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Register a new userAcoustic Nature of the Boson Peak in Vitreous Silica
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New temperature dependent inelastic x-ray (IXS) and Raman (RS) scattering data are compared to each other and with existing inelastic neutron scattering data in vitreous silica (v-SiO_2), in the 300 - 1775 K region. The IXS data show collective propagating excitations up to Q=3.5 nm^-1. The temperature behaviour of the excitations at Q=1.6 nm^-1 matches that of the boson peak found in INS and RS. This supports the acoustic origin of the excess of vibrational states giving rise to the boson peak in this glass.
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The position and strength of the boson peak in silica glass vary considerably with temperature $T$. Such variations cannot be explained solely with changes in the Debye energy. New Brillouin scattering measurements are presented which allow determining the $T$-dependence of unrelaxed acoustic velocities. Using a velocity based on the bulk modulus, scaling exponents are found which agree with the soft-potential model. The unrelaxed bulk modulus thus appears to be a good measure for the structural evolution of silica with $T$ and to set the energy scale for the soft potentials.
We computer model a free-standing vitreous silica bilayer which has recently been synthesized and characterized experimentally in landmark work. Here we model the bilayer using a computer assembly procedure that starts from a single layer of amorphous graphene, generated using a bond switching algorithm from an initially crystalline graphene structure. Next each bond is decorated with an oxygen atom and the carbon atoms are relabeled as silicon. This monolayer can be now thought of as a two dimensional network of corner sharing triangles. Next each triangle is made into a tetrahedron, by raising the silicon atom above each triangle and adding an additional singly coordinated oxygen atom at the apex. The final step is to mirror reflect this layer to form a second layer and then attach the two layers together to form the bilayer. We show that this vitreous silica bilayer has the additional macroscopic degrees of freedom to easily form a network of identical corner sharing tetrahedra if there is a symmetry plane through the center of the bilayer going through the layer of oxygen ions that join the upper and lower layers. This has the consequence that the upper rings lie exactly above the lower rings, which are tilted in general. The assumption of a network of perfect corner sharing tetrahedra leads to a range of possible densities that we have previously characterized in three dimensional zeolites as a flexibility window. Finally, using a realistic potential, we have relaxed the bilayer to determine the density, and other structural characteristics such as the Si-Si pair distribution functions and the Si-O-Si bond angle distribution, which are compared to the experimental results obtained by direct imaging.
Hyper-Raman scattering spectra of vitreous B$_2$O$_3$ are reported and compared to Raman scattering results. The main features are indexed in terms of vibrations of structural units. Particular attention is given to the low frequency boson peak which is shown to relate to out-of-plane librations of B$_3$O$_3$ boroxol rings and BO$_3$ triangles. Its hyper-Raman strength is comparable to that of cooperative polar modes. It points to a sizeable coherent enhancement of the hyper-Raman signal compared to the Raman one. This is explained by the symmetry of the structural units.
We report a molecular dynamics simulation study of the sound waves in vitreous silica in the mesoscopic exchanged momentum range. The calculated dynamical structure factors are in quantitative agreement with recent experimental inelastic neutron and x-ray scattering data. The analysis of the longitudinal and transverse current spectra allows to discriminate between opposite interpretations of the existing experimental data in favour of the propagating nature of the high frequency sound waves.
Experimental results on the density of states and on the acoustic modes of glasses in the THz region are compared to the predictions of two categories of models. A recent one, solely based on an elastic instability, does not account for most observations. Good agreement without adjustable parameters is obtained with models including the existence of non-acoustic vibrational modes at THz frequency, providing in many cases a comprehensive picture for a range of glass anomalies.
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