CONTRIBUTIONS ORALES


7. Thermodynamic properties of liquid metals measured by picoseconds acoustics
S.Ayrinhac*, M.Gauthier, M.Morand , G.Le Marchand, F.Bergame and F.Decremps
Institut de minéralogie, de physique des matériaux et de cosmochimie (IMPMC), Université Pierre et Marie Curie, Paris, France

Measuring and understanding physical properties of liquid metals are a challenging issue. Most of the liquid metals are highly reactive and melt at high temperature, so they impose technical requirements. In addition, they are disordered systems so they are considered to be complex from a theoretical point of view.
Recent technical achievements in picosecond acoustics associated with a diamond anvil cell permit to measure elastic properties at high pressures [1,2,3]. The acoustic waves are generated by picoseconds optical pulses at one side of the sample from a point source; they are detected on the opposite side and visualized by surface phonon imaging. The analysis of the images gives the sample thickness and the sound velocity. From sound velocities we extract the equation of state ?(P,T) by a numerical procedure.
Those achievements have been applied to metals which melt at low temperature, such as liquid mercury[2] and liquid gallium. In the case of l-Ga, we will discuss the possible existence of a liquid-liquid phase transition around 300 K and 2 GPa[4,5]. This technique can be also applied to alkali metals, considered as simple metals at ambient pressure but which get intriguing properties at high pressures.

References
[1] F. Decremps et al, Geo. Res. Lett. 41 1459 (2014)
[2] S. Ayrinhac et al, J. Chem. Phys. 140, 244201 (2014)
[3] F. Decremps et al, Ultrasonics 56 129 (2015)
[4] R. Poloni et al, Phys. Rev. B 71 184111 (2005)
[5] R. Li et al, Appl. Phys. Lett. 105 041906 (2014)
*simon.ayrinhac@impmc.upmc.fr
Keywords: picosecond acoustics, thermodynamic properties, liquid metals, liquid-liquid phase transitions





















4. Experimental knowledge of the equation of state (density-pressure-temperature relation) in liquid metals is important to determining their overall thermodynamic properties, or to test the validity of theoretical assumptions developed through the study of these systems. In the case of liquid mercury, often seen as a hard sphere fluid, its density is well known experimentally as a function of temperature and ambient pressure, but there is no data on the variation in pressure beyond 1.2 GPa. In this presentation, I will discuss recent results about the sound velocity in liquid mercury up to 7 GPa and 240°C by the picosecond acoustics technique in diamond anvil cells. A thermodynamic analysis of these data permit to determine the equation of state of liquid mercury at high density.


1. Hypersound damping in vitreous silica by a picosecond optical technique
A. Devos[1], M. Foret[2], S. Ayrinhac[2], P. Emery[2], B. Rufflé[2],
[1]Institut d’Electronique, de Micro´electronique et de Nanotechnologie, UMR CNRS 8250, Villeneuve d’Ascq,
[2]Laboratoire des Colloïdes Verres et Nanomatériaux
The attenuation of longitudinal acoustic phonons up to frequencies nearing 250 GHz is measured in vitreous silica with a picosecond optical technique. Taking advantage of interferences on the probe beam, difficulties encountered in early pioneering experiments are alleviated. Sound damping at 250 GHz and room temperature is consistent with relaxation dominated by anharmonic interactions with the thermal bath, extending optical Brillouin scattering data. Our result is at variance with claims of a recent deep-UV experiment which reported a rapid damping increase beyond 100 GHz. A comprehensive picture of the frequency dependence of sound attenuation in v-SiO2 can be proposed.
A. DEVOS et al., PRB 77, 100201(R) (2008)
B. RUFFLE et al., PRL 100, 015501 (2008)



POSTERS


7. Etude Brillouin de la silice sous pression
Ayrinhac Simon, Foret Marie, Ruffle Benoit, Vacher Rene

La silice vitreuse possede des anomalies elastiques bien connues mais encore mal comprises. Nous disposons au laboratoire d’un spectrometre Brillouin haute resolution capable de mesurer la frequence et la largeur avec une excellente precision. Les mesures en diffusion Brillouin de la vitesse du son mettent en evidence un minimum a basse temperature au voisinage de 50 K et un minimum a haute pression a environ 2 GPa. L’attenuation du son nous permet d’acceder aux processus dynamiques car les ondes sonores se couplent aux d´efauts structuraux du reseau d´esordonne. On peut observer la signature de ces d´efauts dans l’attenuation et expliquer ainsi le minimum des vitesses en temp´erature, qui revele une augmentation de la vitesse non relaxee [1]. Nous avons adapt´e notre montage a une CED [2] pour mesurer l’attenuation en fonction de la pression. Nous observons en-dessous de 6 GPa un maximum de l’attenuation tres important assoce a un minimum de la vitesse.
[1] R. Vacher et al., PRB 72 (2005) 214205
[2] R. Vacher et al., PRB 74 (2006) 012203


















6. Densification or plasticity of silica glass
S. Ayrinhac, B. Rufflé, M. Foret, R. Vacher, E. Courtens, M.Arai

Permanently densified silica glass, d-SiO2, can be obtained by submitting v-SiO2 to high pressure at elevated temperature. We studied by Brillouin light scattering a series of samples of densities values of 2.31, 2.46 and 2.60 g/cm3, besides v-SiO2 with 2.20 g/cm3. Densification has an enormous effect on both sound velocity and attenuation that is discussed. Brillouin scattering being a rather local probe, it can be used to test the sample homogeneity. Raman measurements on the same samples are also presented.






















3. Mesures de largeur en diffusion Brillouin de la lumière dans la silice vitreuse
S.ayrinhac, M.foret, B.rufflé, R.vacher Et E.courtens
Laboratoire des Colloïdes, Verres et Nanomatériaux, Université Montpellier 2, 34095 Montpellier

Il existe assez peu de mesures d'atténuation dans les verres aux fréquences hypersoniques. En effet les faibles largeurs sont difficilement mesurables avec les instruments Brillouin conventionnels. Nous avons construit à Montpellier un spectromètre qui réponds à ce besoin. Cet instrument fournit une résolution d'au moins 3 MHz à la fois sur la position et la largeur du pic Brillouin. Nous présentons les premières mesures obtenues dans la silice vitreuse sur trois décades de température. Les résultats sont discutés sur la base de la "viscosité du réseau" c'est-à-dire le couplage entre les hypersons et les modes qui forment le bain thermique. Deuxièmement, en vue de mesurer la largeur sous hautes pressions, nous avons étudié l'élargissement additionnel produit par les effets de taille finie. Nous montrons que dans la silice vitreuse, la largeur commence à augmenter de façon visible quand l'épaisseur de l'échantillon chute en-dessous de 100 microns.


















1. Brillouin light-scattering linewidth measurements from silica glass
Simon Ayrinhac, Marie Foret, Benoît Rufflé, René Vacher and Eric Courtens
Laboratoire des Colloïdes, Verres et Nano-matéraiux, UMR CNRS n°5587, Université Montpellier II, F-34095 Montpellier Cedex 5, France

There exist relatively few damping data from glasses at hypersonic frequencies because (small) Brillouin linewidths are usually difficult to measure in usual Brillouin instruments. We constructed in Montpellier a Brillouin spectrometer appropriate for these demanding experiments. This instrument provides a resolution better than 3 MHz resolution on both phonon position and linewidth. Here, we present at first new measurements of acoustic damping and velocity in vitreous silica from 300 K to 1200 K. Results are discussed on the basis of "network viscosity" i.e. the anharmonic coupling of hypersound to the modes that forms the thermal bath. Secondly, in view of linewidth measurements under high pressure, we studied the additional broadening produced by finite sample size effects. We show that in silica glass, the width starts to increase measurably when sample thickness falls below 0.2 mm.












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