Phonons and photons in crystals

The simultaneous propagation of phonons and photons in an insulator is discussed from both phenomenological and quantum mechanical points of view. A phenomenological form of the energy of an insulator is first supposed from which is obtained the equation of propagation of modes involving nuclei displacements and an electric field. This equation is then studied, mainly in the vicinity of q=0, in order to show how various limits lead to different types of propagation. The phenomenological equations are then justified from a microscopic point of view. The proof goes in two steps. A linear screened response function of the electrons in a solid is first assumed; one then proves that all the coefficients entering into the phenomenological equations may be obtained from the sole knowledge of this response function and of the charge of the nuclei. The existence of the response function is then justified from a many-body point of view. Finally, the necessary relations between the phenomenological coefficients are proved. Some other possible applications of the microscopic equations are also discussed at the end of the paper.     

Phonons on surfaces: The importance of structure and adsorbates

Recent progress in detecting thermal energy vibrations at solid surfaces will be reviewed. Phonons on clean, well-ordered metals will be discussed as well as vibrations within ordered and disordered adlayers of adsorbed atoms and molecules. Simple lattice dynamical models will be used to demonstrate the interconnection between vibrations, structure, and force fields at a surface.     

Water Structure and Supergigahertz Phonons

A brief review of recent publications devoted to studies of the water microstructure is presented. Special emphasis is placed on techniques of x-ray molecular scattering, which allow one to determine the propagation speed and attenuation of acoustic phonons inside the water cluster as well as the fundamental frequency of oscillations of this cluster. Alternative ways of studying the water structure by the techniques of nonlinear optics are proposed.     

Phonons and fractons in sol-gel alumina: Raman study

We report Raman scattering from the boehmite,γ-, δ- andα-phases of the alumina gel. Samples are characterized by transmission and scanning electron microscopy, X-ray diffraction and density measurements. The main Raman line in the boehmite phase is red-shifted as well as asymmetrically broadened with respect to that in the crystalline boehmite, signifying the nanocrystalline nature of the gel. Raman signatures are absent in theγ- andδ-phases due to the disorder in cation vacancies. We also show that low frequency Raman scattering from the boehmite phase resembles that from a fractal network, characterized in terms of fraction dimension . Taking Hausdorff dimension D of the boehmite gel to be 2.5 (or 3.0), the value of is 1.33±0.02 (or 1.44±0.02), which is close to the theoretically predicted value of 4/3.     

Phonons and electrons at metal surfaces

Some aspects of the coupling between phonons and electrons and the interaction between electrons at metal surfaces are reviewed. Surface science techniques as diverse as electron energy loss spectroscopy, angle-resolved photoelectron spectroscopy, and scanning tunnelling microscopy are employed to study these interactions. Electron–phonon and electron–electron coupling are discussed in terms of renormalized phonon dispersion relations, and increased decay rates of electronic excitations. PACS 63.20.Kr; 68.35.Ja; 68.37.Ef; 68.43.Pq; 72.10.Fk; 72.15.Qm; 73.20.At     

Phonons in SiAs: Raman scattering study and DFT calculations

We present experimental Raman scattering results on single‐crystal silicon monoarsenide (SiAs). Based on a comparison between Raman measurements and first‐principles density functional theory calculations, we found evidence that SiAs will occur in a monoclinic crystal structure rather than an orthorhombic one as has been discussed in the literature. Further, we provide a detailed discussion of the vibrational properties of the monoclinic structure. Copyright © 2011 John Wiley & Sons, Ltd.     

Reflectivity measurements of Eccosorb

The reflectivity of two types of Eccosorb, LS-22 and AN-72, has been measured in the wavelength interval 0.5–3 mm. The results show that the reflectivity increases sharply for angles of incidence greater than 40°.     

Comment on “mössbauer studies of the 6.2 keVγ-rays of 181 Ta in Ta-dichalcogenides”

The reported discrepancy between TDPAC and Mössbauer measurements of the electric field gradient at Ta in 2H-TaS₂ is shown to be an artifact. Combining both results a new value for the 482 keV state quadrupole moment of ¹⁸¹Ta is derived: Q(5/2) = (+)2.36(5) b.     

Phonons from powder diffraction: a quantitative model-independent evaluation

We describe a model-independent approach for the extraction of detailed lattice dynamics information from neutron powder diffraction data, based on a statistical analysis of atomistic configurations generated using reverse Monte Carlo structural refinement. Phonon dispersion curves for MgO extracted in this way are shown to reproduce many of the important features found in those determined independently using neutron triple-axis spectroscopy. By means of molecular dynamics simulations, we quantify the extent to which the diffraction data are sensitive to lattice dynamics in this system.     

Phonons intrans-polyacetylene: delocalized bond—bond interactions

Delocalized force constants arising from extended π-electronic states are derived for an infinitetrans-polyene chain in the internal coordinate space. This formalism is particularly suitable for the extension of Wilson's GF technique to the calculation of the phonon dispersion curves in semi-conducting polymer chains.     

Phonons and the electronic gap in FeSi

The partial phonon densities of states of iron atoms in the intermetallic compound FeSi have been measured in the temperature range 46–297 K using nuclear resonant inelastic scattering of synchrotron radiation. A significant phonon softening with increasing temperature has been established. The greatest phonon softening for iron atoms is shown to occur in the region of long-wavelength acoustic phonons, for the acoustic branches near the boundary of the Brillouin zone, and for the low-lying weakly dispersive optical branches. The results obtained are analyzed in terms of the views that relate the change in the phonon density of states of iron atoms to the temperature evolution of the electronic density of state for the compound.     

Reprint of : Absorbing/Emitting Phonons with one dimensional MOSFETs

We consider nanowires in the field effect transistor device configuration. Modeling each nanowire as a one dimensional lattice with random site potentials, we study the heat exchanges between the nanowire electrons and the substrate phonons, when electron transport is due to phonon-assisted hops between localized states. Shifting the nanowire conduction band with a metallic gate induces different behaviors. When the Fermi potential is located near the band center, a bias voltage gives rise to small local heat exchanges which fluctuate randomly along the nanowire. When it is located near one of the band edges, the bias voltage yields heat currents which flow mainly from the substrate towards the nanowire near one boundary of the nanowire, and in the opposite direction near the other boundary. This opens interesting perspectives for heat management at submicron scales: arrays of parallel gated nanowires could be used for a field control of phonon emission/absorption.     

Phonons in surface photovoltage of GaAs

Oscillatory surface photovoltage is reported in GaAs at 4.2°K, characterized by two series of minima. Dominating series is attributed to the capture of photoexcited electrons by surface states with emission of phonons. Second, weak series coincides with oscillations in photoconductivity.     

On Virtual Phonons,Photons, and Electrons

A macroscopic realization of the peculiar virtual particles is presented. The classical Helmholtz and the Schrödinger equations are differential equations of the same mathematical structure. The solutions with an imaginary wave number are called evanescent modes in the case of elastic and electromagnetic fields. In the case of non-relativistic quantum mechanical fields they are called tunneling solutions. The imaginary wave numbers point to strange consequences: The waves are non-local, they are not observable, and they are described as virtual particles. During the last two decades QED calculations of the solutions with an imaginary wave number have been experimentally confirmed for phonons, photons, and electrons. The experimental proofs of the predictions of non-relativistic quantum mechanics and the Wigner phase time approach for the elastic, electromagnetic and Schrödinger fields will be presented in this article. The results are zero time in the barrier and an interaction time (i.e. a phase shift) at the barrier interfaces. The measured tunneling time scales approximately inversely with the particle energy. Actually, the tunneling time is given only by the barrier boundary interaction time, as zero time is spent inside a barrier.