Evidence of anomalous dispersion of the generalized sound velocity in glasses

Abstract

The dynamic structure factor, S(Q, ω), of vitreous silica, has been measured by inelastic X-ray scattering in the exchanged wavevector (Q) region Q=4÷16.5 nm −1 and up to energies ω=115 meV in the Stokes side. The unprecedented statistical accuracy in such an extended energy range allows to accurately determine the longitudinal current spectra, and the energies of the vibrational excitations. The simultaneous observation of two excitations in the acoustic region, and the persistence of propagating sound waves up to Q values comparable with the (pseudo-)Brillouin zone edge, allow to observe a positive dispersion in the generalized sound velocity that, around Q≈5 nm −1 , varies from ≈6500 to ≈9000 m/s: this phenomenon was never experimentally observed in a glass.

Elastic anomalies at terahertz frequencies and excess density of vibrational states in silica glass

“…These modes can be associated with the low-frequency peak that appears in the IXS spectra at higher wave vectors. 34,35 The failure of a simple Debye approach to describe the temperature dependence of the BP also supports the conclusion that other excitations accumulate in frequency at the BP.…”

“…51 The observed difference among the studied systems seems thus to reside in the relative weight of the two mechanisms in determining the BP intensity. The branch of nondispersing excitations is particularly intense in the case of vitreous silica 34,35 and represents an important part of the BP. In systems like glycerol and sorbitol the contribution to the BP excess of this feature, if present, is negligible and the softening of the sound waves dominates the BP intensity.…”

“…The presence of nondispersing modes with energies around that of the boson peak has indeed been detected by means of IXS for wave vectors Q > 4 nm −1 . 34,35 Excess vibrations were identified also from the analysis of neutron spectra in the form of librations of corner-sharing tetrahedra. 1 The situation in vitreous silica appears thus noticeably different from that in other glasses, like glycerol 22 and sorbitol, 25 where a computation based on Eq.…”

“…3 The observation of these elastic anomalies requires a very high accuracy which has been attained only in recent years, thanks to the continuous development of the IXS technique. The particular case of vitreous silica has been deeply studied by means of IXS in the past, [26][27][28][29][30][31][32][33][34][35] but these fine features of the dynamics were hidden by the noise in the first experiments.…”

Thermal conductivity accumulation in amorphous silica and amorphous silicon

“…This approach has been previously used to predict effective dispersion curves of disordered and amorphous materials experimentally [18,26,28,63] and numerically [41,42,[64][65][66]. The structure factor at a wave vector κ κ κ is defined as [2] …”