Due to the loose structure of amorphous SiO2 presenting with many voids, small atomic species have the possibility to penetrate in this compound. It has been recently shown[1,2] that helium can be "dissolved" into silica glass, strongly modifying its properties, whereas molecular hydrogen cannot [2]. For example its compressibility is strongly reduced when using helium as a pressure transmitting medium in comparison with that measured with the standard methanol-ethanol mixture[1,2]. It is well known that the thermomechanical properties of a-SiO2 exhibit anomalous behaviors. In Ref. 4, it was shown that the onset of the anomalous temperature hardening of the elastic modulus strongly depends on pressure suggesting a dynamical origin of the structural changes that produce the hardening. Here we study the anomalous softening of the elastic modulus in function of pressure. We have measured the longitudinal and transverse acoustic modes by Brillouin scattering in the range 0 – 6 GPa, using a diamond anvil cell with helium as a pressure transmitting medium. We found that due to the penetration of helium into silica, the anomalous elastic behavior disappears almost totally. This should allow shedding light on the microscopic origin of the thermomechanical anomalies of silica. [1] T. Sato, N. Funamori, T. Yagi, Nature Comm. 2, 245 (2011) [2] G. Shen, Q. Mei, V.B. Prakapenka, P Lazor, S. Sinogeikin, Y. Meng, C. Park, Proc. Nat. Acad. Sci. 108, 6004 (2011) [3] S. Rau, S. Baessler, G. Kasper, G. Weiss, S. Hunklinger, Ann. Physik 4, 91 (1995) [4] S. Ayrinhac, B. Rufflé, M. Foret, H. Tran, S. Clément, R. Vialla, R. Vacher, J. C. Chervin, P. Munsch, A. Polian, Phys. Rev. B 84, 024201 (2011)

Due to the loose structure of amorphous SiO2 presenting with many voids, small atomic species have the possibility to penetrate in this compound. It has been recently shown[1] that helium can be "dissolved" into silica glass, strongly modifying its properties, whereas molecular hydrogen cannot. For example its compressibility is strongly reduced when using helium as a pressure transmitting medium in comparison with that measured with the standard methanol-ethanol mixture[1]. It is well known that a-SiO2 presents a sound velocity pressure dependence anomaly with a deep minimum situated around 2 GPa coincident with a maximum in the hypersound attenuation[2, 3]. In Ref. 3, it was shown that the anomalous hardening is of dynamical origin and therefore that this coincidence is not intrinsic to the material. In the present study, using high resolution Brillouin scattering (confocal Fabry-Pérot) as well as standard Sandercock type tandem Fabry-Pérot interferometer, we measured the longitudinal and transverse sound velocities, and the sound attenuation in a-SiO2 in a diamond anvil cell using helium as a pressure transmitting medium up to 6 GPa, and compare the obtained results with those obtained using methanol-ethanol mixture. We show that the sound velocity minimum is shifted toward low pressure, the minimum depth being strongly reduced, whereas the maximum of the attenuation is shifted toward higher pressure. These results will be discussed in the same framework as Ref. 3. On the other hand, it is well known that the thermomechanical properties of a-SiO2 exhibit anomalous behaviors. In Ref. 3, it was shown that the onset of the anomalous temperature hardening of the elastic modulus strongly depends on pressure suggesting a dynamical origin of the structural changes that produce the hardening. Here we study the anomalous softening of the elastic modulus in function of pressure. We have measured the longitudinal and transverse acoustic modes by Brillouin scattering in the range 0 – 6 GPa, using a diamond anvil cell with helium as a pressure transmitting medium. We found that due to the penetration of helium into silica, the anomalous elastic behavior disappears almost totally. This should allow to shed light on the microscopic origin of the thermomechanical anomalies of silica. [1]. T. Sato, N. Funamori, T. Yagi, Nature Comm. 2, 245 (2011) [2]. S. Rau, S. Baessler, G. Kasper, G. Weiss, S. Hunklinger, Ann. Physik 4, 91 (1995) [3]. S. Ayrinhac, B. Rufflé, M. Foret, H. Tran, S. Clément, R. Vialla, R. Vacher, J. C. Chervin, P. Munsch, A. Polian, Phys. Rev. B 84, 024201 (2011)

We have performed new Brillouin scattering measurements of the velocity and attenuation of hypersonic waves as a function of pressure P in vitreous silica [1]. The hypersoundattenuation coefficient exhibits a sharp maximum located at 2 GPa which appears to coincide with the well-known minimum in sound velocity. The results are analyzed together with previous literature data. Internal friction measurements are described satisfactorily using a model including dissipative effects of anharmonicity and thermally activated relaxation of structural defects. From this model, the associated velocity changes were calculated, and subtracted from the data to obtain the bare velocities (the so-called unrelaxed velocities) as a function of T at different P. v¥ (T) is almost constant at low T, then increases linearly above a temperature onset o T . We find evidence for an increase of o T with pressure P [1]. This is in agreement with numerical simulations in which the observed hardening is related to changes of the ring conformations with T [2,3]. [1] S. Ayrinhac, B. Rufflé, M. Foret, H. Tran, S. Clément, R. Vialla, R. Vacher, J.-C. Chervin, P. Munsch, A. Polian, Phys. Rev. B84, 024201 (2011). [2] L. Huang and J. Kieffer, Phys. Rev. B69, 224203 (2004). [3] Y. Liang, C.~R. Miranda, and S. Scandolo, Phys. Rev. B75, 024205 (2007).

We report on high-resolution Brillouin scattering experiments performed under hydrostatic pressures ranging from 0 to 10 GPa in vitreous silica. The hypersound-attenuation coefficient exhibits a sharp maximum located at 2 GPa, which appears to coincide with the well-known minimum in sound velocity. Two main processes contribute to sound damping: the thermal relaxation of local defects and the anharmonic interactions with thermal vibrations. By analyzing the temperature dependence of the velocity and subtracting the changes produced by the above mechanisms, the unrelaxed velocity is found. It is constant at low temperatures and increases anomalously above a temperature onset. We show that the onset strongly depends on pressure, which is in agreement with a dynamical origin of the structural changes that produce the hardening.[1] [1]S. Ayrinhac, B. Rufflé, M. Foret, H. Tran, S. Clément, R. Vialla, R. Vacher, J. C. Chervin, P. Munsch, and A. Polian, “Dynamical origin of anomalous temperature hardening of elastic modulus in vitreous silica”, Phys. Rev. B 84, 024201 (2011)

Recently we proposed an alternative way of measuring hypersound attenuation in silica thin films using ultrafast acoustics [1-2]. This way, the attenuation of longitudinal acoustic phonons up to 300 GHz has been measured in vitreous silica [3]. Our method is based on the emission of a short acoustic pulse from the absorption of an ultrashort optical pulse in a thin metallic layer deposited on top of the sample. The high frequency content of the pulse is detected in the substrate after propagation in silica using another laser pulse. By comparing the signal obtained for various film thicknesses, we can extract sound attenuation in silica in the sub-terahertz range. Multiple interference effects in the multilayer are analyzed and fully taken into account[3]. Our accurate acoustic attenuation results are found to follow rather well a model combining thermally activated relaxations (TAR) and anharmonicity. In the present work, we first show how to reach higher and higher frequencies. For that it is needed to improve both acoustic emission and detection. Concerning emission, it is governed by the thin metallic layer deposited on top which converts the ultrashort optical pulse in an acoustic pulse. Usually Al is preferred due to its efficiency in light-sound conversion. We will also report the results obtained using the same experimental scheme but at low temperatures. On cooling, the TAR contribution is expected to first increase, and this should be rather easily observable. As shown, an important difficulty to overcome is the thermal heating induced with the laser technique. We will present different experimental evidences which attest that the sample is at the expected low tempetature and then some hypersound attenuation measurements.

New Brillouin scattering measurements of velocity and attenuation of sound in the hypersonic regime are presented. The data are analyzed together with the literature results at sonic and ultrasonic frequencies. As usual, thermally activated relaxation of structural entities describes the attenuation at sonic and ultrasonic frequencies. As already shown in vitreous silica, we conclude that the damping by network viscosity, resulting from relaxation of thermal phonons, must be taken into account to describe the attenuation at hypersonic frequencies. In addition, the bare velocity obtained by subtracting to the experimental data the effect of the two above mechanisms is constant for temperatures below 250~K, but increases almost linearly above, up to the glass transition temperature. This might indicate the presence of a progressive local polyamorphic transition, as already suggested for vitreous silica.

High-resolution Brillouin scattering is used to achieve 3-dimensional maps of the longitudinal acoustic mode frequency shift in soda-lime silicate glasses subject to Vickers indentations. Assuming that residual stress-induced effects are simply proportional to density changes, residual densification fields are obtained. The density gradient is nearly isotropic, confirming earlier optical observations made on a similar glass. The results show that Brillouin micro-spectroscopy opens the way to a fully quantitative comparison of experimental data with predictions of mechanical models for the identification of a constitutive law.