O. Pilla

Nature of the short wavelength excitations in vitreous silica: An X-ray Brillouin scattering study

The dynamical structure factor (S(Q,E)) of vitreous silica has been measured by Inelastic X-ray Scattering varying the exchanged wavevector (Q) at fixed exchanged energy (E) - an experimental procedure that, contrary to the usual one at constant Q, provides spectra with much better identified inelastic features. This allows the first direct evidence of Brillouin peaks in the S(Q,E) of SiO_2 at energies above the Boson Peak (BP) energy, a finding that excludes the possibility that the BP marks the transition from propagating to localised dynamics in glasses.

The low energy excess of vibrational states in v-SiO2: the role of transverse dynamics

A numerical simulation study of the density dependence (ρ = 2.2–4.0 g cm−3) of the high energy collective dynamics in vitreous silica at mesoscopic wavevectors (Q = 1–18 nm−1) is reported. The dynamic structure factor, S(Q,ω), and the density of states, ρ(E), have been determined in the harmonic approximation via the system eigenvalues and the eigenvectors, in turn obtained by the direct diagonalization of the dynamical matrix. The BKS interaction potential employed is capable of reproducing the experimentally observed excess of states (boson peak), and its density dependence. The numerical simulation also indicates a strong density dependence of the transverse excitation dispersion relation, ΩT(Q), at large Q. Specifically, ΩT(Q) is found to flatten at high Q to a value that increases with increasing density. The parallel between the density dependent flattening of ΩT(Q) and the density dependence of the boson peak suggests that the latter feature arises from the high Q portion of the transverse branch. This hypothesis is in line with both the interpretation by Elliott and co-workers (Taraskin et al 2001 Phys. Rev. Lett. 86 1255), who assign the boson peak to a phenomenon in glass reminiscent of the lowest energy Van Hove singularity in the companion crystal, and the Buchenau et al (1986 Phys. Rev. B 34 5665) assignment of the boson peak to the localized hindered rotation of SiO2 tetrahedra.The study of the effects of the density variations on the vibrational dynamics in vitreous silica is presented. A detailed analysis of the dynamical structure factor, as well as of the current spectra, allows the identification of a flattened transverse branch which is highly sensitive to the density variations. The experimental variations on the intensity and position of the Boson Peak (BP) in v-SiO2 as a function of density are reproduced and interpreted as being due to the shift and disappearance of the latter band. The BP itself is found to correspond to the lower energy tail of the excess states due to the piling up of vibrational modes at energies corresponding to the flattening of the transverse branch.

Evidence of anomalous dispersion of the generalized sound velocity in glasses

The dynamic structure factor S(Q,ω), of vitreous silica, has been measured by inelastic x-ray scattering in the exchanged wave-vector (Q) region Q=4-16.5 nm - 1 and up to energies ∞ω= 115 meV in the Stokes side. The unprecedented statistical accuracy in such an extended energy range allows us to accurately determine the longitudinal current spectra and the energies of the vibrational excitations. The simultaneous observation of two excitations in the acoustic region and the persistence of propagating sound waves up to Q values comparable with the (pseudo-)Brillouin-zone edge allow us to observe a positive dispersion in the generalized sound velocity that, around Q5 nm - 1 , varies from 6500 to 9000 m/s; this phenomenon was never experimentally observed in a glass.

Structural and dynamical consequences of density variation in vitreous silica

We present structural and dynamical results of molecular dynamics simulation of vitreous silica undergoing a hydrostatic compression and decompression cycle at room temperature. Structural results as a function of density compare fairly well with experiments as well as with the longitudinal and transverse sound velocity pressure dependence. A shift of the boson peak (BP) toward higher energies and its simultaneous weakening is observed as in experiments. A detailed study of the dispersion of the glass vibration is presented at several densities and for the densified state. Evidence of phonon-like character with two distinct pseudo-periods is shown for longitudinal and transverse dynamics. The relationship between the BP vibrations and the correlation length scale indicated by the first sharp diffraction peak is discussed.

Raman scattering from fractals: Simulation on large structures by the method of moments

We have employed the method of spectral moments to study the density of vibrational states and the Raman coupling coefficient of large 2- and 3- dimensional percolators at threshold and at higher concentration. We first discuss the over-and under-flow problems of the procedure which arise when -like in the present case- it is necessary to calculate a few thousand moments. Then we report on the numerical results; these show that different scattering mechanisms, all {\it a priori} equally probable in real systems, produce largely different coupling coefficients with different frequency dependence. Our results are compared with existing scaling theories of Raman scattering. The situation that emerges is complex; on the one hand, there is indication that the existing theory is not satisfactory; on the other hand, the simulations above threshold show that in this case the coupling coefficients have very little resemblance, if any, with the same quantities at threshold.

Trap depth in Tl+-like phosphors: Deduction of Jahn-Teller coupling constants

Abstract The relationship between trap-depth and Stokes shift in impurities with the sp electronic configuration is generalized and explained. A new simple method is developed to estimate the Jahn-Teller coupling constants from spectroscopic and trap-depth data.

Vibrational dynamic of ‘strong’ glasses: the case of v-SiO2 and v-GeO2

The vibrational dynamic of vitreous germania (v-GeO2) with respect to that of vitreous silica (v-SiO2) has been investigated by means of several inelastic and elastic scattering experiments. These amorphous systems belong to the large family of continuous random network forming glasses. In spite of their topological similarity, some dynamical differences have been singled out. We present experimental data on light Raman scattering at room temperature as well as new elastic and inelastic neutron scattering. Transversal-acoustic character of the vibrations responsible for the density of states excess in the Boson peak energy range is found. 2003 Elsevier B.V. All rights reserved.

Energy landscape, two-level systems, and entropy barriers in Lennard-Jones clusters

We develop an efficient numerical algorithm for the identification of a large number of saddle points of the potential energy function of Lennard- Jones clusters. Knowledge of the saddle points allows us to find many thousand adjacent minima of clusters containing up to 80 argon atoms and to locate many pairs of minima with the right characteristics to form two-level systems (TLS). The true TLS are singled out by calculating the ground-state tunneling splitting. The entropic contribution to all barriers is evaluated and discussed.

Energy transport in disordered oscillatory systems

Abstract Several unexpected features of energy transport in one-dimensional regular and disturbed systems are presented. In the classical case the propagation strongly depends on the type of initial excitation in both the disturbed and disordered chain. The phenomena of localization and superdiffusion are exemplified. For temperature excitation a local concept of transport description is presented and applied both numerically and analytically. Finally, it is shown that a one-dimensional sequence of softened springs in a two- or three-dimensional lattice generates a dynamical instability of the low-frequency mode density, which has consequences for the thermodynamic and transport behaviour.

X-ray and neutron scattering studies in vitreous silica: Acoustic nature of vibrational dynamics in the mesoscopic range

Abstract We report X-ray and neutron scattering results in vitreous silica. As in earlier data, elear evidence is found for sound wave modes at the energies of the boson peak. We have observed by X-ray experiments a elear dispersion of Brillouin peaks and we have detected experimental evidence of inelastic scattering in the second Brillouin zone. By neutron scattering experiments we have found coherence effects in S(Q, E). These effects are consistent with average acoustic phonon dispersion curves, which are similar in character to the dispersion curves found in crystals but, obviously, broadened by the topological disorder.