Soft Report - Dipartimento di Fisica - Sapienza

Scientific Report – Non Equilibrium Dynamics and ComplexityBrillouin visible and ultraviolet light scattering measurementsin v-SiO 2 and silica porous systemsIt is known that a planewave excitation can propagatein a disordered structure only when the wavelength ismuch longer than the scale spanned by microscopicinhomogeneities; as the wavelength shortens, the waveis increasingly distorted and scattered. The question asto the causes of attenuation far from the longwavelength limit, is unlikely to have a single answer,and different mechanisms have been suggested.Depending on the physical origin of the acoustic energydissipation, the attenuation can be stronglytemperature dependent (dynamic) or temperatureindependent (static). In order to clarify the interplaybetween different mechanisms, is necessary toinvestigate as thoroughly as possible, besides thetemperature evolution, also the frequency evolution ofthe acoustic attenuation.Brillouin Light Scattering (BLS) and Inelastic X-RayScattering (IXS) experiments do not cover the wholefrequency and q range from GHz to THz, andinvestigations within the frequency gap which separatesthese techniques could be useful to discriminatebetween the different hypotheses.With this purpose, thanks to a newly availablespectrometer HIRESUV, Ultraviolet Brillouin lightscattering(BUVS) experiments are possible in anunexplored frequency region. We have investigated twoprototype systems v-SiO 2 and silica porous systems.For v-SiO 2, considered as the prototype strong glass,the comparison of the new data with those obtained byIXS and BLS indicates [1]:1) the existence of a crossover among differentattenuation mechanisms: from a dynamic onecharacteristic of low frequency region to a static one,which dominates in the high frequency regime.2) necessarily somewhere in the intermediate range,the acoustic attenuation have to grow faster thanquadratically with q (see fig.1).As regard porous systems, investigating the attenuationof phonons with different wavelength in homogeneoussystems, is equivalent to investigate the attenuation ata given wavelength varying the size of the microscopicinhomogeneities. We have studied samples withdensities between 500 and 2200 kg/m 3 , as aΓ(GHz)1000 BUVS T=300KIXS T=1050KIXS T=300K100 POT T=300KBLS T=300K1010.10.01 0.1 1Q(nm -1 )Fig. 1 Log-log plot of Brillouin widths as a function ofthe exchanged wave vector q, obtained at roomtemperature by BLS, BUVS, IXS, POT. The dashedlines, indicating the q 2 law, are guides for the eye.consequence of a controlled sintering procedure leadingto modifications of their “texture” and of the networkconnectivity. A dynamic to static transition in theattenuation has been found by Brillouin light scatteringmeasurements (see Fig.2) [2]. The static or dynamicalorigin of the attenuation is ascribed to the interplaybetween the mean pore sizes of the samples and theprobe wavelength. This hypothesis is confirmed by thebehaviour of attenuation investigated at twowavelength (see insets of fig.2). A crossover length a*related to the pore size has been determined. For themean pore size smaller than a* the largest contributionto the absorption comes from the attenuation due todynamic mechanisms, such as relaxation processes andanharmonic coupling. For pore size larger than a*, therapidly growing sound attenuation can be attributed tothe disorder due to inhomogeneities produced by pores.HWHM (GHz)0.10.01Densified xerogelΓ (MHz)Γ (MHz)3002001000500400300200λ= 514.5 nmα-quarz0 5 10 15a * (UV)a * (BLS)λ= 266.0 nm0 5 10 15L1E-3p(nm)0 200 400 600 800 1000 1200Temperature (K)aerogel1xerogel1v-SiO 2Fig. 2.Temperature behaviours of the half width halfmaximum of the Brillouin peak in systems withgrooving pore size. In the inset HWHMs of visible andultraviolet Brillouin peaks versus the mean pore size.References[1] P.Benassi et al. Phys. Rev. B 71, 172201 (2005).[2] S. Caponi et al. Phys. Rev. B 70, 214204 (2004).[3] S. Caponi Phil. Mag., 84, Nos.13–16,1423 (2004).Authors:S. Caponi (a), P. Benassi (b) R. Eramo (c), A. Fontana(a), A. Giugni (b), M. Nardone (b), M. Sampoli (d), andG. Viliani (a)-(a) Dipartimento di Fisica and INFM-CRSSoft, Universita` di Trento, Italy; (b) Dipartimento diFisica, and INFM-CRS Soft, Università dell’Aquila, Italy;(c) Dipartimento di Fisica and LENS and INFM-CRS Soft,Università di Firenze, Italy; (d) Dipartimento diEnergetica and INFM-CRS Soft, Università di Firenze,ItalySOFT Scientific Report 2004-0664