Sound attenuation in a unexplored frequency region: Brillouin ultraviolet light scattering measurements inv−SiO2

Abstract

We report Ultraviolet Brillouin light scattering experimental data on v-SiO2 in an unexplored frequency region, performed with a newly available spectrometer up to exchanged wavevector q values of 0.075 nm −1 , as a function of temperature. The measured attenuation scales on visible data following a q 2 behavior and is temperature-dependent. Such temperature dependence is found in a good agreement with that measured at lower q, suggesting that its origin is mainly due to a dynamic attenuation mechanism. The comparison between the present data with those obtained by Inelastic X-ray Scattering suggests the existence of a cross-over to a different attenuation regimes. Our present understanding of the sound attenuation mechanisms in vitreous systems is poor compared to that of crystalline materials although this topic has attracted the interest of several researchers from both experimental and theoretical points of view [1]. In particular, the nature of vibrational dynamics of disordered systems, as derived from the study of the "low-frequency" excitations (in the hypersonic range), has been a highly debated subject in the last years [2,3,4,5,6]. Vitreous silica (v-SiO 2 , or amorphous quartz) has been extensively studied as prototype of a strong glass. Nevertheless, the experimental results reported in literature had conflicting and controversial interpretations [7,8,9,10,11]. To achieve an overall characterization of this particular system is crucial to understand the vibrational dynamics of the vitreous state. It is well known that a plane wave excitation can propagate freely in a disordered structure only when the wavelength is much greater than the scale spanned by microscopic inhomogeneities. As the wavelength shortens the wave is attenuated, distorted, and scattered with increasing magnitude. As a matter of fact, the waves can even become over-damped when the wavelength approaches the interparticle spacing and the excitations become more directly affected by the microscopic disorder characteristic of the glass structure. The question about the nature of excitations far from the long wavelength limit, is unlikely to have a single answer [12,13,14]. Indeed different mechanisms have been suggested in order to explain the attenuation of an acoustic plane wave excitation namely: attenuation induced by topological disorder, thermally activated processes, anharmonic effects, two-level systems (TLS) processes [15]. In general, anharmonic effects are particularly relevant at relatively high temperatures while the importance of the two-level systems is confined at very low temperatures (T < 5 ÷ 10 K). In the intermediate range 10 K < T ≪ T g the TLS effects are not important and disordered induced mechanisms, thermally activated processes and anharmonic effects play the most important role in acoustic attenuation. Anyway, sound propagates through glasses and longitudinal and transverse phonons are found to be still well defined at relatively large wave-vectors (up to 5 nm −1 ) with a linear relationship between frequen...

Acoustic behaviour of normal and densified vitreous GeO2

“…The attenuation and the velocity of sound waves in glasses are an important subjects mainly because distinct and competing mechanisms are responsible for the behaviour observed over different temperature and frequency ranges [1][2][3][4]. In particular, the origin and the nature of low-energy excitations, which are responsible of anomalies in the acoustic properties of [5,6], is a central question in the physics of disordered solids.…”

Low-temperature phonon damping in vitreous silica explored by UV Brillouin spectroscopy

“…This supports the description of the Akhiezer mechanism in terms of equation (3) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 [12] and US, circles [9]. [15], HiResUv, full circles (present work) and full squares [16], BLS, crosses [26] and dots [20]. …”

“…[12]; stars: US, 748 MHz [29]; crosses [26] and points [20]: BLS, 35 GHz. The continuous lines are the best fitted functions to the five US data sets with the model of equation (2) GHz, circles (present work and [16]). The continuous line is obtained using equations (2) and (3) at 35 GHz, as discussed in the text.…”

“…In v-SiO 2 at ultrasonic frequencies (MHz) the attenuation, as a function of temperature, shows a well-defined peak, while at hypersonic frequencies (GHz) this temperature dependence, although still characterized by a maximum located at around 150 K, clearly shows a plateau at higher temperatures. The wave vector dependence of the attenuation at room temperature in the region of frequency explored by BLS and Brillouin ultraviolet light scattering (BUVS) has been recently observed [16] to scale with the square of the frequency and thus with the square of the wavevector, being the dispersion curve linear in this region. A 2 q dependence of the attenuation was observed also at much higher frequencies ( 1 ν -10 THz) by means of inelastic X-rays scattering (IXS).…”

“…In particular the topic of sound attenuation has been the subject of a great amount of experimental work accomplished with different techniques (see ref. [11][12][13][14][15][16] to mention only the most recent ones). In v-SiO 2 at ultrasonic frequencies (MHz) the attenuation, as a function of temperature, shows a well-defined peak, while at hypersonic frequencies (GHz) this temperature dependence, although still characterized by a maximum located at around 150 K, clearly shows a plateau at higher temperatures.…”