Eric Courtens

The vibrational modes of glasses

Abstract Our recent experimental observations on the harmonic vibrational modes of glasses are presented. Emphasis is placed on normal and densified silica. These results are discussed within the broader current knowledge, including thermal properties and other spectroscopic data that are critically assessed. We find that propagating acoustic modes enter a regime of strong scattering as their wavelength is reduced, and that this leads to an Ioffe–Regel crossover at frequencies of the order of the terahertz, corresponding to wavelengths of several nanometers. At similar frequencies, an excess in the density of states of optical modes, generally called the Boson peak, is observed. Hyper-Raman spectroscopy on these modes clearly shows that in silica they are due to the rocking of small groups of tetrahedra. These findings provide unique and unexpected information on the structure of glasses at the extended 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.

Anharmonic versus relaxational sound damping in glasses. II. Vitreous silica

The temperature dependence of the frequency dispersion in the sound velocity and damping of vitreous silica is reanalyzed. Thermally activated relaxation accounts for the sound attenuation observed above 10 K at sonic and ultrasonic frequencies. Its extrapolation to the hypersonic regime reveals that the anharmonic coupling to the thermal bath becomes important in Brillouin-scattering measurements. At 35 GHz and room temperature, the damping due to this anharmonicity is found to be nearly twice that produced by thermally activated relaxation. The analysis also reveals a sizeable velocity increase with temperature which is not related with sound dispersion. A possible explanation is that silica experiences a gradual structural change that already starts well below room temperature.

Nature of the Boson peak of silica glasses from hyper-Raman scattering

Abstract Hyper-Raman scattering from the Boson peak has been performed in vitreous silica and in a silica-based lead glass. A comparison with Raman scattering and infrared measurements suggests that in v-SiO 2 the low- ω excitations are associated with rotational motions of SiO 4 tetrahedra. These modes dominate the vibrational density of states in that frequency range. In the lead glass, despite a strong modification of the SiO 4 network as compared to pure silica, the results indicate that the low-frequency density of states might also be dominated by hyper-Raman active modes.

Anharmonic versus relaxational sound damping in glasses. I. Brillouin scattering from densified silica

This series discusses the origin of sound damping and dispersion in glasses. In particular, we address the relative importance of anharmonicity versus thermally activated relaxation. In this first article, Brillouin-scattering measurements of permanently densified silica glass are presented. It is found that in this case the results are compatible with a model in which damping and dispersion are only produced by the anharmonic coupling of the sound waves with thermally excited modes. The thermal relaxation time and the unrelaxed velocity are estimated.

Is there an unusual condensation in quantum paraelectrics

Abstract Quantum paraelectrics are incipient ferroelectrics whose transition is hindered by the zero-point quantum motion. Recently however, electronparamagnetic-resonance measurements uncovered a phase-transition-like anomaly in the quantum paraelectric SrTiO3 at T q ≈ 37 K. The observed line shifts are accompanied by no measurable splitting or broadening, suggesting that the possible transition would be towards a dynamical state preserving the crystal symmetry. This new “Muller state” generated considerable interest. In this report, we concentrate on data obtained on oxide perovskites. Light and neutron scattering spectroscopies confirmed the existence of T q in SrTiO3 and revealed several anomalies among which a new branch whose origin remains unclear. A new quasielastic doublet was also found, both in SrTiO3 and KTaO3, which is produced by second sound. Although some measurements were interpreted in terms of a coherent quantum state, there seems to be so far no direct evidence for such a state, while ...

Hyper-Raman scattering from vitreous boron oxide: coherent enhancement of the boson peak.

Hyper-Raman scattering spectra of vitreous B(2)O(3) are compared to Raman scattering ones. Particular attention is given to the low-frequency boson peak which relates to out-of-plane rigid librations of planar structural units, mostly boroxols. While the Raman strength can be accounted for by the motions of single units, the hyper-Raman signal exhibits a unequaled enhancement due to coherent librations of several boroxols. This important distinction is explained by the different symmetry properties of the polarizability and hyperpolarizability tensors of the structural units.

Hyper-Raman scattering analysis of the vibrations in vitreous boron oxide

Hyper-Raman scattering has been measured on vitreous boron oxide,

Do high-frequency acoustic vibrations propagate in structurally disordered solids?

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The crossover from propagating to strongly scattered acoustic modes of glasses observed in densified silica

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Nature of the hyper-Raman active vibrations of lithium borate glasses

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