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(122) Production(s) de VACHER R.
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High-frequency acoustic modes in silica via ultrafast optical techniques 
Auteur(s): Huynh Agnès, Péronne Emmanuel, Perrin Bernard, Ruffle B., Vacher R., Foret M.
Conference: Forum Acusticum (Lyon, FR, 2020-12-07)
Ref HAL: hal-03240299_v1
DOI: 10.48465/fa.2020.0723
Exporter : BibTex | endNote
Résumé: We use the advent of reliable ultrafast optical techniques to explore acoustic modes in glasses in the difficult but crucial frequency region between around 0.1 and 1 THz. Brillouin scattering of light allows to measure velocity and absorption of sound up to frequencies around 100 GHz and coherent inelastic x-ray scattering (IXS) is limited to frequencies larger than 1 THz. A rather universal property of glasses is that a large excess of modes exists, with a maximum density of states near 1 THz, forming the so-called boson peak. The acoustic modes are expected to be strongly affected as their frequency nears the boson peak. The onset of this effect, which is expected to be dominant in the 0.5 to 1 THz range, is observed here for the first time in v-SiO2 as a function of temperature.
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Temperature dependence of hypersound attenuation in silica films via picosecond acoustics
Auteur(s): Foret M. , Huynh A., Peronne E., Ruffle B., Perrin B., Lafosse X., Lemaitre A., Vacher R.
Conference: 16th International Conference on Phonon Scattering in Condensed Matter (Phonons 2018) (Nanjing, CN, 2018-05-30)
Ref HAL: hal-01940549_v1
Exporter : BibTex | endNote
Résumé: We reinvestigate sound dispersion and attenuation in a SiO2 layer as a function of temperature over the range 20–300 K by picosecond acoustics [1]. A pulse-echo scheme is used, where a broadband strain-pulse (centered at 120 GHz) is detected in an Al transducer after propagating back and forth through the SiO2 layer. The acoustic attenuation coefficient α within the SiO2 layer is evaluated by fitting the echoes to a mismatch model including an effective local law for the frequency dependence of attenuation over the band of the pulse. In this way, the T dependence of α in SiO2 layers could be extracted in this work for the first time. Results are found to follow rather well a model combining coupling to thermally activated relaxation mechanisms and interactions with thermal vibrations. This leads to a non-trivial variation of the attenuation coefficient with frequency and temperature. The number density of relaxing defects in the SiO2 layer is found to be slightly higher than that in bulk v-SiO2. In contrast, similar anharmonic contribution to acoustic absorption is observed in both systems. The velocity variations are also measured and are compared to the dynamical velocity changes deduced from the sound attenuation.
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Elastic and plastic transformations of vitreous silica underpressure
Auteur(s): Foret M., Ruffle B., Weigel C., Vacher R.
Conférence invité: Journées de la Matière Condensée, 2018 (Grenoble, FR, 2018-08-27)
Ref HAL: hal-01940534_v1
Exporter : BibTex | endNote
Résumé: The talk focuses on the thermodynamical properties of vitreous silica submitted to high pressures in a diamond anvil cell as obtained directly from Brillouin Light Scattering experiments or indirectly from standard relations. The analysis reveals non-negligible differences between static and dynamic compressibilities which are mostly related to the existence of thermally activated relaxational processes. Estimate of the residual densifications after complete cycles of compression/decompression is discussed.
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