Preface

Summary

Because matter is composed of light and heavy particles – namely, electrons and nucleons – the properties of solid materials naturally divide into two main categories: electronic and vibrational. For the most part, the rapid motion of electrons governs the binding and elasticity of a solid, its conductivity, and its optical and magnetic properties. In contrast, the more sluggish motion of the atomic nuclei determines the vibrational properties of a solid. The descriptions of electronic and nuclear motion in solids are generally developed in terms of wave motion. Such waves in solids are most easily understood for well-ordered systems such as crystals, where the atoms are arranged in regular arrays.

This book is about vibrational waves in crystals – more specifically, those classified as acoustic waves. These are the waves that depend on the elasticity of the medium and whose frequencies extend from zero to a few terahertz (1 THz = 10¹² Hz). Experimental probes of the motion of acoustic waves in solids traditionally range from ultrasonics to thermal conductivity. The major focus of this book is the propagation of phonons – the tiny packets of vibrational energy associated with heat and typically observed in the frequency range from 10¹⁰ to 10¹² Hz. The propagation of ultrasonic waves in the megahertz range will also be examined here using imaging techniques initially developed for high-frequency phonons.

Phonons govern the thermal properties of nonmetallic solids. At normal temperatures their motion is highly diffusive.