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

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 frequency of 18 cm(-1) and is identified with the Boson peak. It is suggested that the latter can serve as a signature for configurational excitations of the ideal glass structure, i.e. the topologically diverse defects of the glassy solid state. The excess vibrational heat capacity associated with this generation of low frequency modes with structural excitation is shown to be responsible for about 60% of the jump in heat capacity at T-g, most of the remainder coming from configurational excitation.