Résumé
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 T0 . We find evidence for an increase of T0 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).