Locating the nonergodicity-parameter anomaly near the liquid-to-glass crossover temperature in CaKNO3 by Brillouin scattering

Collective acoustic modes in liquids: a comparison between the generalized-hydrodynamics and memory-function approaches

The most familiar approaches used to describe the dynamical structure factor from adiabatic density fluctuations in liquids are based on generalized hydrodynamics and on the memory function, respectively. We show that, contrary to the common belief, the two approaches are not fully equivalent. In particular, models based on the memory function of a normalized damped oscillator fail in reproducing the correct experimental spectral profiles of systems close to the relaxation process. The discrepancy is due to misleading interpretation of the theoretical memory-function expressions, producing an unavoidable mixing of spectral contribution at different wave vectors when the theory is forced beyond its limits of validity.

Relaxational Dynamics and Strength in Supercooled Liquids from Impulsive Stimulated Thermal Scattering

Impulsive stimulated thermal scattering (ISTS), a time-domain light scattering technique, provides a more than 6-decade time range from sub-ns to many ms. It permits characterization of the structural relaxation dynamics and determination of the relaxation strength or Debye-Waller factor in supercooled liquids, and thus allows testing of the mode coupling theory of the liquidglass transition. ISTS experiments were performed on glass formers salol, butylbenzene, and the molten salt [Ca(N03)]0.4[KNO3]0.6. The relaxational dynamics and the Debye-Waller factorfq=0 were obtained. A square-root anomaly was observed in fq=0 (T) at a crossover temperature Tc for all three materials, consistent with the prediction of mode coupling theory.

Role of structural relaxations and vibrational excitations in the high-frequency dynamics of liquids and glasses

We present theoretical investigation on the high-frequency collective dynamics in liquids and glasses at microscopic length scales and in the terahertz frequency region based on the mode-coupling theory for ideal liquid-glass transition. We focus on recently investigated issues from inelastic-x-ray-scattering and computer-simulation studies for dynamic structure factors and longitudinal and transversal current spectra: the anomalous dispersion of the high-frequency sound velocity and the nature of the low-frequency excitation called the boson peak. It will be discussed how the sound mode interferes with other low-lying modes present in the system. Thereby, we provide a systematic explanation of the anomalous sound-velocity dispersion in systems--ranging from high temperature liquid down to deep inside the glass state--in terms of the contributions from the structural-relaxation processes and from vibrational excitations called the anomalous-oscillation peak (AOP). A possibility of observing negative dispersion--the decrease of the sound velocity upon increase of the wave number--is argued when the sound-velocity dispersion is dominated by the contribution from the vibrational dynamics. We also show that the low-frequency excitation, observable in both of the glass-state longitudinal and transversal current spectra at the same resonance frequency, is the manifestation of the AOP. As a consequence of the presence of the AOP in the transversal current spectra, it is predicted that the transversal sound velocity also exhibits the anomalous dispersion. These results of the theory are demonstrated for a model of the Lennard-Jones system.

Universality of the dynamic crossover in glass-forming liquids: a "magic" relaxation time

Analysis of experimental data on the structural relaxation time tau(alpha) in various glass formers revealed its universality at the critical temperature T(c) of the mode-coupling theory. In most glass formers studied ln tau(alpha)(T(c))=-(6.5-7.5). Possible reasons for such a universality are discussed.

Glass transition and density fluctuations in the fragile glass former orthoterphenyl

High-resolution Brillouin light scattering is used to measure the dynamic structure factor of the fragile glass former orthoterphenyl (OTP) in a wide temperature range around the glass transition region and up to the boiling point. The whole set of spectra is described in terms of a phenomenological generalized hydrodynamic model. In the supercooled phase, we show the contemporary existence of the structural process, whose main features come out to be consistent with the results obtained with other spectroscopies, and of a secondary, activated process, which occurs on the 10(-11) s time scale and has a low activation energy (E(f)(a)=0.28 kcal/mol). This latter process, which is also present in the glassy phase and seems to be insensitive to the glass transition, is attributed to the coupling between the density modes and intramolecular degrees of freedom. In the normal liquid phase, the two processes merge together, and the resulting characteristic time is no longer consistent with those derived with other spectroscopies. The analysis points to the conclusion that, for what concerns the long-wavelength density fluctuations in fragile glass formers such as OTP, the universal dynamical features related to the glass transition come out clearly only in the supercooled phase and at frequencies lower than approximately 10(6) Hz.

Brillouin scattering study of molten zinc chloride

Polarized and depolarized Brillouin scattering experiments on molten ZnCl2 were performed between 300 and 600 degrees C in different geometries. VV spectra measured in backscattering and small angle scattering were analyzed with conventional viscoelastic theory using either a Debye or a Cole-Davidson model for the memory function. We also analyzed in the same way the temperature dependence of the transverse Brillouin lines detected in a 90 degrees VH geometry. We show that the Cole-Davidson memory function yields a consistent interpretation of all the spectra. The resulting shear and longitudinal relaxation times are equal within their error bars, and are about 2.5 times smaller than the alpha relaxation time previously determined. The static shear viscosity values deduced from the analysis of the propagating transverse waves agree, at all temperatures, with the measured viscosity values.

Intramolecular origin of the fast relaxations observed in the brillouin light scattering spectra of molecular glass formers

The Brillouin light scattering spectra of the o-terphenyl single crystal are compared with those of the liquid and the glass phases. This shows (i) the direct evidence of a fast relaxation at frequencies nu approximately 5 GHz in both the single crystal and the glass; (ii) a similar temperature dependence for the attenuation of the longitudinal sound waves in the single crystal and the glass; and (iii) the absence of coupling between the fast relaxation and the transverse acoustic waves. These results allow us to assign such a relaxation to the coupling between the longitudinal acoustic waves and intramolecular vibrations, and therefore to exclude any relationship between it and the glass transition.