Interaction of quasilocal harmonic modes and boson peak in glasses

The direct proportionality relation between the boson peak maximum in glasses, $\omega_b$, and the Ioffe-Regel crossover frequency for phonons, $\omega_d$, is established. For several investigated materials $\omega_b = (1.5\pm 0.1)\omega_d$. At the frequency $\omega_d$ the mean free path of the phonons $l$ becomes equal to their wavelength because of strong resonant scattering on quasilocal harmonic oscillators. Above this frequency phonons cease to exist. We prove that the established correlation between $\omega_b$ and $\omega_d$ holds in the general case and is a direct consequence of bilinear coupling of quasilocal oscillators with the strain field.Comment: RevTex, 4 pages, 1 figur

Conduction thermique des verres aux basses températures

The thermal conductivities of the vitreous modifications of GeO2, B2O3, BeF2, As2S3, As2Se3, GeS2 and CaMgSi2O 6, and of the crystalline form (diopside) of CaMgSi2O 5 have been measured between 1.5 and 30 K. The conductivity of the polycristalline specimen is proportional to T3 over a wide temperature range as expected for boundary scattering. All the glasses show a shoulder in the conductivity-temperature curve K(T) near 10 K but the shape of this feature depends markedly on the material. For B2O 3 and BeF2 the shoulder is weak, for GeO2, GeS 2 there is a distinct plateau, and for As2S3, As2Se3, and perhaps for CaMgSi2O6, a maximum and minimum appear in K(T). This behaviour has important consequences for understanding the scattering processes determining K(T), since it implies an explicitly temperature dependent process, and a new mechanism of 2 phonon interaction with the two level defects is discussed.Les conductions thermiques des verres GeO2, B2O 3, BeF2, As2S3, As2Se 3, GeS2, CaMgSi2O6, verre et polycristal (diopside), ont été mesurées entre 1,5 K et 30 K. La variation de la conductivité de l'échantillon cristallin est en T3 dans un grand domaine de température ce qui se comprend aisément par la diffusion des phonons sur les frontières des cristallites. Tous les verres présentent un épaulement dans les courbes K(T) au voisinage de 10 K : pour B2O 3 et BeF2 cet épaulement est peu prononcé, pour GeO2, GeS2 apparaît un véritable plateau tandis que pour As2S3, As2Se 3 et peut-être CaMgSi2O6 la courbe K(T) présente un maximum suivi d'un minimum. Ce type de comportement a des conséquences importantes pour la compréhension du mécanisme de diffusion des phonons puisqu'il implique un temps de relaxation dépendant de la température. Un processus nouveau de couplage phonons-défauts à 2 niveaux est proposé et discuté