Phonon localization in glasses

Abstract

A universal but unexplained characteristic of glasses is the temperature-independent region or plateau in the thermal conductivity for temperatures 1--30 K. Examination of low-temperature thermal conductivity data for a wide variety of glasses indicates that the mean free path of the average thermal phonon becomes less than the phonon wavelength at temperatures in the vicinity of the plateau. Comparison with similar data in aggregates suggests that Ioffe-Regel localization of the vibrational modes occurs for phonons whose wavelength and mean free path are on the order of a structural correlation length of the material. Typical correlation lengths in glasses determined by this argument are 20--50 A\r{}. The universal plateau in bulk glasses is therefore associated with phonon localization due to strong scattering from inhomogeneities on this length scale.