Phonons heat transport at an atomic well boundary in ultrathin solid films

The effect of the central-cell corrections on the shallow donor states in Si spherical quantum dot is studied within the effective mass approximation. Finite step-like spatial confining potential, Coulomb and image charge potentials arising from the dielectric mismatch at the interface of the media are taken into account. We found that it is possible to tune the impurity energies by varying the dot radius and dielectric constant of the barrier material. In the strong confinement regime, due to the enhanced weight of the donor wave functions on the impurity atoms, large values of the chemical shifts for typical donors in Si compared to the ones in bulk are obtained. The calculated size-dependence of the effective Bohr radius in donor doped nanocrystals is in reasonable accord with electron spin resonance measurements on Si quantum dots embedded in insulating glass matrices. We conclude that both the dielectric mismatch and central-cell corrections must be considered in the study of these systems in order to obtain satisfactory agreement with experimental data.     

Phonons in mixed crystals

A concept is presented which gives qualitative information concerning the phonon behaviour of ionic mixed crystalsAB _1–x C _x at certain symmetry points on the zone boundary and consequently through the zone to the center. The model assumes that the end-member phonons and the impurity-modes ofAB:C andAC:B are known and uses the assumption that mixed crystal phonons behave symmetry-wise as the corresponding end-member phonons. The concept is demonstrated for several cases of mixed crystals.     

Phonons in disordered solids

The effects of crystalline disorder on the frequency distribution and lifetimes of phonons are investigated. As the disorder parameter g characterizing the scattering of normal modes is increased to g>g_c, the normal mode spectrum becomes unstable against arbitrarily small nonlinearities while the ground state (if any) becomes increasingly degenerate. The bandwidth of the acoustic phonon spectrum increases with g, from a value ω_Dat g = 0 to 1.24ω_D at g_c. The speed of sound at long wavelengths decreases with increasing g; at threshold (g_c) it has decreased by some 29.3% from its value at g = 0. For g < g_c the scattering lifetime of long-wavelength normal modes satisfies Rayleigh's law (1/τ = A(_g)ω⁴). At g_c the damping satisfies a different law: 1/τ = Bω². Similar effects are obtained for the optic mode phonons. For these reasons, it appears that g_c marks the threshold of amorphous behavior, although the regime g >g_c is beyond the scope of our model.     

Phonons in mixed crystals

Dispersion relations for the optic and acoustic phonons in mixed crystals are derived as a function of concentration. The dispersion curves show localized as well as band modes. It is shown that our theory leads to the MREI model of Chang and Mitra when the wave vector q = 0.     

Phonons in fcc strontium

A more realistic model, requiring a few parameters to explain the interactions among distant neighbours and valid for various crystallographic structures, is developed to study phonon dispersion in the bulk fcc metals. The model free from usual fitting procedure is employed to obtain phonon-dispersion in the bulk of fcc strontium crystal which has attracted scant attention so far. The computed results showing good agreement with the recent experimental results lend reliability and credibility to the theory.     

Phonons in carbon nanotubes

A broad review of the unusual one-dimensional properties of phonons in carbon nanotubes is presented, including phonons in isolated nanotubes and in crystalline arrays of nanotubes in nanotube bundles. The main technique for probing the phonon spectra has been Raman spectroscopy and the many unique and unusual features of the Raman spectra of carbon nanotubes are reviewed. Also included is a brief review of the thermal properties of carbon nanotubes in relation to their unusual phonon dispersion relations and density of states.     

Phonons in mixed crystals

A concept is presented which gives qualitative information concerning the phonon behaviour of ionic mixed crystalsAB _1–x C _x at certain symmetry points on the zone boundary and consequently through the zone to the center. The model assumes that the end-member phonons and the impurity-modes ofAB:C andAC:B are known and uses the assumption that mixed crystal phonons behave symmetry-wise as the corresponding end-member phonons. The concept is demonstrated for several cases of mixed crystals.     

Phonons in red HgI2

A critical analysis of existing experimental and theoretical information on the lattice dynamics of the layered crystal α-HgI₂ is performed. It is shown that a satisfactory understanding of the character of the interatomic interaction and characteristic features of the phonon spectrum of this material can be obtained in a simple phenomenological model with eight parameters, assuming that E _g/B_1g degeneracy of the 29 cm⁻¹ Raman line is of a two-phonon nature. Fiz. Tverd. Tela (St. Petersburg) 40, 537–541 (March 1998)     

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 in mixed valence systems

The effect of valence changes due to f?d electronic transitions on phonons is studied. A large electronic density of states at the Fermi-energy in the intermediate valence state causes significant softening of the longitudinal acoustic and optic phonons and large phonon line-widths. These phonon renormalizations due to strong electron-lattice coupling are largest for phonon wavevectors q along the (111)-direction and maximal at the zone-boundary for LO modes and nearly half-way towards the zone-boundary in the case of LA mode. The results agree well with recent neutron scattering data on Sm_0.75Y_0.25S.     

Phonons in Quantum-Dot Quantum Well

Phonon modes of AlAs/GaAs/AlAs and GaAs/AlAs/metal Pb quantum-dot quantum wells (QDQW‘s)with the whole scale up to 90 A are calculated by using valence force field model (VFFM) based on group theory.Their optical frequency spectra are divided into two nonoverlapping bands, the AMs-like band and the GaAs-like band,originated from and having frequency interval inside the bulk AlAs optical band and bulk GaAs optical band, respectively.The GaAs-LO (F)-like modes of QDQW‘s that have maximum bulk GaAs-LO (F) parentages in all modes covering the whole frequency region and all symmetries have always A1 symmetry. Its frequency is controllable by adjusting the structure parameters. In AlAs/GaAs/AlAs, it may be controlled to meet any designed frequency in GaAs-like band.The results on GaAs/AMs/metal Pb QDQW‘s show the same effect of reducing in interface optical phonons by using the metal/semiconductor interface revealed ever by macroscopic model. The frequency spectra in both GaAs-like and AlAs-like optical phonon bands are independent of the thickness of Pb shell as long as the thickness of Pb shell is no less than 5 A. Defects at metal/AlAs interface have significant influence to AMs-like optical modes but have only minor influence to GaAs-like optical modes. All these results are important for the studying of the e-ph interaction in QD structures.     

Phonons in an entropic crystal

Hard spheres crystallize due to purely entropic forces. The underlying excluded-volume interaction is completely anharmonic and the nature of the phonon spectrum is therefore of interest. To measure the single-particle motion and the phonon spectrum by dynamic light scattering we have used a collection of novel techniques including multispeckle cross correlation on a CCD chip and the growth of large single crystals using a temperature gradient. The random hexagonal close packed crystal has a dispersion relation closer to hcp than to fcc.     

Phonons in titanium doped vitreous silica

In this paper we report the infrared and Raman studies of titanium doped silicate glasses for a number of Ti concentrations. New modes associated with the titanium groups are identified. The observed shifts and broadenings in the spectra of vitreous silica are discussed in the context of random network models. The “anomalous” decrease in density with the addition of titanium is attributed to an increase in the average intertetrahedral angle of the SiO₄ network.     

Phonons in Excited State for Polydiacetylene

A simple model is used to evaluate the phonon frequencies in both ground and excited state for polydiacetylene. The spring constants K^σ_i in the σ-bond skeleton are distinct from the experimental effective spring constants K_i. The results are compared with experimental date.     

Phonons in graphite studied by eels

We have studied the phonon spectrum of Graphite using Reflection Electron Energy Loss Spectroscopy (REELS), measuring high frequency dispersion curves in regions of the Brillouin Zone which have not been accessed in previous experiments. The results are compared with calculated (bulk) phonon dispersion curves.