Acoustic and Thermal Properties of Glasses: Defect Modes and Vibrational Anharmonicity

Low energy vibrational excitations represent an universal feature of amorphous solids and cause anomalies in the acoustic and thermal properties glasses. The excitations associated to the local mobility of structural defects, the defect modes, originate a vibrational and relaxation dynamics in glasses which differ considerably from those of their crystalline counterparts. Unfortunately, the microscopic nature of structural defects is unknown mainly because the anomalies observed in the physical properties of amorphous solids are quite independent of their microscopic structure and stoichiometry. Some experimental results concerning the acoustic and thermal properties of glasses are provided with the aim to illustrate the possible physical nature of the defect modes. Moreover it will be shown that, besides tunneling and classical activation of structural defects, vibrational nharmonicity is playing a significant role in governing the acoustic behaviors.