No abstract

We report the observation, by means of high-resolution inelastic x-ray scattering, of an unusually large temperature dependence of the sound attenuation of a network glass at terahertz frequency, an unprecedentedly observed phenomenon. The anharmonicity can be ascribed to the interaction between the propagating acoustic wave and the bath of thermal vibrations. At low temperatures the sound attenuation follows a Rayleigh-Gans scattering law. As the temperature is increased the anharmonic process sets in, resulting in an almost quadratic frequency dependence of the damping in the entire frequency range. We show that the temperature variation of the sound damping accounts quantitatively for the temperature dependence of the density of vibrational states. DOI: 10.1103/PhysRevLett.112.125502 PACS numbers: 63.50.Lm, 62.30.+d, 62.65.+k, 66.70.Hk When the wavelength of a sound wave is sufficiently large with respect to the average size of the elastic modulus fluctuations, an amorphous solid behaves in a way similar to a continuous elastic medium. Since the elasticity of a glass is typically correlated on the nanometer scale [1][2][3], the wave propagation is influenced by the heterogeneous elasticity of the material when the frequency reaches the terahertz range [3]. A proper understanding of the propagation of elastic waves in random media is of great interest for many diverse fields. Examples are the attenuation of seismic waves in Earth's lithosphere [4,5] or the ability to tune the thermal conductivity of a thermoelectric material [6,7]. From a more fundamental perspective, the main unsolved problem concerns the nature of the vibrational modes of glasses in the few terahertz frequency range and the associated thermal transport properties [8][9][10]. The reduced density of vibrational states (RDOS) gðωÞ=ω 2 deviates from the Debye prediction and presents a peak, usually termed the boson peak (BP). This excess of states is often related to the known low temperature anomalies of the specific heat and of the thermal conductivity [8].Various models have been proposed to explain the BP anomaly, models that we can classify in three main groups, without the claim of being exhaustive. A first group includes those models, such as the soft potential model [11,12] and the jamming scenario [13], that associate the BP to quasilocalized soft modes. A second class of models relates the excess of states to the elastic heterogeneities of the glass. For instance, Ref. [14] suggests the appearance of localized modes close to the elastic heterogeneities, while the model of Refs. [15,16], recently extended to also take into account the role of anharmonic interactions [17], demonstrates that a distribution of random elastic constants can give rise to a peak in the RDOS. As a third class we mention those models where the BP is explained as a broadened first van Hove singularity, shifted to lower frequency by the disorder [18] and by the difference in mass density between the glass and the corresponding crystalline structure [19,20].The...

Polymeric fibers were produced from a polyaluminocarbosilane obtained by reacting polycarbosilane and an aluminum alkoxide modified with a p-ceto ester. The pre-ceramic fibers were converted into amorphous Si-Al-0-N ceramic fibers after a pyrolysis process under flowing ammonia at 1000°C. "Si and "AI magic-angle spinning nuclear magnetic resonance investigations were performed to characterize the conversion process of the polymeric precursor fibers into the ceramic product. However, because of the amount of matter required for the MAS-NMR experiments, the heat treatment applied for fibers was done on powders assuming identical evolution for both materials. Tensile strength was measured at various stages of the pyrolysis process and related to the corresponding structural evolution.

We report new inelastic Raman and neutron scattering spectra for glasses with different degree of fragility, v-SiO2 , v-GeO2, (AgI)0.5(Ag2O-B2O3)0.5, (AgI)x(AgPO3)1−x; the data are compared for each sample to obtain the Raman coupling function C(ω). The study indicates a general linear behaviour of the C(ω) near the Boson peak maximum, and evidence a correlation between vibrational and relaxational properties, already observed in recent publications.

No abstract