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, Si${\mathrm{O}}_2$ and C${\mathrm{a}}_{0.4}$${\mathrm{K}}_{0.6}$(N${\mathrm{O}}_3$${)}_{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.

• Received 9 May 1996

DOI:https://doi.org/10.1103/PhysRevLett.78.2405