G. Viliani

Connected Network of Minima as a Model Glass: Long Time Dynamics

A simple model to investigate the long time dynamics of glass formers is presented and applied to study a Lennard-Jones system in supercooled and glassy phases. According to our model, the point representing the system in the configurational phase space performs harmonic vibrations around (and activated jumps between) minima pertaining to a connected network. Exploiting the model, in agreement with the experimental results, we find evidence for (i) stretched relaxational dynamics, (ii) a strong T dependence of the stretching parameter, and (iii) breakdown of the Stokes-Einstein law. [S0031-9007(98)07786-2]

Relaxation Processes in Harmonic Glasses

A relaxation process, with the associated phenomenology of sound attenuation and sound velocity dispersion, is found in a simulated harmonic Lennard-Jones glass. We propose to identify this process with the so-called microscopic (or, instantaneous) relaxation process observed in real glasses and supercooled liquids. A model based on the memory function approach accounts for the observation and allows one to relate to each other (1) the characteristic time and strength of this process, (2) the low frequency limit of the dynamic structure factor of the glass, and (3) the high frequency sound attenuation coefficient, with its observed quadratic dependence on the momentum transfer.

The Raman coupling function in amorphous silica and the nature of the long-wavelength excitations in disordered systems

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

Low-Energy Raman Scattering from Silver Particles in Alkali Halides

Raman spectra of silver particles in NaCl and KI are reported, which show the particles to have spheroidal shape and undergo quadrupolar surface vibrations. From the energies of the observed modes the shape and dimension of the metallic particles are deduced.

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.

Networking Properties of Cyclodextrin-Based Cross-Linked Polymers Probed by Inelastic Light-Scattering Experiments

An integrated experimental approach, based on inelastic light-scattering techniques, has been here employed for a multilength scale characterization of networking properties of cyclodextrin nanosponges, a new class of cross-linked polymeric materials built up from natural oligosaccharides cyclodextrins. By using Raman and Brillouin scattering experiments, we performed a detailed inspection of the vibrational dynamics of these polymers over a wide frequency window ranging from gigahertz to terahertz, with the aim of providing physical descriptors correlated to the cross-linking degree and elastic properties of the material. The results seem to suggest that the stiffness of cross-linked polymers can be successfully tuned by acting on the type and the relative amount of the cross-linker during the synthesis of a polymer matrix, predicting and controlling their swelling and entrapment properties. The proposed experimental approach is a useful tool for investigating the structural and physicochemical properties of polymeric network systems.

Potential energy landscape and long-time dynamics in a simple model glass

We analyze the properties of a Lennard-Jones system at the level of the potential energy landscape. After an exhaustive investigation of the topological features of the landscape of the systems, obtained by studying small size samples, we describe the dynamics of the systems in multidimensional configurational space by means of a simple model. This considers the configurational space as a connected network of minima where the dynamics proceeds by jumps described by an appropriate master equation. Using this model we are able to reproduce the long-time dynamics and the low temperature regime. We investigate both the equilibrium regime and the off-equilibrium one, finding those typical glassy behaviors usually observed in the experiments such as (i) a stretched exponential relaxation, (ii) a temperature-dependent stretching parameter, (iii) a breakdown of the Stokes-Einstein relation, and (iv) the appearance of a critical temperature below which one observes a deviation from the fluctuation-dissipation relation as a consequence of the lack of equilibrium in the system.

Low-Frequency Surface-Enhanced Raman Scattering from Fractal Vibrational Modes Localized at the NaCl-Na Colloid Interface

An experimental study of the very-low-energy Raman scattering from Na-colloids in NaCl is presented. The spectra show evidence of fractal structure of the Na-colloids and of surface enhancement by the metal interface. A model is developed which provides information on the fractal and spectral dimensions.

High Frequency Sound Waves in Vitreous Silica

We report a molecular dynamics simulation study of the sound waves in vitreous silica in the mesoscopic exchanged momentum range. The calculated dynamical structure factors are in quantitative agreement with recent experimental inelastic neutron and x-ray scattering data. The analysis of the longitudinal and transverse current spectra allows to discriminate between opposite interpretations of the existing experimental data in favour of the propagating nature of the high frequency sound waves.

Temperature Effect on the Vibrational Dynamics of Cyclodextrin Inclusion Complexes: Investigation by FTIR-ATR Spectroscopy and Numerical Simulation

The vibrational dynamics of solid inclusion complexes of the nonsteroidal anti-inflammatory drug Ibuprofen (IBP) with beta-cyclodextrin (beta-CD) and methyl-beta-cyclodextrin (Me-beta-CD) has been investigated by using attenuated total reflection-Fourier transform infrared FTIR-ATR spectroscopy, in order to monitor the changes induced, as a consequence of complexation, on the vibrational spectrum of IBP, in the wavenumber range 600-4000 cm(-1). Quantum chemical calculations were performed on monomeric and dimeric structures of IBP, derived from symmetric hydrogen bonding of the two carboxylic groups, in order to unambiguously assign some characteristic IR bands in the IBP spectrum. The evolution in temperature from 250 to 340 K of the C horizontal lineO stretching vibration, described by a best-fit procedure, allowed us to extract the thermodynamic parameter DeltaH associated to the binding of IBP with betaCDs in the solid phase. By comparing these results, Me-beta-CD has been shown to be the most effective carrier for IBP.