Soft Report - Dipartimento di Fisica - Sapienza

Scientific Report – Non Equilibrium Dynamics and ComplexityBrillouin ultraviolet light scattering on vitreous silicaSound absorption properties of amorphous solidshave been widely investigated in the last decades;these systems are characterized by a much largersound attenuation coefficient when compared to thecorresponding crystals, and the mechanismsinvolved in sound absorption are still poorlyunderstood. Vitreous silica is a strong glass, andmany techniques have been used to investigate itsdynamics.Fig. 2 - DHO-deconvoluted signal obtained by fittingthe Stokes peak. Dashed: laser, T= 230 K, Q =0.078 nm -1 , peak position: 287.5 µeV, FWHM = 3.0µeV; continuous: synchrotron, room temperature, Q= 0.11 nm -1 , peak position: 438.5 µeV, FWHM = 5.3µeV.An example of experimental spectrum is reported inFig. 1, while examples of deconvoluted Brillouinspectra are shown in Fig. 1, at exchanged momentaQ = 0.078 (laser) and 0.11 (synchrotron) nm -1 .Fig. 1 - Experimental spectrum (circles) withsynchrotron excitation.Ultrasonic attenuation and Brillouin light scatteringshow that, in the respective ranges (kHz-GHz), thesound attenuation is temperature dependent and,thus, due to dynamical processes (typically,anharmonicity). More recently, the use of inelastic X-ray scattering (IXS) at much higher frequencies(THz) showed a T-independent attenuation inducedby the presence itself of structural disorder.Theoretical models and numerical simulations havebeen proposed to describe the transition from thedynamical regime to the static one that dominates inthe region investigated by IXS.We have recently performed [1] measurements inthe intermediate region; the measurements werecarried out at the new inelastic ultraviolet beam line(IUVS) of the Elettra synchrotron radiation facility inTrieste. The IUVS beam line operates usingsynchrotron radiation, with wavelength tunable inthe previously unexplored range 260-110 nm andwith a very high photon flux. A relative energyresolution of 1.1 * 10 -6 was achieved. Alternatively,the instrument can be used with an ultraviolet lasersource, i.e. a frequency-doubled 488 nm single modeAr laser. Backscattering geometry was used, with ascattering angle of about 176 degrees.The Brillouin spectrum directly provides the dynamicstructure factor, S(Q,E), whose width of is related tothe attenuation of the acoustic excitations. Thesound attenuation, C, measured with the laser (Q =0.078 nm -1 ) and with the synchrotron (Q = 0.11 nm -1 ) agree, within a relative error of 10%, with a Q 2law extrapolated from the BLS data; therefore, thedynamic regime persists at least up to Q= 0.11 nm -1 ,indicating anharmonicity as a likely mechanism. Thelatter, should saturate around frequencies of theorder of 100 GHz. The sound absorption coefficientfor exchanged momenta Q > 0.11 nm_1 is expectedto depart from the Q2 dependence. Experiments in awider range of exchanged Q are in progress.References[1] G. Baldi et al., J. Non-Cryst. Sol. 351, 1919(2005).Authors:G. Baldi(a), S. Caponi (a), L. Comez (c), S. Di Fonzo(b), D. Fioretto (c), A. Fontana (a), A. Gessini (b), C.Masciovecchio (b), M. Montagna (a), G. Ruocco (d),S.C. Santucci (b), G. Viliani (a) - (a) Dipartimentodi Fisica and INFM-CRS Soft, Universita` di Trento,Trento, Italy; (b) Sincrotrone Trieste, Basovizza,Trieste, Italy; (c) Dipartimento di Fisica and INFM-CRS Soft, Universita` di Perugia, Perugia, Italy; (d)Dipartimento di Fisica and INFM-CRS Soft,Universita` di Roma ‘La Sapienza’, Roma, Italy.SOFT Scientific Report 2004-0654