Quasi-elastic scattering in vitreous silica: A Raman and neutron scattering study A. Fontana a, * , F. Rossi a , S. Caponi a , E. Fabiani b , U. Buchenau c , A. Wischnewski c a Istituto Nazionale per la Fisica della Materia, Dipartimento di Fisica, Universita ` di Trento, 38050 Povo, Trento, Italy b Institut Laue Langevin, B.P. 156, Grenoble, France c Institut fu ¨ r Festko ¨ rperforschung, FZ Ju ¨ lich, Postfach 1913, D-52425 Ju ¨ lich, Germany Available online 23 May 2005 Abstract Measurements of Raman scattering from vitreous silica (spectrosil) in the temperature range 12–1200 K are analyzed and com- pared with inelastic neutron scattering measurements done on the same sample. The results show a temperature dependence of the quasi-elastic scattering. Comparing this study with acoustical attenuation measured at 30 GHz by Brillouin light scattering, we found that the mechanism giving rise to quasi-elastic scattering in vitreous silica seems to be the same as the one causing the acoustic attenuation. Ó 2005 Elsevier B.V. All rights reserved. 1. Introduction The search for a satisfactory description of the low energy (<100 cm 1 ) vibrational dynamics of amorphous solids has been subject of both theoretical and experi- mental studies [1,2] in the last years. As a consequence, some of the fundamental questions in this field have found a satisfactory and accepted answer [1,2]. How- ever, in spite of these efforts, some issues about the dynamics at the atomic scale and the origin of the Ôuni- versalÕ features concerning the density of vibrational states in glasses have not yet found a generally accepted explanation [1]. Among them are two effects observed both in inelastic neutron scattering [3–6] and first order Raman scattering [7,8] at frequencies <100 cm 1 : (i) the existence of a structure-less band in the 20–100 cm 1 (2– 10 meV) range called the boson peak (BP), a deviation from the usual crystal-like Debye of the density of states at low-energies (ii), the observation of a quasi-elastic scattering, QES, at lower frequencies (<20 cm 1 ), usu- ally referred to as extra-scattering [8,9], which has a tem- perature dependence stronger than the Bose–Einstein population factor [9]. Both features are common to all glassy materials. The origin of the BP seems to be a low-energy excess of vibrational states in glasses with respect to their crystalline counterparts, while the QES has been tentatively ascribed to relaxation mechanisms [8,10]. In vitreous materials it is possible to distinguish two main relaxation processes: slow (a) dynamics below the GHz regime and a fast (b) dynamics in the GHz re- gime. The a process is a cooperative effect and is temper- ature dependent especially near the glass transition (T g ). It is not measurable in the present experiment, while the b processes emerge at temperatures below T g . In general, the presence of an active relaxation process affects the width of the Brillouin peak as well as the shape of the susceptibility function. The susceptibility at high fre- quencies can be obtained by low frequency Raman scattering. Even if the microscopic mechanism, which is the source of the QES, is not well understood, it is now 0022-3093/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.04.040 * Corresponding author. Tel.: +39 461 881 559; fax: +39 461 881 696. E-mail address: afontana@science.unitn.it (A. Fontana). www.elsevier.com/locate/jnoncrysol Journal of Non-Crystalline Solids 351 (2005) 1928–1931