Phonon scattering in quasi-one-dimensional structure

We introduce a model to study a symmetric nanocontact, whereby its mechanical properties can be analyzed via the vibration spectra. The model system consists of two groups of triple semi-infinite atomic chains joined by atoms in between. The matching method theoretical approach is used to calculate the coherent reflection and transmission scattering probabilities, the characteristic vibration Green functions and densities of states (DOS), for the vibration components of the individual atomic sites that constitute a complete representation of the nanocontact domain boundaries. The nanocontact observables are numerically calculated for different cases of elastic hardening and softening, to investigate how the local dynamics can respond to changes in the microscopic environment on the nanocontact domain. The analysis of the vibration spectra and the DOS demonstrate the fluctuations, related to Fano resonances, due to the coherent coupling between traveling phonons and the localized vibration modes in the nanocontact domain.     

Phonon scattering by paraelastic defects

The relaxation timeτ(ω) for the scattering of the lattice waves of frequencyω by the paraelastic defects has been calculated by using the Feynman graph method, applied to a previous theoretical model [8]. A simple expression for the resonance component ofτ(ω)⁻¹ has been derived.

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Zusammenfassung Die Relaxationszeitτ(ω) für die Streuung der Gitterwellen der Frequenzω an den paraelastischen Defekten wurde nach der Methode der Feynmanschen Graphen berechnet, wobei ein früheres theoretisches Modell [8] angewendet wurde. Ein einfacher Ausdruck für den Resonanzteil vonτ(ω)⁻¹ wurde hergeleitet.

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Résumé Nous avons calculé le temps de relaxationτ(ω) des ondes du réseau de fréquenceω, qui sont diffusées par des défauts para-élastiques, en appliquant la méthode des graphes de Feynman à un modèle théorique antérieur [8]. Une simple expression pour la partie résonante deτ(ω)⁻¹ a été dérivée.

     

Multiple scattering of plasmons at rough surfaces

We investigated the polarization and angle dependence of the scattered light intensity of surface plasma oscillations (SPO) propagating along rough silver surfaces. The rough silver films were prepared using the methods of pre-evaporating CaF₂ and silver on heated substrates. The scattered light intensity of both systems is explained by surface scattering effects. Differences of the light intensities between the rough silver films prepared by the two methods mainly were observed in “restlight intensities”, which cannot be described by first-order scattering theories. We explain these differences by higher order scattering effects.     

Phonon coupling in inelastic atom-surface scattering

Inelastic scattering of a thermal He beam from a LiF surface has been observed in terms of velocity and intensity distributions as a function of scattering angle. The results allow a correlation with theoretical predictions assuming single Rayleigh phonon coupling for both phonon creation and annihilation processes. A marked decrease of inelastic intensities with momentum or energy transfer is demonstrated, which depends strongly on the temperature of the scattering surface.     

Electron scattering and optical absorption at semiconductor surfaces

With the aid of a simple dielectric theory a discussion is given of the relation between the line shape observed on optical absorption due to interband transitions involving surface states and the corresponding electron energy-loss profile.     

Phonon density of states of tetracyanoethylene from coherent inelastic neutron scattering at Dhruva reactor

Inelastic neutron scattering experiments to determine phonon density of states of coherent scattering samples of polycrystalline complex solids are generally intensity-limited and therefore are feasible only at high flux facilities. Phonon density of states of the monoclinic phase of tetracyanoethylene at 300 K, obtained using the medium resolution triple axis spectrometer at the new Indian medium flux reactor Dhruva are reported here. The raw data is converted to the “neutron weighted” phonon density of states by applying suitable corrections. Comparison made with results from a theoretical calculation based on a semirigid molecule model of lattice dynamics is fair. Results from Dhruva are also consistent with that obtained (to be published) at the high flux pulsed neutron source (ISIS) of the Rutherford Appleton Laboratory in United Kingdom.     

Diffusive scattering of high-frequency phonons at free silicon surfaces

Comparing measurements of high-frequency phonon reflection at the uncovered and optically polished (100)-silicon surface, with calculations considering phonon focusing, reveal complete diffusive scattering with at most 4% specular reflection contribution. Two possible mechanisms causing diffusive scattering are discussed.     

Multiple scattering of low-energy rare-gas ions at solid surfaces

We have studied the multiple scattering of He⁺, Ne⁺, and Ar⁺ from a TaC(001) surface in the energy region of the order of 1 keV. The experimental data revealed large differences between the energy spectra of these ions. The spectral peak corresponding to quasi-double scattering is clearly observed for Ar⁺ but not observed for Ne⁺. The multiple scattering effect appearing in the energy spectra of rare-gas ions is discussed on the basis of electron exchange between these ions and solid surfaces.     

Monte Carlo simulation of inelasting surface scattering; Phonon transfer model applied to Br2 and no scattering from graphite surfaces

The inelastic scattering results in the form of angular distributions for Br₂-graphite (Holmlid et al.) and NO-graphite (Ertl et al.) have been analyzed using a Monte Carlo simulation technique and a one-phonon transfer model, sampling from the distribution of phonons perpendicular to the surface. The Br₂ results can be well reproduced by the calculations, and so can the lobe peak position for NO scattering at high surface temperature. For NO scattering at low surface temperature both phonon gain and loss processes contribute to the observed lobe, and its angular width is well described in this way. The full lobe can only be explained by two-phonon difference processes, however. If the phonons are assumed to follow Boltzmann instead of the correct Bose-Einstein statistics, improved fits are found in some cases. This probably indicates that dynamical effects, as for example a varying energy transfer probability, are important during the phonon transfer. The experimentally observed quasi-cosine shape of the lobes below the specular direction is well reproduced by a loss process with no excitation function, using only the distribution of phonon frequencies in the surface layer.     

Phonon modes at the 2H-NbSe2 surface observed by grazing incidence inelastic x-ray scattering

We have determined the dispersion of acoustic and optical surface phonon modes 2H-NbSe2 at the by inelastic x-ray scattering under grazing incidence conditions. Already, at room temperature, an anomaly is observed close to the charge density wave -vector position located at about one-third along the Gamma-M direction of the Brillouin zone. Our results indicate that the anomaly for the surface mode occurs at a lower energy than that measured in bulk sensitive geometry in the same experiment, showing evidence of a modified behavior in the uppermost layers. We demonstrate that inelastic x-ray scattering in grazing incidence conditions provides a unique tool to selectively study either surface or bulk lattice dynamics in a single experiment.     

Defects at surfaces and interfaces — A scattering theoretical approach

A theory for isolated defects at surfaces and interfaces of semiinfinite solids is presented and applied to vacancies at or near surfaces. Vacancy-induced changes in the surface electronic structure of a simple cubic s-band model are discussed in detail and are found to converge very slowly to corresponding bulk vacancy results when the defect is moved into the solid. Results for Ga and As vacancies at GaAs surfaces are discussed in connection with recent experimental data.     

Elastic scattering of ions by atomic strings at solid surfaces

Comparison of elastic energy losses for ions scattered from a continuous atomic “string” and from a discrete atomic row yields a concept of an effective number of binary atomic collisions. The string approximation is shown to be valid when this effective number is greater than three, in agreement with experiment.     

Phonon eigenvectors in Si determined by inelastic neutron scattering

We have determined the eigenvectors of longitudinal phonons with wavevectors in the direction in Si at 12 K from inelastic neutron scattering intensities. The eigenvectors obtained from different model and quantum-mechanical calculations are at variance. Comparison of experimental and theoretical results shows that of the various theoretical predictions the ones from the bond-charge model are in best agreement with experiment. Internal-strain constants from these models and from experiment are compared.