL line in Ag and Pt will be discussed as a case in point. Various methods have been employed to determine the electron wavevectorK of the direct transitions to the bands accessible to photon energieshv=16.9 eV and 21.2 eV: the disappearance and appearance angle method, the symmetry method, the triangulation method and the gap emission method. It is found that the experimentalE(K) points agree well within ±0.5 eV with a recently ab initio calculated conduction band structure. We further have measured Ag(111) normal emission AREDCs in the photon energy range 11 eV≦hv≦25 eV and have found that the intensity behaviour with photon energy of the direct transitions compares well with calculated bulk momentum matrix elements for Ag along Λ. A summary of the experimentalE(K) data for both the conduction and valence bands in Ag is given and compared with a recent local density calculation. Similar investigations for Pt are presented.

Band structure of platinum from angle resolved photoemission experiments

Abstract Angle-resolved and angle-dispersed ultraviolet photoelectron spectroscopy (ARUPS) has been used to determine the experimental band structure of platinum at various points along the ΓX, ΓKX and ΓL directions in the Brillouin zone. Various methods have been employed to obtain the E(k) points, among them absolute methods which determine the energy and the momentum independently without any assumption about the final state dispersion. The experimental points are compared with a self-consistent relativistic band structure calculation for energies below and above the Fermi energy. Good agreement between theory and experiment is found.

Mechanical elastic constants and diffraction stress factors of macroscopically elastically anisotropic polycrystals: the effect of grain-shape (morphological) texture

The effect of the grain-shape (‘morphological’) texture of a polycrystal on the mechanical elastic constants and diffraction (X-ray) stress factors is investigated. To this end, the Eshelby–Kröner grain interaction model originally devised for polycrystals consisting of spherical grains is extended to ellipsoidal grain morphology. Results obtained for the mechanical elastic constants show that a polycrystal consisting of ellipsoidal grains with their principal axes aligned along common directions (i.e. when an ideal grain-shape texture occurs) is macroscopically elastically anisotropic. Also the diffraction (X-ray) stress factors are affected by the grain-shape texture; they reflect the macroscopic elastic anisotropy by resulting in nonlinear so-called sin2 ψ plots. In general, a grain-shape texture can have a moderate effect on the mechanical elastic constants and a pronounced effect on the diffraction elastic constants, depending on the crystal symmetry and single-crystal elastic anisotropy.

Self-Consistent Calculation of the X-Ray Elastic Constants of Polycrystalline Materials for Arbitrary Crystal Symmetry

For the calculation of load or residual stresses from measured strain data by means of x-rays or neutrons, the x-ray elastic constants are required. Usually they are calculated from the corresponding single crystal data assuming some models such as Voigt(strain compatibility), Reuss(stress equilibrium), Hill(arithmetic average of Voigt and Reuss) or Kroner. All these models, however, make assumptions of the coupling of the grains. Historically, the Voigt and Reuss limits have been investigated first assuming constant strain respectively constant stress in the grains. Hill proved that the average elastic constants for a macroscopically isotropic aggregate should fall within the limits imposed by the Reuss and Voigt models Although the Kroner model is often recommend for the calculation of the x-ray elastic constants, it assumes the polycrystalline material to behave as a model or perfect disorder. In this paper, the finite element method is used to guarantee both, stress equilibrium and strain compatibility across the grain boundaries. The polycrystalline material is assumed to behave as a three-dimensional mixture of macroscopic isotropic and fully orthotropic grains under elastic loading where the relative ratio of both types of grains can be varied between 0 and 100%. The macroscopically isotropic bulk values are calculated by an iteration procedure which was first proposed by Kneer. The method is applicable to materials with a random texture by calculating the hkl dependent quasiisotropic x-ray elastic constants as well as to specimens with a given texture using the information of the Orientation Distribution Function (ODF) by calculating the corresponding hkl- and orientation dependent stress factors. All crystal structures from cubic to triclinic are supported. The method is demonstrated on monoclinic SiO 2 with a random texture.

Electronic structure investigation of Cu(001)-p(2 × 2) S using angle-resolved photoemission and inverse photoemission

Abstract Angle-resolved photoemission (ARPE) and inverse photoemission (ARIPE) are used to measure the dispersion of sulfur-induced electronic surface bands of the Cu(001)-p(2 × 2)S surface. Polarized synchrotron and rare-gas resonance radiation is used to identify the symmetry and the atomic origin of the three sulfur 3p-like bonding states between −4.9 and −5.4 eV below the Fermi energy, as well as of occupied antibonding Cu 3d-like states split off from the bulk d states to higher energy. The surface-band structure of the odd-symmetry states along the \ gD line of the Brillouin zone is interpreted in terms of a simple tight-binding LCAO picture, which takes into consideration only nearest-neighbour interactions between S 3p orbitals in the fourfold hollow adsorption site and neighbouring Cu3d orbitals in the surface plane. Two unoccupied S-induced states at +1.4 and +12.5 eV are detected. Using photoionization cross-sections, found in the literature, the latter is ascribed to S 3d. The work function (deduced from ARPE spectra) in dependence of the sulfur coverage is also given.

Dependence of the X‐Ray Elastic Constants on the Diffraction Plane

The calibration constants that link the peak-shift measured with X-rays to the macroscopic stress acting on the material are termed X-ray elastic constants. These terms contain both material parameters such as the elastic stiffness or compliance terms and configurational parameters caused by the Miller indices of the diffracting plane of the corresponding crystal structure. The configurational parameters can cause significant variations on the X-ray elastic constants. In this paper a new theoretical model is proposed for the calculation of the average X-ray elastic constants from single crystal data and its extensions will be compared and discussed with respect to the known Voigt and Reuss model. It uses a modification of the Voigt assumption and takes into account the fact that not all grains in a polycrystalline aggregate contribute to the data obtained by diffraction but only those which are selected by Braggs law for a given reflection. It is shown that this restriction yields compliances that depend on the crystallographic orientation of the diffracting crystallites. The new model is demonstrated for polycrystalline cubic, hexagonal and tetragonal materials but also can be extended to any crystal structure. It is called constraint Voigt (CV) model.

Angle-Resolved Photoemission and Band Structure of Copper

The triangulation method, the appearance angle method and for the first time the surface emission method have been used for the determination of the electronic band structure of copper along the [001], [110] and [111] direction from ARPES spectra taken on the (110) and (111) faces. The experimentally derived bands are compared with first-principles calculations. For the bands up to energies of 20 eV above the Fermi energies it is found that Burdicks band calculation is in good agreement with experiment. Together with published normal emission data an almost complete experimental band structure below the Fermi energy is derived which clearly shows the spin-orbit splitting of the d-bands.

Band structure of Ag, Au and Pt along the Δ line near X from angle resolved photoemission spectroscopy (ARUPS) : Location of transitions via evanescent gap surface states

Abstract The zero-slope (ZS)_method and the triangulation method of angle resolved photoemission for the location of transitions in -space have been used on (111) and (110) faces of Ag, Au and Pt in the ΓLK(1 1 0) mirror plane with hν = 21.2 eV. It is found that the experimental ZS-points correspond to emission from around the X110-point is due to excitation into evanescent states localized on the Γ111-Δ-X110 line, allowing an energy-mapping of the occupied bands. A comparison with calculated bands is given.

Identification of ventricular tachycardias by means of fast wavelet analysis

Discrimination of ventricular tachycardias at slow rate from sinus tachycardia still remains a challenge for implantable antitachycardia devices. It was the purpose of this study to assess the feasibility of endocardial signal processing using fast wavelet analysis (FWA) as a novel approach for discrimination of ventricular tachycardia (VT) and normal sinus rhythm (NSR). During an electrophysiological study, bipolar endocardial signals were obtained from the right ventricle in patients with known monomorphic VT. FWA was applied to single endocardial activations. The distribution of energy in the time-scale domain clearly discriminated between endocardial signals obtained during NSR and VT in each particular patient when using an individually adjusted wavelet. In patients with multiple VTs, identification of each VT with different morphology was also achievable.