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Accueil du site > Colloïdes, Verres et Nanomatériaux > Equipe : Physique des Verres > Thème : Structure, vibration, relaxations dans les systèmes désordonnés

Thème : Structure, vibration, relaxations dans les systèmes désordonnés

Présentation

In the last years important progress has been made on long-standing fundamental questions such as those related to sound properties in glasses and their relation with the universal low-temperature anomalies. A comprehensive picture of the nontrivial frequency dependence of sound attenuation in oxide glasses can now be proposed. It is controlled by three processes which in increasing order of the frequency at which they dominate are : the thermal relaxation of defects, relaxation via anharmonic interactions with the thermal bath, and a remarkably rapid increase (α Ω4) towards a Ioffe-Regel crossover at THz frequencies which we have evidenced in two oxide glasses. The latter mechanism relates to the boson peak and the low temperature plateau generally observed in the thermal conductivity. The close relation between the Ioffe-Regel crossover and the boson peak implies a resonance of these rather local low-lying optic modes with acoustic modes near the Ioffe-Regel limit.

Current researches

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PERMANENTS NON PERMANENTS

Hypersound damping in vitreous silica measured by a picosecond optical technique

Involved researchers : M. Foret, S. Ayrinhac (PhD), B. Rufflé (Collaboration A. Devos, IEMN, Université Lille) 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 (...)

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The boson peak and its relation with acoustic attenuation in glasses

Involved researchers : B. Rufflé, E. Courtens and R. Vacher (Collaboration D.A. Parshin Université Saint Petersbourg Russie) Experimental results on the density of states and on the acoustic modes of glasses in the THz region are compared to the predictions of two categories of models. A recent one, solely based on an elastic instability, does not account for most observations. Good agreement without adjustable parameters is obtained with models including the existence of non-acoustic (...)

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Glass-specific behavior in the damping of acousticlike vibrations

Involved researchers : B. Rufflé, G. Guimbretière (PhD), E. Courtens, and R. Vacher (Collaboration G. Monaco, ESRF, Grenoble) High frequency sound is observed in lithium diborate glass, Li2O-2B2O3, using Brillouin scattering of light and x-rays. The sound attenuation exhibits a non-trivial dependence on the wavevector, with a remarkably rapid increase towards a Ioffe-Regel crossover as the frequency approaches the boson peak from below. An analysis of literature results reveals that the (...)

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Anharmonic vs. relaxational sound damping in silica glasses

Involved researchers : M. Foret, R. Vacher, E. Courtens 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 (...)

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Hyper-Raman scattering analysis of the vibrations in borate glasses

Involved researchers : B. Hehlen, G. Simon, E. Courtens, R. Vacher Hyper-Raman scattering has been measured on vitreous boron oxide, v-B2O3. This spectroscopy, complemented with Raman scattering and infrared absorption, reveals the full set of vibrations that can be observed with light. A mode analysis is performed based on the local D3h symmetry of BO3 triangles and B3O3 boroxol rings. The results show that in v-B2O3 the main spectral components can be successfully assigned using this (...)

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Small ring density and inter-tetrahedra bond angle of permanently densified silicas : Femtosecond and Raman spectroscopy study

Involved researchers : B. Hehlen, M. Foret (Collaboration with P. Langot, J. Burgin, and F. Vallée, CPMOH-Bordeaux) Normal and permanently densified silica glasses (SiO2) have been investigated using a time resolved pump-probe technique and spontaneous Raman spectroscopy. Comparison of the amplitudes of the time- and frequency-domain responses reveals a much larger increase in concentration of three-membered rings as compared to four-membered ones upon densification. The variation of the (...)

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Glass and environment

In the forthcoming ecological era, new European environmental regulations (horizon 2020) will impose the design of new “green” materials with lower energetic production costs. In this context, the Glass Physics group at the LCVN (both experimental and simulation) has successfully applied in 2008 to the ANR-call “energetic efficiency et reduction of greenhouse gases in industrial systems“, within a partnership including three university laboratories and one major glass-maker, Saint-Gobain. The (...)

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Structure and Dynamics in Nanoconfined Self-Assembled Polymers

Involved researchers : A.C. Genix (collaboration : J. Colmenero, A. Arbe, San Sebastian –SPAIN) We are exploiting the selectivity of neutron scattering combined with isotopic substitution to study the structure and dynamics of poly(n-alkyl methacrylates). In these polymers, the alkyl side groups of different repeat-units seem to self-assemble forming nanodomains (called polyethylene (PE)-like). Moreover, the higher order PnMAs show two distinct glass-transitions which presumably correspond (...)

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Investigation of the polymer dynamics in silica/polymer nanocomposites

Involved researchers : A.C. Genix, J. Oberdisse, C. Dupas (collaboration : F. Ganachaud, Ecole de Chimie – Montpellier / M. Tyagi, NIST – USA / J. Colmenero, A. Arbe, San Sebastian – SPAIN) We are interested to study the effects of silica nanoparticles on both the structure and dynamics of the polymer matrix. To prepare the samples, we exploit an original way of synthesis developed in Montpellier (Ouzo effect). Using the backscattering spectrometer at NIST, we have followed the proton (...)

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Ferroelectric relaxors

Collaboration with J. Hlinka, Academy of Sciences of the Czech Republic Praha) (P. Papet, J. Rouquette, and J. Haines, Institut Charles Gehrardt, Montpellier) One major interest of Hyper-Raman is its high sensitivity to all polar vibrations including soft polar modes, and also to excitations that are inactive in both infrared absorption and Raman scattering. These specificities provide hyper-Raman with a very powerful tool for the investigation of the local and average structure of (...)

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Innovative experimental setups

[BLSwww.lcvn.univ-montp2.fr/inst...] for fast data acquisition using imaging on a CCD. Hyper-Raman spectroscopy (HRS) Hyper-Raman scattering (HRS) is a non-linear process in which two incident photons scatter one photon after interaction with an excitation in the material. The main interest of this spectroscopy is that its selection rules are different from those of Raman scattering and infrared absorption. Although the theory was established 45 years ago, this spectroscopy develops very (...)

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