Charge transfer from chemical shifts at (110) surfaces of III–V compound semiconductors

Surface charge-transfer are calculated from surface core-level shifts determined by Eastman et al. and Taniguchi et al. by using soft X-ray photoemission spectroscopy. A simple electrostatic model which was first applied by Shevchik et al. to the analysis of bulk data is adapted for surfaces of compound semiconductors. The charge transferred from cations to anions is found to be the same in the bulk and at the (110) surfaces of GaP, GaAs, GaSb, and InSb.     

Localized phonons due to the adsorption of a monolayer of atoms on a crystal surface

Localized surface phonons due to a monolayer of adsorbed atoms on a free surface are studied with a simple crystal model. Surface localized modes inside a ‘surface gap’ between the bulk bands of frequencies, and inside a bulk band are reported. Resonant surface states well defined in energy are also found.     

Mixed-valence states at a crystal surface: SmS

We present a model calculation, based on the Falicov-Kimball model for metal-insulator transitions, which shows that for a mixed-valence solid with a surface, the average valence of the atoms at the surface may be substantially different from that in the bulk. The effect, which we have calculated only for T = 0 and neglecting hybridization, is due to the different local density of itinerant states at the surface and bulk atoms. Surface states contribute to the valence difference but are not solely responsible for it.     

Phonons and periodons in IV–VI semiconductor superlattices

We have calculated the phonon and periodon dispersion relations in IV–VI semi-conducting bulk PbTe and SnTe and their superlattice structure. The model used here is a one-dimensional lattice which includes harmonic interactions up to second neighbours as well as on-site nonlinear electron-ion interactions at the anion site. We calculate the phonon and periodon dispersion relations in bulk and PbTe-SnTe superlattice for the transverse optic and acoustic modes using the transfer matrix method. Our analysis has predicted correct nature of the folding of acoustic and confinement of optical phonons at various frequency intervals corresponding to pass and stop bands of the superlattices.     

On the equilibrium segregation of interacting impurity atoms towards (100) metal surfaces: I. Theoretical considerations including first- and second-neighbor interactions

Experiments on the segregation of foreign atoms towards single crystal surfaces usually show long-range-order phenomena and/or strong deviations from the Gihbs-McLean behavior both indicating the presence and importance of interactions between the segregating atoms. Model calculations of segregation bulk isosteres for interacting impurity atoms segregating towards (100) metal surfaces are therefore presented based on an extended Ising model using the Monte Carlo method. Interactions between nearest and next-nearest neighbors are considered and bulk isosteres given for a number of combinations of values thereof. In a paper to follow the results of these calculations are to be applied to existing experiments and values for certain interaction enthalpies deduced.     

A systematic semi-microscopic analysis of the scattering of oxygen isotopes

A semi-microscopic theory for the scattering of nuclei near closed shells is discussed in detail and applied to the bulk of the presently available experimental data. The results generally indicate that the single-folding model can be used to extrapolate the effective interactions for heavy-ion scattering away from closed-shell systems. We particularly discuss the importance of the imaginary part of the valence interaction and the effects of higher-order reorientation and two-step transfer processes.     

Surface phonons and superstructures; Some applications

Experiments using the technique of low energy electron diffraction (LEED) have shown that the periodic configuration of the atoms on some clean surfaces for a wide range of crystals is different from what is expected from the bulk configuration. A simple lattice dynamical calculation is presented showing that surface superstructures may exist even if interactions near the surface remain unchanged by the “bond-breaking” procedure. A calculation has been made for surface phonons at the (001) surface of some fcc and bcc transition metals. A model consisting of first and second neighbor central interactions together with angle bending forces is used — specific results have been obtained for surface phonons of Pt, Pd, Cu and Ag for fcc metals as well as Fe and W for bcc crystals. The agreement with neutron measurements for the bulk phonons is good. The (001) surface phonons for the above cited transition metals are obtained for the first time to our knowledge. The effect of variations of the interactions near the surface for Pt has been also investigated showing soft surface phonons for surface forces decreased by about 35% to 40% as compared to their bulk values. This model has been extended to study the vibrations of an adsorbed layer namely in the case concerning rare gazes (Ne, Xe,…). A similar attempt showing possible magnetic surface superstructure is also reported.     

Surface excitations in magnetic systems with a singlet ground state

We have investigated surface excitations in a system wherein the ionic ground state is a magnetic singlet. The pseudospin formalism is employed to account for the crystal-field and exchange interactions between the ions in a Heisenberg ferromagnet with the singlet-triplet crystal-field-only level scheme. The Hamiltonian was studied in the molecular field approximation to find the possible ground states. Surface excitations for the simple cubic structure were investigated for the (001) surface in the random phase approximation. Analytic expressions have been obtained for the thermodynamic Green functions. For a fully magnetized molecular field ground state, there are in general two bulk bands, the spin-wave and the excitonic. Surface modes were found to exist below the spin-wave band, above the excitonic band and between the bands. The dispersion curves can exist only over one or two limited regions of the two-dimensional wave-vector parallel to the crystal surface.     

Sur l'entropie de vibration des défauts dans l'approximation d'einstein

The vibration entropies of bulk or defect atoms of a crystal (vacancies, clean or rough surfaces) are given, in general, with a good precision in the Einstein approximation. In this approximation we compare the Montroll-Potts model to a model with central potentials between first nearest neighbours and to a model with central and second nearest neighbours and also angular interactions in order to show that, for sites having unsymmetrical neighborhood, one has in general to define different Einstein frequencies for different normal modes of vibration. The results in the last model show also the importance of angular forces and central forces between second neighbors. Finally the precision of the calculation of the surface entropy is i if one does an Einstein-like approximation on a dynamical matrix written for each k∥.     

Enhanced Raman characterization of adsorbed water at the electrochemical double layer on silver

Surface enhanced Raman scattering (SERS) from molecular water adsorbed on silver in an aqueous electrolyte is reported. The SERS active water is most probably associated with active sites on the metal which are stabilized by the double layer structure. The line shapes of the primary vibrations associated with the O-H stretch and the H-O-H bend are strong functions of the applied voltage, the electrolyte pH, and the bulk concentration of cations. At least two types of adsorbed water are observed. These data directly demonstrate that the structure of the first layer of water is significantly influenced by interaction with adjacent layers.     

Approximate calculation of surface Debye temperatures

A simple-minded decoupled oscillator model is used to calculate the effective surface Debye temperatures of several low-index fcc and bcc metal surfaces. The surface field is simulated by the pair-wise summation of Morse potentials. Surface relaxations are the essential factor in changing the potential environment of the surface atoms. Acceptable agreement with experiment is found for both bulk and surface Debye temperatures, indicating that the model is an adequate representation of the first-order features of surface atom vibrations.     

A noncontact method for determining surface density of nonporous materials with the limp-wall mass law

A new noncontact technique for determining the surface density or mass per unit area of nonporous, homogeneous membranes and foils of sub-wavelength thicknesses is introduced. Surface densities are determined through application of the limp-wall mass law and through-transmission ultrasonic measurements of bulk waves. The ultrasonic measurements are performed with commercially-available, broadband air-coupled ultrasonic transducers. Surface densities of aluminum foil, brass shim, and plastic sheets are typically found to be within 3% of their accepted values.     

Surface nanocrystallization of an ionic liquid

Surface crystallization at the vapor-liquid interface of the ionic liquid (1-butyl-3-methylimidazolium hexafluorophosphate) is observed in synchrotron x-ray diffraction studies. Sharp Bragg reflections emerge in grazing-angle x-ray diffraction patterns 37?°C above the bulk melting temperature, indicating the presence of a long-range ordered phase at the surface in coexistence with the bulk parent liquid. The unique structure of the vapor-liquid interface where butyl chains attached to the cations are expelled to the vapor side facilitates interionic electrostatic interactions that lead to the crystallization. Our results demonstrate the complexity of ionic-liquid structure with their tendency to spontaneously phase separate into nanodomains with finite correlation lengths in coexistence with the liquid phase. By virtue of interfacial boundary conditions, these nanodomains grow laterally to form quasi-two-dimensional crystals.     

Random copolymers at a selective interface: Many chains with excluded volume interactions

We investigate numerically, using the bond-fluctuation model, the adsorption of many random AB-copolymers with excluded volume interactions at the interface between two solvents. We find two regimes, controlled by the total number of polymers. In the first (dilute) regime, the copolymers near the interface extend parallel to it, while in the second regime they extend perpendicular to it. The density at the interface and the density in the bulk depend differently on the total number of copolymers: In the first regime the density at the interface increases more rapidly than in the bulk, whereas the opposite is true in the second regime. Received 4 March 1998 and Received in final form 22 September 1998     

Charge carrier dynamics in a-Si:H

The transient excess microwave absorption induced by laser pulse excitation of free charge carriers has been investigated. Surface and bulk decay characteristics have been obtained by excitation with 530 and 1060 nm light, respectively. An initial fast second order process and a slow decay attributed to dispersive trapping have been observed.     

Molecular forces and elastic constants of polyethylene single crystals

Polyethylene has an orthorhombic lattice for which nine elastic constants exist; they are obtained in terms of the intra- and intermolecular forces. Constants involved in the 6-12 Lennard-Jones potential approximating the London dispersion type of van der Waals' forces are obtained by computing the crystal potential energy and comparing it with the cohesive energy. First and second nearest-neighbor interactions are considered to establish relationship between the elastic constants and the interaction constants. The latter are obtained in terms of the C—C bond, stretching, bending, and repulsive force constants and the L-J potential constants. A limited type of central force assumption is applied. Values of Young and shear moduli are obtained along the three axes. The value along the chain compares with the experimentally determined and calculated values for oriented polyethylene. Young's modulus along the lateral direction is of the order of Young's modulus of bulk polyethylene, showing that intermo-lecular forces are the ones that determine the Young modulus of bulk polyethylene.     

Lowering Critical Temperature in the Extended Quark Sigma Model at Finite Temperature

A phase transition, a critical temperature, and meson masses are studied in the extended quark sigma model, in which the effective mesonic potential is extended to include eighth-order mesonic interactions. The second derivative of the effective mesonic potential is applied to calculate the effective sigma and pion masses as functions of temperature. We find that the critical temperature assumes a lower value in comparison with that of original quark sigma model. A comparison with recent calculations of lattice QCD is introduced. The behavior of the phase transition remains unchanged when the higher-order mesonic interactions are included. We find that the spontaneous symmetry-breaking condition is necessary to satisfy the Goldstone theorem at low temperatures.     

Quantized electrons and holes in tellurium surfaces

A calculation has been made for surface phonons at the (001) surface of some fcc and bcc transition metals. A model consisting of first and second neighbor central interactions together with angle bending forces is used. The general harmonic approximation is used. A least mean square fit of the bulk phonon spectrum determines the parameters characterizing the interactions. Specific results have been obtained for surface phonons of Pt, Pd, Cu and Ag as well as Fe and W. The effect of varying the force constant near the surface is also reported showing the possibility for the existence of soft surface modes which may account for superstructures.     

On the compressive response of carbon nanotube tangles

The nonlinear bulk compressibility of entangled multiwalled carbon nanotubes is studied. The analogy with textile fibre assemblies is explored by means of the well established van Wyk model. In view of the small diameter of the nanotubes, the possible effect of adhesive van der Waals interactions at tube-tube contacts is analysed. It is found, however, that the contribution of adhesive contacts to the bulk stress should be negligible. Compression experiments are performed on multi-walled carbon nanotubes and show that van Wyk's model is able to describe the response, although the values of the dimensionless parameter k of van Wyk's model were lower than expected. There is indeed no indication that van der Waals interactions play any significant role.     

Bulk,surface and thermal effects in inverse photoemission spectra from Cu(100), Cu(110) and Cu(111)

We have performed a study of empty electronic bulk and surface states on the three low indexed copper surfaces employing momentum resolved inverse photoemission. The bulk electronic features may be well understood in the frame work of the bulk direct transition model using state of the art band structure calculations. Surface states of both, the crystal derived and the image potential induced type have been identified and were found to agree with previous work. Several radiative transitions into unoccupied bands were also investigated at elevated temperatures. Characteristic temperatures of an exponential attenuation law are distinctly different between surface and bulk transitions. However, no systematic behaviour of bulk transitions at different points of the Brillouin zone could be established.