Influence of Tunneling Systems on the Acoustic Properties of Disordered Solids

Abstract

In this chapter we review the acoustic properties of disordered solids at low temperatures, i.e. we discuss the propagation of sound in amorphous materials (dielectrics and metals) as well as in polycrystalline metals in terms of sound attenuation (or internal friction) and sound velocity. Along with the study of thermal (see Chap. 2), dielectric (see Chap. 5), and some optical (see Chap. 6) properties of amorphous or disordered solids at T ≤ 10 K, the results of the investigation of the low-temperature acoustic properties indicate the existence of tunneling entities in the disordered atomic structure with a broad distribution of relaxation times and an almost energy-independent density of states (at least for T < 5 K). There is a general consensus that these low-energy excitations associated with quantum-tunneling processes are responsible for the so-called “glasslike” behavior of the low-temperature properties of disordered solids.