Potential energy, relaxation, vibrational dynamics and the boson peak, ofhyperquenched glasses

Abstract

We describe a combination of laboratory and simulation studies that givequantitative information on the energy landscape for glass-forming liquids. Bothtypes of study focus on the idea of suddenly extracting the thermal energy, so thatthe system obtained for subsequent study has the structure, and hence potentialenergy, of a liquid at a much higher temperature than the normal glass temperatureTg.One type of study gives information on the energy that can be trapped inexperimental glasses by hyperquenching, relative to the normal glass,and on the magnitude of barriers separating basins of attraction on thelandscape. Stepwise annealing studies also give information on the matter ofenergy heterogeneity and the question of ‘nanogranularity’ in liquids nearTg.The other type of study gives information on the vibrational properties of asystem confined to a given basin, and particularly on how that vibrationalstructure changes with the state of configurational excitation of the liquid. Afeature in the low frequency (‘boson peak’) region of the density of vibrationalstates of the normal glass becomes much stronger in the hyperquenched glass.Qualitatively similar observations are made on heating fragile glass-formers intothe supercooled and stable liquid states. The vibrational dynamics findings aresupported and elucidated by constant pressure molecular dynamics/normalmode MD/NM simulations/analysis of the densities of states of differentinherent structures of a model fragile liquid (orthoterphenyl (OTP) in theLewis–Wahnstrom approximation). These show that, when the temperature israised at constant pressure, the total density of states changes in a mannerthat can be well represented by a two-Gaussian ‘excitation across thecentroid’, leaving a third and major Gaussian component unchanging.The low frequency Gaussian component, which grows with increasingtemperature, has a constant peak frequency of 18 cm−1and is identified with the Boson peak. It is suggested that the latter can serve as asignature for configurational excitations of the ideal glass structure, i.e. thetopologically diverse defects of the glassy solid state. The excess vibrational heatcapacity associated with this generation of low frequency modes with structuralexcitation is shown to be responsible for about 60% of the jump in heat capacityat Tg,most of the remainder coming from configurational excitation.