Correlation between density of tunneling states and fragility of glasses

Abstract

Evidence of a correlation between the density of tunneling states of two-level systems and the fragility for a variety of glasses is presented. More fragile glasses tend to have a higher excess low-temperature (TϽ1 K͒ specific heat and thus a higher density of tunneling states. This correlation is consistent with a picture which connects the fragility of glasses to the topology on their potential energy hypersurfaces in configuration space.

Two-order-parameter model of the liquid–glass transition. III. Universal patterns of relaxations in glass-forming liquids

“…This physical picture also qualitatively explains the finding of Zhu [83] that the number density of two-level systems per mole increases with an increase in the fragility, since it should be correlated with the number density of normal-liquid structures, 1 À S, in our model. Thus, we argue that the physical origin of a boson peak is the same as that of the ultrafast process, but essentially different from that of fast b motion.…”

The Boson peak in structural and orientational glasses of simple alcohols: specific heat at low temperatures

“…There are no published data for 2-propanol to our knowledge. In this framework, the large difference in C TLS between ethanol and glycerol (almost one order of magnitude, see Table 1) for two systems of very similar fragility claims against the correlation proposed by Zhu [19]. Moreover, 1-propanol has a contribution to C p from tunneling states almost a factor of 3 higher than that of glycerol, being less fragile Finally, we would like to consider the interstitialcy model proposed by Granato [33,34].…”

“…Zhu [19] has suggested a general correlation between the density of tunneling states and the fragility, presenting data for a variety of glasses. More fragile glasses would have a larger number of minima on the potential-energy hypersurface, hence explaining a higher density of tunneling states and a higher value of C TLS .…”

“…In addition to these phenomenological models, other recent approaches [16,17,18,19,20] have focused on suggesting general scalings, correlations or universalities in the low-temperature properties of glasses which could hint at their microscopic origin.…”

Viscosity scaling of glass forming liquids on the basis of fragility

“…Fragility is an evocative phrase which has forcefully drawn attention towards the inherent relation between bonding and viscosity behaviour [10,11] of glass forming liquids. Several empirical methods [11][12][13][14][15][16][17][18] of quantifying fragility have been discussed in the literature and we feel that most of them are flawed because they are determined using properties measured in the region close to glass transition. They are therefore, subject to inaccuracies arising from the extremely slow kinetics of flow.…”