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Abstract: Enhancement of surface state peaks in angle resolved ultraviolet photoelectron spectra (ARUPS) from the Al(111) surface is studied experimentally and theoretically within the one-step model of photoemission. The resonant enhancement of the surface state emission is explained by the crucial role of elastic scattering of the outgoing electron. Dipole transitions to evanescent states in the final bands of the crystal are shown to determine photoemission at the resonant photon energy. The band structure based explanation is confirmed by the measurements of electron reflectivity and of the fine structure of valence band spectra. The surface sensitivity of ARUPS is shown to depend strongly on the complex band structure of the crystal and to be finely tunable by the choice of photoemitted electron energy. [Copyright &y& Elsevier]

The role of the variational quantum Monte-Carlo method for homogeneous systems and bulk material, on one hand, and finite aggregates like molecules, on the other, is now well established, mainly for the determination of ground state properties. It has been recently shown that it may be also used in surface investigations. Furthermore, dynamic behavior seems to be accessible, too, in view of the numerical resolution currently achieved. This applies also to surface dynamics. Considering the still smaller energy scales as represented by the electron–phonon interaction represents the actual borderline of accessible resolution. Nevertheless, localized electronic excitations could be investigated in bulk solids in order to yield the changes in excitation energy induced by a deformation, which shows thus a possible access to the deformation potentials. [Copyright &y& Elsevier]

Over the past two decades dramatic progress has been made in the development of the theory of X-ray and electron spectroscopies, e.g., X-ray absorption spectra (XAS), core-level X-ray photoemission spectroscopy (XPS), X-ray photoelectron diffraction (XPD), etc. A revolutionary advance was the development of efficient and accurate treatments of high-order, curved-wave electron multiple-scattering for high electron energies above a few hundred eV. These advances were applied first in the theory of X-ray absorption fine structure (XAFS) and subsequently in X-ray photoelectron diffraction (XPD). They also led to efficient ab initio codes which permit a quantitative interpretation of the spectra. Extensions have made it possible to treat magnetic effects, e.g., X-ray magnetic circular dichroism (XMCD) and its analog in XPD. Important progress has recently been made in understanding near-edge spectra, e.g. X-ray absorption near edge spectra (XANES), which often require a full-multiple-scattering treatment. Although such calculations had been highly demanding computationally, fast new approaches based on efficient Lanczos approaches and parallel processing have been developed to overcome this limitation. [Copyright &y& Elsevier]

We describe a new method to calculate the VLEED (very low energy electron diffraction) spectra within the Bloch waves approach. The method is based on the variational solution of the Schrödinger equation for a semi-infinite crystal. Inside the solid the trial LEED function Φ is a linear combination of propagating and evanescent Bloch waves, which are generated by the inverse ELAPW (extended linear augmented plane waves)-kp method. The trial function is smoothly continuous over the whole space, and it satisfies by construction the equation ( Ĥ − E) Φ=0 both in the crystal and in the vacuum half-spaces. In the surface layer the equation δ|( Ĥ − E) Φ|=0 is solved. To illustrate the properties of the method we discuss its application to the 1D case. We have performed a self-consistent band structure calculation of the 1 T chalcogenide VSe2 and obtained from the first principles the normal-incidence target current spectrum (TCS) for its (0001) surface. [ABSTRACT FROM AUTHOR]

The surface sensitivity of electron diffraction and of electron spectroscopies is determined by the imaginary component of the electron self-energy. In crystals, the energy and direction dependence of the electron attenuation and of the escape depth should be taken into account at very low energies. Strong anisotropy of the electron attenuation has been obtained around 20 eV from peak shapes in VLEED intensity profiles from (111) transition metal surfaces. Extension of the local density approximation in the density functional formalism provides quantitative description of the electron self-energy. The one-step model of angular resolved photoemission incorporating the self-energy predicts a strong energy and angle dependence of the escape depth of low energy photoelectrons emitted from GaAs(110). [ABSTRACT FROM AUTHOR]

Photoelectron spectroscopy, in particular the angular resolved photoemission excited by ultraviolet radiation (ARUPS), provides the most direct experimental information about the electron structure of crystals, both of the bulk and of the low-index surfaces. The sensitivity of the method, as well as its difficulties, when applied to GaAs/AlAs superlattices are described. The new periodicity of these man-made crystals in the direction of their growth (e.g., in the layer-by-layer growth in molecular beam epitaxy), is responsible for opening of the new energy gaps (so-called minigaps) in the electron energy bands of crystals forming the superlattice. In addition to the well-known confinement of electrons at the valence and conduction band edges in long-period superlattices, the electron confinement to the interfaces has also been found in the vicinity of minigaps in short-period superlattices. The role of this confinement in the intensities of electrons photoemitted from superlattice surfaces is discussed. Superlattices with different thicknesses in the topmost layers represent systems with a simple change of the surface atomic structure. The predictions of one-dimensional models about a change of the surface-state energy within the band gap with a change of crystal potential termination are tested for the ideally terminated (1 0 0) surface of a very thin superlattice (GaAs)2(AlAs)2. The results of the energy distributions of photoemitted electrons, calculated in the one-step model of photoemission, show that the ARUPS experimental observation of surface-state shifts should be possible, at least in larger minigaps. The results indicate the possibility of a straightforward tuning of the electronic structure of the superlattice surface by geometrical means. [Copyright &y& Elsevier]

Profound gradual changes of surface state energies were predicted for varying surface terminations of the periodic crystal potential in one-dimensional models.[sup 1] This situation can be realized in superlattices with different thicknesses of topmost layers. For the ideally terminated (100) surface of a very thin superlattice (GaAs)[sub 2](AlAs)[sub 2], the shift of the energy of the surface state over the whole minigap in the lower part of the valence band has been found for different terminations of the topmost layer. In the center of the surface Brillouin zone the surface state shift follows model trends. The changes of the energy distribution of photoemitted electrons as determined from the one-step photoemission calculation[sup 2] indicate that experimental observation by the surface-sensitive technique of angle-resolved photoemission should be feasible, and preliminary data indicate this. The results show a straigthforward tuning of surface electron structure by geometrical means. [ABSTRACT FROM AUTHOR]

Increased translational period of crystalline superlattices gives rise to qualitative modifications of the electron band structure E(k): minigaps appear at new Brillouin zone boundaries and band dispersions are reduced into narrower allowed energy bands. Modifications of the wave functions, consisting in electron confinement into one of the two components of a superlattice can affect the intensities of photoemitted electrons. The layer-resolved contributions from a few topmost layers to the photoelectron intensity are evaluated in the one-step model and the importance of the related optical matrix elements is shown. Large number of surface states and resonances connected with superlattice can be expected. This expectation is confirmed by evaluation of the local densities of electron states for the unreconstructed (1 0 0) surface of the 2×2 superlattice. The space distribution of localized states is presented. Energy distribution curves for normally photoemitted electrons are analyzed from the above viewpoints. [Copyright &y& Elsevier]

Quantum Monte Carlo methods are a stochastic approach to directly tackle the manybody problem in solids. They have proven to describe virutually exactly the ground state of correlated bulk systems, like the homogeneous electron gas or solids of C, Ge, Si and GaAs. Especially Variational quantum Monte Carlo calculations using nonlocal ab initio pseudopotentials offer a way to study systematically many-body effects at solid surfaces, safely founded on the variational principle “the lower the energy, the better the wave function”. Here we report on first attempts for the relaxed (110) surface of GaAs, serving as a prototype of semiconductor surfaces. A finite layer geometry is chosen as the boundary condition of the multidimensional stochastic integration scheme. The exact many-body Hamiltonian is cast in a form allowing for rapid evaluation. New parameters in the correlated trial wave function increase the variational freedom necessary to take into account the influence of the surface. Their physical meaning and their statistical significance are discussed in detail. [ABSTRACT FROM AUTHOR]

Attosecond streaking spectroscopy of solids provides direct observation of the dynamics of electron excitation and transport through the surface. We demonstrate the crucial role of the exciting field in electron propagation and establish that the lattice scattering of the outgoing electron during the optical pumping leads to the wave packet moving faster than with the group velocity and faster than the free electron. We solve the time-dependent Schrödinger equation for a model of laser-assisted photoemission, with inelastic scattering treated as electron absorption and alternatively by means of random collisions. For a weak lattice scattering, the phenomenological result that the photoelectron moves with the group velocity dE/dℏk and traverses on average the distance equal to the mean-free path is proved to hold even at very short traveling times. This offers a novel interpretation of the delay time in streaking experiment and sheds new light on tunneling in optoelectronic devices. [ABSTRACT FROM AUTHOR]

Abstract: The electronic structure of (GaAs)2/(AlAs)2(100)-c(4×4) superlattice surfaces was studied by means of angular-resolved photoelectron spectroscopy (ARUPS) in the photon energy range 20–38eV. Four samples with different surface termination layers were grown and As-capped by molecular beam epitaxy (MBE). ARUPS measurements were performed on decapped samples with perfect c(4×4) reconstructed surfaces. An intensive surface state was, for the first time, observed below the top of the valence band. This surface state was found to shift with superlattices’ different surface termination in agreement with theoretical predictions. [Copyright &y& Elsevier]

In order to understand binding mechanisms and the resulting stability of misfit layered compounds, the valence bands of (PbS)1.18TiS2 and (PbS)1.18(TiS2)2 were mapped along the ΓM(AL) direction in reciprocal space parallel to the surface. The results were compared to the experimental and the theoretical band structure of pure TiS2. The changes in the electronic structure were analyzed with regard to the effects that are known to occur in intercalated transition metal dichalcogenides and that can directly be attributed to a charge transfer from the intercalated species to the host material. The theoretical determination of the densities of states for the pure TiS2 and the misfit layered compound (PbS)1.18TiS2 confirms a charge transfer. [Copyright &y& Elsevier]

Angle-resolved photoemission measurements on gallium nitride single crystals and epitaxial thin films with wurtzite structure were performed using synchrotron radiation. Calculated theoretical final state bands were used to determine the corresponding k vectors in reciprocal space using the direct transition model. We were able to identify several previously unobserved features including several surface states and transitions to non-free-electron final states. Significant differences in the surface electronic band structure between thin film and single crystal samples were observed. [Copyright &y& Elsevier]