Measurements of the elastic and inelastic neutron scattering from vitreous silica in frequency range 0.3 to 4 THz with vectors 0.2 5.3 A${\r{}}^{\mathrm{\ensuremath{-}}1}$ are analyzed conjunction heat-capacity measurements on same samples provide a microscopic description low-frequency vibrational modes. The results show that additional harmonic excitations coexist sound waves below 1 THz, these correspond relative rotation ${\mathrm{SiO}}_{4}$ tetrahedra.

The vibrational excitations in vitreous silica above 150 GHz were investigated at 50, 100, and 290 K by inelastic scattering of cold neutrons. A detailed study the frequency wave-vector dependence shows that even lowest frequencies only a small part is due to acoustic phonons. type motion dominating additional low-frequency modes identified as coupled rotation Si${\mathrm{O}}_{4}$ tetrahedra. Possible connections low-temperature glass anomalies are discussed.

The soft-potential model (an extension of the well-known tunneling for two-level states in glasses) has been formulated terms soft-mode eigenvectors order to characterize localization these modes. interaction with high-frequency modes explains absence very small restoring-force constants. A quantitative comparison specific-heat and neutron-scattering data from vitreous silica, amorphous selenium, boron trioxide shows that both systems low-energy vibrational can be explained by same...

The temperature dependence of the inelastic neutron-scattering intensity from vitreous silica has been studied between 50 and 300 K down to frequencies 150 GHz. Above 500 GHz one finds essentially harmonic behavior. Low-frequency anharmonic behavior can be described by a relaxational model using parameters determined ultrasonic measurements. dynamic structure factor shows that relaxation involves coupled rotational jumps ${\mathrm{SiO}}_{4}$ tetrahedra, with jump width atoms smaller than 0.8...

Atomic mean-square displacements for frequencies above 1011 Hz have been determined from neutron time-of-flight data in the glassy, liquid and crystalline phase of selenium. As many other substances, difference between values disordered ordered shows a strong increase with increasing temperature, setting at about glass temperature. The inverse that turns out to be linearly related logarithm viscosity. relation holds below transition temperature up melting covering twenty decades

Neutron time-of-flight data of polybutadiene and polyisoprene in a wide temperature range around the glass transition are reported. The anharmonic part spectra is evaluated using recently developed time domain Fourier transform technique. results corroborate recent finding two different domains dynamics, one above crossover about 2 ps other below it. Two interpretations short behavior considered, namely fast Debye regime \ensuremath{\alpha} relaxation vibrational softening boson peak.

The soft potential model (an extension of the tunneling to include localized vibrations) is shown describe anomalous features specific heat ${\mathit{C}}_{\mathit{P}}$ and thermal conductivity glasses over entire low-temperature range, up an including peak in ${\mathit{C}}_{\mathit{P}}$/${\mathit{T}}^{3}$ second rise above plateau.

Inelastic-neutron-scattering studies of glassy, liquid, and polycrystalline selenium have been performed at temperatures between 100 600 K. A self-consistent data evaluation, taking careful account multiple multiphonon scattering, shows that a complete interpretation is possible in terms temperature-dependent density vibrational states, together with diffusion broadening the elastic line. This states used to calculate thermodynamic properties show about one-third additional entropy liquid...

Low-frequency Raman- and neutron-scattering spectra are compared for the three glass formers polybutadiene, polystyrene, ${\mathrm{SiO}}_{2}$ at different temperatures. One finds similar neutron Raman in frequency range where quasielastic (relaxational) contribution dominates, but a marked quasilinear increase of light-scattering sensitivity above vibrational boson peak. Analysis data shows that there is some intrinsic relation between peak vibrations. These results with predictions models.

The dynamical structure factor S(Q,\ensuremath{\omega}) and the vibrational density of states g(\ensuremath{\omega}) for amorphous silicon have been measured by neutron scattering methods. Computer simulations S(Q,\ensuremath{\omega}), based on a 216-atom structural model, are in substantial agreement with experiment.

We report on neutron scattering observations of the fast picosecond relaxation in two amorphous polycarbonates. Contrary to previous experience, one already observes these relaxations far below glass transition, at temperatures where they cannot be related flow process. An explanation terms low-barrier structural is proposed.

Vibrational spectra for $a$-Si and $a$-Si${\mathrm{H}}_{0.12}$ have been measured by inelastic neutron scattering, directly yielding the phonon density of states former material, partial H vibrations latter. The relevance these results to interpretation infrared Raman is discussed.

New Raman and incoherent neutron scattering data at various temperatures molecular-dynamic simulations in amorphous silica are compared to obtain the coupling coefficient C(ω) and, particular, its low-frequency limit. This study indicates that ω → 0 limit extrapolates a nonvanishing value, giving important indications on characteristics of vibrational modes disordered materials; particular our results indicate even very long wavelength local disorder implies nonregular atomic displacements.

The low-temperature ${\mathrm{T}}^{2}$ rise and the subsequent plateau of thermal conductivity glasses allow one to determine two parameters phenomenological soft-potential model. first these is a weighted average C-bar dimensionless parameter combinations ${\mathrm{C}}_{\mathrm{l}}$ ${\mathrm{C}}_{\mathrm{t}}$ which model shares with tunneling second crossover energy W between states vibrations. Values obtained from ten different agree within experimental error those specific-heat...

Using neutron spin echo and dielectric spectroscopy we have studied the molecular motions of 1-4 polybutadiene in $\ensuremath{\alpha}\ensuremath{-}\ensuremath{\beta}$ relaxation regime. At first peak static structure factor times follow temperature dependence viscosity, while near second Arrhenius law $\ensuremath{\beta}$ is observed. Considering localized on a length scale 1.5 \AA{} with barrier distribution from dynamic regime can be described quantitatively.

We have investigated fast relaxational processes and the much debated boson peak in strong glass former, ${\mathrm{B}}_{2}{\mathrm{O}}_{3},$ from 60 to 1300 K (glass transition temperature 523 K) using coherent inelastic neutron scattering. The wave-vector dependence of dynamic structure factor is different for energies below above peak, at $\ensuremath{\sim}2.5\mathrm{meV},$ indicating a qualitative difference character respective modes. This finding demonstrates that can be decomposed into...

We report on a neutron spin echo investigation of the intermediate scale dynamics polyisobutylene studying both self-motion and collective motion. The momentum transfer (Q) dependences self-correlation times are found to follow Q(-2/beta) law in agreement with picture Gaussian dynamics. In full Q range observation, their temperature dependence is weaker than rheological shift factor. same true for stress relaxation time as seen sound wave absorption. show dependences; at structure factor...

The phonon dispersion curves of fcc Sr and bcc Ba at room temperature 100 K have been determined. Neutron time-of-flight spectra polycrystalline samples over a wide range $Q$ $\ensuremath{\omega}$ measured. were analyzed in terms Born-von K\'arm\'an models. In addition, the elastic constants obtained from low-frequency part spectra.

Isothermal compressibilities and Brillouin sound velocities from the literature allow us to separate compressibility at glass transition into a high-frequency vibrational low-frequency relaxational part. Their ratio shows linear fragility relation discovered by x-ray scattering, though data bend away line higher fragilities. Using concept of constrained degrees freedom, one can show that part follows fragility-independent Lindemann criterion; dependence seems stem The physical meaning this...

The Lindemann criterion is reformulated in terms of the average shear modulus $G_c$ melting crystal, indicating a critical strain which necessary to form many different inherent states liquid. In glass formers with covalent bonds, one has distinguish between soft and hard degrees freedom reach agreement. temperature dependence picosecond mean square displacements liquid crystal shows that there are two separate contributions divergence viscosity decreasing temperature: anharmonic increase...