New insights on the specific heat of glasses

The physical properties of disordered systems are in the focus of a large research effort. In particular, one of the open problems related to glasses is their excess of modes in the THz frequency range over the vibrational contribution predicted by the Debye model. In fact, one could expect the continuum approximation underlying the Debye model to hold in glasses at long wavelengths at least as well as in crystalline solids. Experiments, instead, seem to indicate that the specific heat at low temperature (a few Kelvin) and the density of vibrational states at low frequency (a few terahertz) are very different from the Debye prediction. We present here a detailed analysis of specific heat measurements of vitreous GeO2, a prototype of strong glasses, and of permanently densified vitreous GeO2. Our data give experimental evidence that glasses do not show any excess of vibrational modes when compared to their crystalline counterparts of similar mass density.