Light-scattering spectra of the glasses polystyrene ͑PS͒, polycarbonate ͑PC͒, and Ca 0.4 K 0.6 (NO 3 ) 1.4 ͑CKN͒ are measured in the frequency interval 3Ϫ10000 GHz covering a broad temperature range. The low frequency wing of the fast relaxation spectrum is found to show a power-law behavior with an exponent ␣ϭ0.2Ϫ0.6. The exponent depends on system and temperature. No indication of a crossover to a white noise spectrum, as previously reported and discussed within mode-coupling analyses is found. It is shown that the Gilroy-Phillips model of thermally activated transitions in asymmetric double-well potentials well describes the power-law part of fast relaxation spectra in PS and CKN but fails in the case of PC. The distribution of barrier heights is extracted from the spectra. ͓S0163-1829͑98͒04645-1͔

Low-frequency Raman coupling coefficient of 11 different glasses is evaluated. It is found that the coupling coefficient demonstrates a universal linear frequency behavior near the boson peak maximum and a superlinear behavior at very low frequencies. The last observation suggests vanishing of the coupling coefficient when frequency tends to zero. The results are discussed in terms of the vibration wavefunction that combines features of localized and extended modes.Comment: 8 pages, 9 figure

Quasielastic light scattering (QLS) in the frequency interval 100–1000 GHz is measured in some polymers: polycarbonate, polybutadiene, polystyrene, and poly(methyl methacrylate). To describe the spectra, a model of the fast picosecond relaxation processes responsible for the QLS, which is based on the damping of the boson peak vibrations by the dynamic hole volume fluctuations, is used. Within the frame of the model, the intensity of the fast relaxation process is proportional to the fractional dynamic hole volume (which above the glass transition temperature Tg is known as the fractional free volume). The hole volumes can be measured using the positron annihilation lifetime spectroscopy (PALS). The comparison of the literature PALS data in the four polymers with the QLS shows an apparent correlation between the relaxation strength and the fractional dynamic hole volume in good agreement with the predictions of the model.

No abstract

We present dielectric relaxation (DS) and light scattering (LS) data of several glass formers. Relaxational features are compiled which are not yet properly taken into account by current models. (i) We distinguish two types of glass formers. Type A systems do not show a slow -process whereas type B systems do. A full line-shape analysis of is presented . In type A systems the evolution of the high-frequency wing of the -process is the most prominent spectral change while cooling and leads to an essentially constant loss at . The analysis of of type B systems is carried out within the Williams-Watts approach and we focus on the temperature dependence of the -relaxation strength. (ii) Concerning fast relaxations below as revealed by LS we identify relaxation with a low-frequency power-law behaviour. No indication of a crossover to a white noise spectrum as previously reported and discussed within MCT is found. Analysing this relaxation we recourse to the model of thermally activated transitions in asymmetric double well potentials. We show that the model works well in some cases and the distribution of barrier heights may be extracted, but in other systems pronounced deviations occur.

Low-energy excitations were investigated in amorphous poly(methyl methacrylate) by means of both neutron and Raman scatterings, in a wide temperature interval. The frequency dependence of the light-vibration coupling coefficient was deduced, especially at low temperature where the quasi-elastic contribution is not significant; it was not found to be linear. The vibrational density-of-states anomaly is interpreted in the frame of a non-continuous glassy nanostructure, which is characterized by a log-normal size distribution of domains.

No abstract

Liquid-and solid-like particles in a glass are defined via the Lindemann criterion of melting. Due to structure inhomogeneity the amplitude of the mean-square thermal atomic displacements r 2 varies spatially. This situation defines a percolation problem for liquid-and solid-like clusters. There are two percolation thresholds: the first corresponds to the temperature T1 at which the first infinite cluster of liquid-like particles appears. The second threshold T2 corresponds to a point at which the last infinite cluster of solid-like particles disappears. In the 3D case T1 ≤ Tg ≤ T2 holds. We argue that this interval is relatively broader in strong glass-formers in comparison with fragile ones; this means that the spatial distribution of r 2 and, respectively, of the elastic constants is broader in the former case than in the latter one.Comparison with neutron experimental data on r 2 shows that numerically T1 is close to the Vogel-Fulcher-Tammann temperature T0 and T2 is close to the critical temperature Tc defined within mode coupling theory.