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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; iii) breakdown of the Stokes-Einstein law.Comment: 4 pages (Latex), 4 eps figure

A relaxation process, with the associated phenomenology of sound attenuation and sound velocity dispersion, is found in a simulated harmonic LennardJones 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 to relate to each others: 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.

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.

We report measurements of the sound attenuation coefficient in vitreous silica, for sound waves of wavelength between 50 and 80 nm, performed with the new inelastic UV light scattering technique. These data indicate that in silica glass a crossover between a temperature-dependent (at low frequency) and a temperature-independent (at high frequency) acoustic attenuation mechanism occurs at Q 0:15 nm ÿ1 . The absence of any signature in the static structure factor at this Q value suggests that the observed crossover should be associated with local elastic constant fluctuations. DOI: 10.1103/PhysRevLett.97.035501 PACS numbers: 61.43.Fs, 63.50.+x The sound attenuation in disordered materials and its frequency and wavelength dependence are the result of the interplay between two physical mechanisms: one is due to the anharmonicity of the interparticle interactions, and the other to the structural disorder.The anharmonic attenuation of an acoustic sound wave, identified by its wavelength , frequency , and wave vector Q 2 = , is characterized by a specific, temperature-dependent, relaxation time r [1]. At low frequency (! r < 1) this process dominates the sound absorption through mechanisms such as, e.g., the Akhiezer mechanism [2,3]. Accordingly, the sound attenuation coefficient, as measured by the broadening ÿ of the Brillouin peak in the dynamic structure factor S Q; ! , scales as ! 2 and Q 2 . At high frequency ÿ! r > 1, i.e., Q > Q r 1=v r , where v is the sound velocity, the anharmonic attenuation becomes frequency independent [1][2][3].The sound attenuation associated with topological disorder gives rise to a steeper Q dependence of ÿ Q . If Rayleigh scattering is responsible for this attenuation, ÿ / Q 4 is expected for wavelengths larger than the typical defects size 2 =Q R . For Q > Q R , when the Rayleigh scattering regime is abandoned, one expects that ÿ Q is no longer / Q 4 . Experimentally, for Q larger than 1 nm ÿ1 , all glasses studied so far show ÿ / Q x , with x very close to 2 [4,5].This scenario can be summarized by a three-regime behavior of ÿ Q : (i) at low Q, ÿ Q is determined by anharmonic processes, and ÿ Q / Q 2 up to a first (temperature-dependent) crossover Q r 1=v r ; (ii) an intermediate regime, where the Q dependence of ÿ Q is determined by the system dependent strengths of anharmonicity and structural disorder processes; (iii) a high-Q regime, where ÿ Q is determined by the topological disorder and ÿ Q / Q 2 with a temperature-independent coefficient. This picture is highly debated because it critically depends on the location of Q r and Q R in different glasses. In densified v-SiO 2 , for example, the crossover Q R has been hypothesized to be around 2 nm ÿ1 [6].In the most studied case of vitreous silica, similarly to what happens in many other glasses, both Q r and Q R belong to a Q region which is not directly accessed by traditional scattering probes. In the case of v-SiO 2 , clear evidence is reported for the low-and high-Q quadratic behaviors of ÿ Q , using Brillouin light scat...