Fast dynamics in glasses: Relation between the boson peak and quasielastic scattering

Raman study of TiO2 role in SiO2-Al2O3-MgO-TiO2-ZnO glass crystallization

Tough glass-ceramic material of special mechanical properties with nanosize crystal phases formed by appropriately controlled crystallization was studied by Raman spectroscopy. It was obtained by TiO2 activated crystallization of Mg-aluminosilicate glass of SiO2-Al2O3-MgO-TiO2-ZnO composition. Crystallization was preceded by a change in the TiO2 structural position and state, which is manifested by a changed color of glass from yellow into blue shortly before the glass transformation (Tg) temperature. Raman spectroscopy was applied to explain the mechanism of this process and to establish the role of TiO2 in the early stage of glass crystallization that precedes a complete crystal phase formation. The starting glasses were found in almost complete disorder, since all bands were weak, broad and dominated by a Bose band at about 90 cm-1. After the sample annealing all bands turned out better resolved and the Bose band practically disappeared, both confirming the amorphous structure reorganization process. A multiplet observed in the vicinity of 150 cm-1 we assigned to the anatase and other titania structures that can be considered prime centers of crystallization. Finally, in the closest neighborhood of the Rayleigh line the low frequency mode characterizing nanoparticles was observed. According to this band theory, the mean size of initial titania crystallites is about 10nm for all samples, but the size distribution varies within factor two among them.

Light-induced giant softening of network glasses observed near the mean-field rigidity transition

The longitudinal acoustic (LA) mode of bulk GexSe1-x glasses is examined in Brillouin scattering (BS) over the 0.15<x<1/3 range using near band-gap radiation (lambda=647.1 nm). The LA-mode frequency (nu(LA)) softens with increasing laser power in an athermal and reversible manner by nearly 30% (=Delta nu(LA)/nu(LA)) near x=x(c)=0.19(1) or mean coordination number, r(c)=2+2x(c)=2.38(2), close to the mean-field rigidity percolation transition (r(t)=2.40). BS is a bulk probe of elasticity, and photosoftening is maximized here when network stress is minimized near the elastic phase transition.

On the nature of a glassy state of matter in a hydrated protein: Relation to protein function

Diverse biochemical and biophysical experiments indicate that all proteins, regardless of size or origin, undergo a dynamic transition near 200 K. The cause of this shift in dynamic behavior, termed a "glass transition," and its relation to protein function are important open questions. One explanation postulated for the transition is solidification of correlated motions in proteins below the transition. We verified this conjecture by showing that crambin's radius of gyration (Rg) remains constant below approximately 180 K. We show that both atom position and dynamics of protein and solvent are physically coupled, leading to a novel cooperative state. This glassy state is identified by negative slopes of the Debye-Waller (B) factor vs. temperature. It is composed of multisubstate side chains and solvent. Based on generalization of Adam-Gibbs' notion of a cooperatively rearranging region and decrease of the total entropy with temperature, we calculate the slope of the Debye-Waller factor. The results are in accord with experiment.