Potential energy, relaxation, vibrational dynamics and the boson peak, of hyperquenched glasses

Abstract

We describe a combination of laboratory and simulation studies that give quantitative information on the energy landscape for glass-forming liquids. Both types of study focus on the idea of suddenly extracting the thermal energy, so that the system obtained for subsequent study has the structure, and hence potential energy, of a liquid at a much higher temperature than the normal glass temperature T g. One type of study gives information on the energy that can be trapped in experimental glasses by hyperquenching, relative to the normal glass, and on the magnitude of barriers separating basins of attraction on the landscape. Stepwise annealing studies also give information on the matter of energy heterogeneity and the question of 'nanogranularity' in liquids near T g. The other type of study gives information on the vibrational properties of a system confined to a given basin, and particularly on how that vibrational structure changes with the state of configurational excitation of the liquid. A feature in the low frequency ('boson peak') region of the density of vibrational states of the normal glass becomes much stronger in the hyperquenched glass. Qualitatively similar observations are made on heating fragile glass-formers into the supercooled and stable liquid states. The vibrational dynamics findings are supported and elucidated by constant pressure molecular dynamics/normal mode MD/NM simulations/analysis of the densities of states of different inherent structures of a model fragile liquid (orthoterphenyl (OTP) in the Lewis-Wahnstrom approximation). These show that, when the temperature is raised at constant pressure, the total density of states changes in a manner that can be well represented by a two-Gaussian 'excitation across the centroid', leaving a third and major Gaussian component unchanging. The low frequency Gaussian component, which grows with increasing temperature, has a constant peak

Correlations between Vibrational Entropy and Dynamics in Liquids

“…The amplitude of the correlations between fragility and jump in specific heat implies that the mechanism at play must include the softening under heating of a limited number of soft degrees of freedom, and is not simply a change of elastic moduli. This likely corresponds to the well-known softening of the Boson peak, which has been shown experimentally to affect significantly the thermodynamics of some liquids [18]. Assuming that this is the case I find a reasonable estimate for the magnitude of the vibrational specific heat.…”

“…The latter effect does not appear in high frequency dynamic heat capacities studies, as it requires changes of configurations that take place on the time scale τ to occur, but does contribute to the jump of specific heat characterizing the glass transition, whose amplitude is known to strongly correlate to the liquid fragility [16]. The configurational fraction of the jump in specific heat has been estimated using calorimetry in quenched and annealed glasses [15], measurements of elastic moduli [17], densities of states [18][19][20] and non-linear dielectric susceptibilities [21]. Overall, a great variation was found among glasses, with a configurational fraction ranging from 15% to 80% and apparently decaying with the liquid fragility [21].…”

“…An excess of soft modes with respect to the Debye model for the density of states, the so-called Boson peak [22], shifts toward higher frequencies as the system is cooled [18,24,25]. Those modes presumably contribute significantly to the change of mean square displacement and to the vibrational specific heat [18,19]. If the increase of u 2 under heating is mostly due to the softening of a limited number of low-frequency modes, the gain in entropy is diminished because the short time scale dynamics becomes more correlated, as can be computed directly from Eq.(7).…”

“…In this "elastic" view of the glass transition the vibrational entropy most directly reflects the liquid dynamics, in consistence with the present analysis. Empirical evidences for such a stiffening range from vibrational spectra [18,24,25], mean square displacements [2,3] and elastic moduli [12,31] measurements. In all cases strong correlations with the dynamics are found.…”

Influence of confinement on the vibrational density of states and the Boson peak in a polymer glass

“…An enhancement of the Boson peak has also been observed experimentally in mineral glasses for hyperquenched samples when compared to their annealed counterparts. 21 In our study, the minimized bulk and film systems can be considered to be hyperquenched glasses. However, the differences between the film and bulk glasses arise solely on account of the confinement since the all the configurations in our study have been prepared using the same protocol.…”