The vibrational modes of glasses

Our recent experimental observations on the harmonic vibrational modes of g lasses are presented. Emphasis is placed on normal and densified silica. Th ese results are discussed within the broader current knowledge, including t hermal properties and other spectroscopic data that are critically assessed . We find that propagating acoustic modes enter a regime of strong scatteri ng as their wavelength is reduced, and that this leads to an Ioffe-Regel cr ossover at frequencies of the Order of the terahertz, corresponding to wave lengths of several nanometers. At similar frequencies, an excess in the den sity of states of optical modes, generally called the Boson peak, is observ ed. Hyper-Raman spectroscopy on these modes clearly shows that in silica th ey are due to the rocking of small groups of tetrahedra. These findings pro vide unique and unexpected information on the structure of glasses at the e xtended length scale, about which so little is known otherwise. The strong elastic inhomogeneity found at this scale might be decisive in determining glass properties, and even stability, and this will justify further studies . (C) 2001 Elsevier Science Ltd. All rights reserved.