The low energy excess of vibrational states in v-SiO₂: the role of transverse dynamics

Abstract

The study of the effects of the density variations on the vibrational dynamics in vitreous silica is presented. A detailed analysis of the dynamical structure factor, as well as of the current spectra, allows the identification of a flattened transverse branch which is highly sensitive to the density variations. The experimental variations on the intensity and position of the Boson Peak (BP) in v-SiO2 as a function of density are reproduced and interpreted as being due to the shift and disappearance of the latter band. The BP itself is found to correspond to the lower energy tail of the excess states due to the piling up of vibrational modes at energies corresponding to the flattening of the transverse branch.

Pressure Effect on the Boson Peak in Deeply Cooled Confined Water: Evidence of a Liquid-Liquid Transition

“…For both measured temperatures, as pressure increases, M and V increase, while H decreases. These dependences on pressure, or density, agree with the observations in other experimental [29][30][31]35,37,44] [40,41]. Particularly, as the pressure changes from 3 to 4 kbar, all of these quantities undergo larger changes.…”

“…The boson peak is a broad peak observed at frequencies ∼2-10 meV in the inelastic neutron [27][28][29][30][31], nuclear inelastic [32][33][34][35], and Raman [36][37][38][39] scattering spectra of disordered materials and supercooled liquids. Its origin is widely believed to be related to the transverse dynamics of the material [32][33][34]40,41]. Moreover, both theoretical and experimental studies assign the boson peak in glass to a phenomenon reminiscent of the van Hove singularity of the transverse phonon of the crystal counterpart [33,42,43].…”

Low temperature specific heats of permanently densified glassy GeO2

“…Permanent densification makes it possible to study indirectly the effects of pressure on physical properties, such as the thermodynamic measurements, for which it is usually difficult to carry out in situ high pressure experiments. Remarkable changes are in fact observed in the linear specific heat [11], the hump in C/T 3 and the frequency of the Boson Peak [12][13][14][15], and also in the internal friction [16] and the sound velocity [17]. Measurements on permanently densified samples can provide information on the microscopic mechanisms governing the LEE, mainly when the observations are compared with the microscopic structural changes.…”

Brillouin and boson peaks in glasses from vector Euclidean random matrix theory

“…Thus, hydrodynamics predicts a completely washed-out Brillouin peak, in plain contradiction with experiments. A drastically different approach is to consider these excitations (whose inverse frequency is much smaller than the structural relaxation time) as harmonic vibrations around a quenched atomic structure, a point of view supported by recent molecular dynamics simulations [21,22,23]. Given the presence of well formed local structures (SiO 2 tetrahedra, for instance) a most natural approximation in this context is to consider that the oscillation centers form a crystalline structure, the disorder in the atomic positions being mimicked by randomness in their interaction potential [8,10] (disordered lattice models [7]).…”