Origin of boson peak in glasses: Role of atomic soft-mode excitations

Abstract

In the present article, glasses are considered, for which THz frequency vibrational anomalies include both an experimentally observed boson peak and, above it, a ``high-frequency'' sound with well-defined acousticlike excitations. The phenomena are strongly related to an Ioffe-Regel crossover from a weak inelastic scattering of interacting acoustic and soft-mode (nonacoustic) vibrational excitations to a strong scattering and to an associated hybridization of both types of excitations. A theoretical soft-mode-dynamics model of the anomalies, actually containing a single material parameter, is presented, which is complete in the sense that it takes into account both the Ioffe-Regel crossover and a recently found ``vibrational instability'' effect due to elastic interactions between soft-mode vibrations. The basic vibrational density of states and the reduced density of states are calculated. The results appear to show that the qualitative features and scale estimations of the experimentally studied inelastic scattering intensities in the glasses under discussion can be described by the present complete soft-mode-dynamics model. The latter is compared with other recent theoretical models of the boson peak in glasses.