V. Mazzacurati

Numerical study of the low-frequency atomic dynamics in a Lennard-Jones glass

Abstract We present a computer simulation study of a monatomic glass model. The investigated system consists of up to 32000 ‘argon’ atoms, interacting via a Lennard-Jones potential. Its dynamical properties have been investigated both by the time evolution of the atomic trajectories during molecular dynamics runs at low temperature and by the normal mode analysis (NMA) in the inherent configuration. In the NMA, we used both the direct diagonalization of the dynamical matrix and the spectral moment method. The vibrational density of states (DOS). the dynamic structure factors, the Raman spectrum and the spatial patterns of the eigenmodes have been calculated. The Raman spectrum shows the characteristic bump, that is the so-called ‘boson peak’, at frequencies where the DOS presents an excess of states with respect to the Debye behaviour. By the analysis of the previous dynamical quantities and from the direct inspection of the pattern of the eigenvectors, we derive a consistent picture of the atomic dynamic...

On the shape of the dynamic structure factor in glasses

Abstract The peculiar shape of the dynamie structure factor of glasses has been investigated thoroughly by light scattering and computer simulation techniques. The experimental spectra show a pronounced asymmetry with respect to their peak frequencies. Similar asymmetries have already been observed by simulation on argon glass in a range of exchanged momentum about ten times higher than that probed by light scattering. New simulation on a large sample of silica glass also shows similar features. Moreover the calculated dynamie structure factors appear as the straight continuation at higher momentum values of the experimental light scattering data. The qualitative independence of this phenomenon from the glass type together with the analysis of the eigenvector pattems in argon glass suggest that this asymmetric shape is a mark of the topological disorder.

Study of the dynamic structure factor of strong glasses

Abstract The dynamic structure factors (DSFs) of several strong glasses (SF6, SF10, BK7, SUPRASIL) measured by Brillouin light scattering spectroscopy are reported. Spectra have been collected, at and above room temperature, at two scattering angles, θ=90° and θ=180° corresponding to exchanged wavevector q values ranging from 0.0256 to 0.0448 nm−1. In particular we find that the isotropic spectral lineshapes are in all cases well described by the simple hydrodynamic theory of an amorphous solid. The width of the Brillouin peaks are found to be consistent with the predicted q2 dependence at both investigated temperatures. This damping is however found to account only partially for the strong asymmetry of the Brillouin line clearly visible on a logarithmic intensity scale. As a matter of fact there is an excess intensity in the very low frequency plateau underlying the central component. The height of this plateau and hence the entire lineshape is well reproduced if a relaxation process is taken into account in the hydrodynamic equations. Owing to the intense elastic scattering we are able to determine unambiguously only the ratio between amplitude and characteristic time of this process which quantifies the sound dispersion to be of the order of a few percent in all samples. The temperature dependence of the parameters indicates that this relaxation cannot be attributed to thermally activated relaxation phenomena. These general findings favorably compare with molecular dynamics simulation results on similar systems.

A high resolution ultraviolet monochromator HIRESUV for the study of disordered materials in the mesoscopic regime

We present the state of the art in the realization of a new spectrometer especially designed for high resolution and high-contrast scattering experiments in the ultra-violet (UV) spectral region. Different scattering techniques, employing neutrons, X-rays or visible light are compared with each other for transferred energies and momenta. The importance of high-resolution measurements in the UV range is stressed with particular attention devoted to the study of disordered materials.

Induced Light Scattering from Electrically Disordered Solids

In this paper, current theories of Disorder Induced Light Scattering (DILS) are applied to mixed alkali halide crystals. It is recognized that Second Order Raman Scattering (SORS) is a relevant part of the measured spectrum of (KCl)0.5(KBr)0.5. The calculation of one-phonon and two-phonon scattering is reviewed and the SORS calculated for mixed KCl-KBr. This calculation allows one to identify unambiguously the spectral shape of the DILS scattering and to prove the validity of DILS predictions in these systems.