Strain-field scattering of fast-transverse acoustic phonons by dislocations in LiF

The mean-free-paths of transverse acoustic phonons generated in surface hot spots and propagating ballistically along [100] in LiF are calculated exactly for the case of scattering by the static strain fields of edge and screw dislocations of Burger's vector $\text{1}\text{2}\text{a [}\text{1}\text{10}\text{]}$. Due to wavevector conservation the screw dislocations are shown not to scatter such phonons at all while the edge dislocations lead to mean-free-paths greater than 12 cm for dislocation densities ?2×10⁸ cm^-2 and frequencies ?5×10¹¹ Hz. This is consistent with the observation by Northrop et al. (1982) of the ballistic propagation of such heat-pulse phonons across deformed crystals of 1 cm dimensions.     

Surface phonons in superlattices

We report the existence of surface localized phonons for a superlattice consisting of alternating slabs (parallel to the surface) of two different crystals. The superlattice has a larger periodicity in the direction perpendicular to the slabs and therefore many phonon branches in the folded Brillouin zone. In the gaps existing between these phonon branches appear the surface localized modes.     

Surface analysis of LiF films by electron stimulated desorption and x-ray photoelectron spectroscopy

Electron stimulated desorption and X-ray photoelectron spectroscopy studies of evaporated LiF films show that the films are Li-rich in the surface region (Li/F atomic ratio ∼ 3:1) with a contaminant overlayer which includes hydrocarbons and water. Substantial amounts of hydrogen are also directly bonded to LiF surface. Exposure of the films to RF-activated Ar plasma results in reduction of the Li/F stoichiometry by a factor of ∼ 2 and the removal of carbon from the surface. H₂O and possibly other oxygen-containing species remain.     

Light modes in thin polyurethane and LiF films

Light modes are excited in thin polyurethane films (freely supported) and LiF films (bounded by a silver film). They are registered by the radiation which is emitted from the rough film surface. The dispersion relation together with the observed position of the s and p modes allows one to determine the refractive index and thickness of the polyurethane film whereas in the case of LiF one needs a small anisotropy of the optical properties (≈ 1%) of the film to fit the observed data. The refractive indices could be determined within an error of about 10^?4.     

Coherent phonons and their properties

The optical phonons in semimetals, semiconductors, and superconductors were studied by the light reflection techniques with femtosecond time resolution and by the method of spontaneous Raman scattering. During measurements in the time domain, the phonon system is converted into a coherent state by the first ultrashort laser pulse and then probed at a variable delay by the second pulse. In this case, the phonons are shown to occur in a nonclassical state in which their fluctuational properties, different in various quadratures, are described by periodic functions of time. A comparison of the results obtained in the time and frequency domains gives evidence that the energies of thermal and coherent phonons coincide, while their dephasing and energy relaxation times are different.     

Amplification of surface phonons in piezoelectric semiconductor films

A quantum theory of surface phonon amplification with frequencies above 1 GHz is presented for a piezoelectric semiconductor film. Numerical analysis is made for an n-type GaAs layer grown epitaxially on a semi-insulating GaAs substrate.     

Surface and interface phonons and related topics

We analyze the state of the art of surface and interface phonons with an emphasis on their relevance to many aspects of surface and interface physics.     

Inelastic scattering of helium from bulk longitudinal acoustic phonons in LiF(001)

The analysis of some recent time-of-flight spectra reported by Doak for He inelastic scattering from LiF(001) along the 〈110〉 azimuth reveals important contributions from bulk longitudinal acoustic (LA) phonons. The large intensity due to LA phonons compared to that due to Rayleigh waves is shown to be the result of resonance enhancement. The independent determination of the coupling to LA phonons and to Rayleigh waves permits an estimate of the atom-surface, potential softness.     

Surface phonons in GaAs(110)

Surface phonon dispersion curves have been measured along the <001>, <11&#x0304;2>, and <11&#x0304;0> azimuths of GaAs(110). Features of note include a very low frequency (5.5 meV at zone boundary) surface acoustic mode in the first two directions; this may arise through the known (1 × 1) reconstruction of this surface. A higher frequency surface mode (7.3–8.8 meV, depending on azimuth) is seen in all directions. The helium scattering intensities are greatly influenced by bound state resonances. A careful survey of the selective adsorption signatures in extremely high resolution scans of polar angle, azimuthal angle, and incident beam wavevector allows the bound state energies to be determined with some confidence. Initial results indicate energies of roughly 1, 2, and 4 meV.     

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 modification of diamond-like carbon films with perfluorooctyl functionalities and their surface properties

Photolysis of perfluoroazooctane with diamond-like carbon (DLC) films led to the surface modification to introduce perfluorooctyl functional groups, confirmed by means of FT-IR, XPS, Raman and TOF-SIMS measurements. The DLC films modified with fluorine moieties showed reduction of the surface energy evaluated by contact angle to water, as compared with pristine DLC film. The contact angle of chemically modified DLC film is 105°, comparable to that of polytetrafluoroethylene (PTFE). By monitoring with XPS, we found that the results on the value of fluorine/carbon ratio of fluorinated DLC films depending on irradiation time are consistent with those of contact angle. Chemical modification of DLC films with perfluorooctyl functionalities also led to improvement of their frictional properties. The friction coefficient of the modified film is 0.05 under vacuum condition, whereas that of the pristine film shows very high value (>1).     

Surface plasmon resonance of sputtered Ag films: substrate and mass thickness dependence

The influence of substrate on the tunability of the localized surface plasmon resonance wavelength, _SPR, of silver island films was investigated by magnetron sputtering the films on various substrates. It reveals that increasing refractive index of the substrate, n_sub, not only leads to redshift, but also an extension of the tunable range in _SPR. Such effects can be greatly enhanced by increasing the mass thickness of the metallic islands. The resonance wavelength of an island film can be readily adjusted from the visible to the near infrared region of the electromagnetic spectrum. The sensitivity factor, _SPR/_av, where _av is the interisland dielectric constant, was found to increase approximately linearly with the silver mass thickness up to 4 nm, from 60 nm to about 160 nm per dielectric constant unit. PACS 61.46.+w; 81.07.-b; 36.40.GK     

Parametric generation of 48 GHz phonons in Gd films

Phonon frequency doubling in gadolinium films has been observed and is understood by considering parametric excitation of the normal mode states by a r.f. magnetic field. As a result, dynamic processes involving higher order magnetoelastic terms must be considered.     

Transmission of high-frequency phonons through thin glass films

By means of phonon spectroscopy we measured the transmission of high-frequency phonons through thin glass films in the frequency range from 100 to 300 GHz. The films were prepared by thermal oxydation of silicon single crystals. Our data obtained from films with different thicknesses suggest that the observed phonon attenuation is due to scattering processes at the silicon-glass interface and not to a bulk effect in the glass film. The phonon mean free path at 300 GHz turns out to be larger than 4microm. We find evidence for the absence of inelastic scattering processes.     

Propagation of ballistic phonons in crystals and their detection with a detachable bolometer

Ballistic phonons were generated in single-crystalline sapphire samples by scanning the crystal surface with the electron beam in a scanning electron microscope equipped with a low-temperature stage. The phonons were detected using a superconducting thin film (granular Al) bolometer configuration which could be detached from the sample crystal (allowing repeated use). We have applied these experimental techniques for obtaining phonon images of laser-drilled holes in the sample crystals, achieving a spatial resolution of better than 10 m.     

The dependence of surface tension of solid nanoscale films on their thickness

This paper presents a theoretical method to calculate the surface energy dependence on thickness of nanofilms based on the Landau theory. For the first time, it is shown that the surface energy of thin films having free surfaces is greater than the surface energy of macroscopic objects. For nano-objects having free surfaces, it is stated that their interior order parameter is always less than that of macroscopic solids of the same composition. It is obtained that the surface energy of thin films increases with decrease in their thickness passing its maximum meaning. A further decrease in the solid film thickness leads to a monotonic decrease in the surface energy.