New insights on the specific heat of glasses

The physical properties of disordered systems are in the focus of alarge research effort. In particular, one of the open problems relatedto glasses is their excess of modes in the THz frequency range over thevibrational contribution predicted by the Debye model. In fact, onecould expect the continuum approximation underlying the Debye model tohold in glasses at long wavelengths at least as well as in crystallinesolids. Experiments, instead, seem to indicate that the specific heat atlow temperature (a few Kelvin) and the density of vibrational states atlow frequency (a few terahertz) are very different from the Debyeprediction. We present here a detailed analysis of specific heatmeasurements of vitreous GeO2, a prototype of strong glasses, and ofpermanently densified vitreous GeO2. Our data give experimental evidencethat glasses do not show any excess of vibrational modes when comparedto their crystalline counterparts of similar mass density.