Phonon localization in aggregates.

The concept of non-propagating phonon modes occupying part of the phonon spectrum is implicit in several proposals for the vibrational properties of disordered materials. Localized modes have been suggested as scattering centers in an attempt to explain a long-standing puzzle in glasses-- the temperature- independent thermal conductivity in the range 1-30K which is found universally in bulk glasses. Experiments on the transport of thermal energy through aggregates of small particles at low temperature are discussed. A transition to a weakly temperature dependent thermal conductivity, or plateau, appears at the temperature at which the thermal phonon wavelength equals the particle size. This is interpreted as evidence of phonon localization on a length scale determined by the structural correlation length.