Low-temperature anomalies in strong and fragile glass formers

The low-temperature anomalies of glass forming systems are analyzed focusing on the concept of fragility. It is shown that two extremes on the fragility plot, SiO₂ and Ca_0.4K_0.6(NO₃)_1.4, show a strong difference in specific heat in the range of excess vibrational contribution. This difference cannot be explained in the framework of the soft potential model in its present state. A relation between the excess vibrational contribution and the amount of structural fluctuations is suggested. It is shown that the suggested relation explains the found difference in low-temperature specific heat between strong and fragile glass formers.