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Laser-assisted photoemission from a solid is considered within a numerically exactly solvable one-dimensional model of a crystal. The effect of the inelastic scattering and of the finite duration of the pump pulse on the photoelectron dynamics is elucidated. The phenomenological result that the photoexcited wave packet moves with the group velocity dE/dk and traverses on average the distance equal to the mean free path is found to hold for energies far from the spectral gaps of the final state band structure. On the contrary, close to a spectral gap the photoelectron is found to move during the excitation by the pump pulse with a velocity higher than the group velocity., 5 pages, 3 figures

Circular dichroism in angular distribution was observed for the valence band photoelectron from nonmagnetic and nonchiral material Si(001) with the incident photon energy in UPS region. The obtained two-dimensional angular distributions are to a certain extent consistent with an analysis of simple electronic dipole transition. Calculated angular distributions based on highly accurate one-step model showed better agreement considering photoelectron diffraction effect.

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Physical review / B 63(3), 035107 (2001). doi:10.1103/PhysRevB.63.035107, Published by Inst., Woodbury, NY

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Physical review letters 83(26), 5551 - 5554 (1999). doi:10.1103/PhysRevLett.83.5551, Angle-resolved photoemission spectroscopy (ARPES) has been commonly applied to evaluate the shape of Fermi surfaces by employing simple criteria, such as maximum photoemission intensity at the Fermi level or discontinuity in the momentum distribution function, for the determination of the Fermi vector kF parallel to the surface. Here we show that these criteria may lead to large uncertainties, in particular for narrow band systems. We develop a reliable method for the determination of Fermi vectors employing high resolution ARPES at different temperatures. The relevance and accuracy of the method is discussed on data of the quasi-two-dimensional system TiTe2., Published by APS, College Park, Md.

Physical review / B 55(16), 10392 - 10399 (1997). doi:10.1103/PhysRevB.55.10392, We report on a complete determination of the experimental and theoretical valence- and conduction-band structure of WSe2. The results of combined angle-resolved photoemission and inverse photoemission are discussed in the context of fully relativistic linear-muffin-tin-orbital and extended linear-augmented-plane-wave calculations. We measure an indirect band gap of 1.2 eV, and find the valence-band maximum to be located at the center of the Brillouin zone and the conduction-band minimum at 0.55ΓK., Published by Inst., Woodbury, NY

The impact ionization rate and its orientation dependence in k space is calculated for ZnS. The numerical results indicate a strong correlation to the band structure. The use of a q-dependent screening function for the Coulomb interaction between conduction and valence electrons is found to be essential. A simple fit formula is presented for easy calculation of the energy dependent transition rate., 9 pages LaTeX file, 3 EPS-figures (use psfig.sty), accepted for publication in PRB as brief Report (LaTeX source replaces raw-postscript file)

Physical review / B 41(5), 2982 - 2990 (1990). doi:10.1103/PhysRevB.41.2982, Normal-emission photoemission spectra from the (001) surface of 1T-titanium diselenide have been calculated and compared to experimental energy-distribution curves recorded for various photon energies in the range from 12 to 28 eV. The theory is based on a single-particle one-step model for the emission process. The initial state of the photoelectron is described by means of an empirical tight-binding method, the surface being included via a Green’s-function formalism, while a low-energy electron-diffraction scheme is used for the characterization of the final state. Dipole matrix elements are computed numerically. In comparison to the experimental data, the variation of the relative intensities of the photoemission maxima with photon energy, and to some extent these relative intensities themselves, are correctly reproduced by the calculation., Published by Inst., Woodbury, NY

A combined experimental and theoretical study of very-low-energy electron diffraction at the (0001) surface of 2H-NbSe2 is presented. Electron transmission spectra have been measured for energies up to 50 eV above the Fermi level with k|| varying along the GammaK line of the Brillouin zone. Ab initio calculations of the spectra have been performed with the extended linear augmented plane wave k-p method. The experimental spectra are interpreted in terms of three-dimensional one-electron band structure. Special attention is paid to the quasi-particle lifetimes: by comparing the broadening of the spectral structures in the experimental and calculated spectra the energy dependence of the optical potential Vi is determined. A sharp increase of Vi at 20 eV is detected, which is associated with a plasmon peak in the Im(-1/epsilon) function. Furthermore, the electron energy loss spectrum and the reflectivity of NbSe2 are calculated ab initio and compared with optical experiments. The obtained information on the dispersions and lifetimes of the unoccupied states is important for photoemission studies of the 3D band structure of the valence band., 17 pages, 11 Postscript figures, submitted to Phys. Rev. B

(Keine Zusammenfassung in deutscher Sprache vorhanden.) (No summary in German language.) In this thesis, the software package KSAP (Kieler Software for Angular Resolved Photoemission) is used to calculate within the one-step model the spectra of the angle resolved photoemission spectroscopy for different III/V semiconductor surfaces. The numerical results are not only used to compare the experimental and theoretical spectra qualitatively. Moreover, the information from of a full calculation such as the matrix elements, complex band structure and density of states allow to analyse the microscopic details of the measured photocurrent. Gallium nitride has attracted considerable interest because of its application for blue light-emitting diodes and lasers. This thesis presents the photoemission spectra calculated for different surfaces of the cubic and the wurtzit phases of GaN. The direct comparison between experimental and theoretical spectra shows a better agreement, as could be expected by a comparison between an experimental band mapping and calculated band structures. In particular, for the case of GaN(0001) the numerical freedom is used to calculate spectra series in normal emission based on different theoretical band structures. The systematic comparison with experimental results allows the accurate determination of the valence band width, which permits the assessment of the accuracy of different ab initio band structure calculations for GaN. Furthermore, a shift in the binding energy of about 1.0 eV between the corresponding theoretical and experimental structures was observed. This difference could be related to the uncertainty in the experimental determination of the valence band maximum. Total energy calculations predict for GaN a stable 1x1 reconstruction of the (001) and (0001) surfaces respectively, which are stabilized through the metallic Ga bonds, a property rather different compared to semiconductors like GaAs. Within the scope of the photoemission calculations, some features of the measured spectra are clearly related to emission from these characteristic gallium layers, which underline the reliability of the used surface model. With GaAs(001)-c(4x4) surfaces and (GaAs)_n(AlAs)_m Superlattices the calculations are extended to systems with large unit cells parallel and longer periodicity perpendicular to the surface, respectively. For the GaAs(001)-c(4x4) surfaces, it is shown that in experiment the expected periodicity of a surface state is lost, which is related to the interaction between the contributions of the bulk and the surface to the photocurrent. It was demonstrated through the calculated photoemission spectra of (GaAs)_n(AlAs)_m Superlattices that photoemission, usually expected to be surface sensitive, can detect the change of the bulk periodicity. The features of the spectra associated to the change of the bulk periodicity are a lower dispersion of emission which are related to direct transitions, the occurrence of minigaps, which are detectable in the spectra, and the energetical position of surface states within these minigaps, which is strongly affected by the surface termination. Furthermore, the theoretical analysis allows to determine the contribution of the atomic layers below the surface to the photocurrent. It is shown that for a single emission, small differences in the photon energy changes drastically the depth of these layers. The latest effect is analysed in more detail for InAs. Experimental techniques allow to prepare samples with a well defined initial state at the upper conduction band edge. The photoemission spectra, which was measured in constant initial state mode, can be related to escape depth by theory. This shows that for low excitation energy, as used in the ultraviolet photoelectron spectroscopy, the band structure effects in the final states strongly affect the escape depth. Therefore, the well known universal curve of escape depth should be used carefully in this energy region.

The aim of this work was to understand how alkali metal atoms intercalate into transition metal dichalcogenides (TMDCs). To this end, a detailed investigation into the geometrical and electronic effects on TMDCs caused by adsorption and intercalation of alkali atoms was carried out by means of ab initio density-functional theory. The geometrical and electronic properties of clean TiSe_2 (0001) surface were studied. Also, the adsorption sites and diffusion of alkali atoms on TiSe_2 (0001) surface were computed and compared with those obtained at the van der Waals gap. The electron charge distribution caused by alkali atoms on TiSe_2 (0001) surface was analysed. The alkali coverage dependence of the physical properties adsorption energy, work function, and valence electron distribution was also studied. The question of whether the alkali atoms can reach the van der Waals gap from the plain surface through the atomic planes was addressed. Because of the huge energy barrier involved in this process (3.81 eV), this type of intercalation can be excluded. Thus, the experimentally found intercalation processes must be associated with defects, such as steps or dislocations at the surface. A model of alkali intercalation into TMDCs was obtained, namely via a (1 0 -1 0) step.

Materialeigenschaften ab initio, d.h allein aus den Ordnungszahlen der beiteiligten Elemente im Periodensystem, zu berechnen, ist eine der großen Herausforderungen der theoretischen Festkörperphysik. In dieser Arbeit wird ein Formalismus entwickelt, der die Gesamtenergie komplexer Festkörper (mit vielen Atomen in der Einheitszelle) effizient zu berechnen erlaubt. Dadurch können statische Größen wie Dichte, Härte oder Symmetrie und dynamische wie Diffusion beschrieben werden. Angewendet wird das Verfahren auf zwei Beispiele nanotechnologischer Relevanz. An einem neuartigen Schichtkristall KCu2Se2 werden die Fragen der chemischen Bindung, der optischen Eigenschaften und der Symmetriebrechung diskutiert. Die Verwendung des röhrenförmigen Kristall Cetineit als Molekularsieb wird anhand der Diffusion untersucht.

Es werden die elektronischen und optischen Eigenschaften eines kristallinen Festkörpers unter Berücksichtigung der Oberfläche mit ab-initio Methoden berechnet. Die Behandlung der Oberflächeneffekte in einer Halbraum-Geometrie erzwingt die Darstellung der Wellenfunktionen und der Dielektrischen Funktion (DK) in einer lokalen Basis. Anhand von Volumenkristallen wird die Effizienz von LCAO- Basisfunktionen demonstriert. Die Erweiterung der atomaren Orbitale mit unbesetzten Orbitalen und 'off-site'-Funktionen (die im zwischen-atomaren Raum lokalisiert sind) ergibt eine sehr gute Beschreibung der Valenz- und Leitungsbänder bis zu Energien von einigen 10 eV für einfache Metalle und Halbleiter. Die Darstellung der Hamiltonmatrix des Halbraums in diesen Basisfunktionen erlaubt die iterative Bestimmung der Greenschen Matrix und damit der Oberflächenbandstruktur. Es werden selbst-konsistent berechnete Ergebnisse von ab-initio Rechnungen für die relaxierte GaAs(110) und die Cu(111) Oberfläche vorgestellt. Die Berechnung der DK von GaAs(110) in der Random-Phase-Approximation aus der Greenschen Funktion des Halbraums führt auf ein RAS-Spektrum, das in guter Übereinstimmung mit Superzellen-Rechnungen und dem Experiment ist.

(Keine deutschsprachige Zusammenfassung vorhanden.) Quantum Monte Carlo (QMC) methods are a stochastic approach to directly tackle the many-body problem in solids. Especially variational quantum Monte Carlo (VMC) calculations using nonlocal ab initio pseudopotentials offer a way to study many-body effects in real materials systematically, safely founded on Ritz' variational principle 'the lower the energy, the better the wave function'. In this work the method is extended to study dynamical and structural aspects in solids and solid surfaces from first principles, with practical calculations for phonons and the electron-phonon coupling in gallium arsenide (GaAs) and relaxation at the (110) surface of GaAs. Within a frozen phonon scheme, the frequency of an optical phonon is determined with an accuracy of a few meV in very good agreement with experiment. An optical deformation potential approach is chosen for the electron-phonon coupling, showing that the interaction between excited states and distortions of the ion lattice is statistically resolvable within the VMC method. As for the surface systems, taking the boundary condition for the simulation from a finite-layer geometry, the Hamiltonian is cast in a layer-resolved form and evaluated with a two-dimensional Ewald summation technique. The exact cancellation of all jellium contributions to the total energy is ensured. In the trial wave function a confinement term is introduced to compensate for the smoothing effect of the Jastrow factor. A realistic surface model of GaAs(110) is compared with the ideal geometry for different system sizes, showing a consistent finite size dependence of the total energy per atom. With the resulting optimal wave function further surface specific information is extracted to further analyze the many-body effects at solid surfaces.