Nonlinear effects in the propagation of high-frequency sound in normal conductors
The basic results of the theory of nonlinear acoustic phenomena in metals and semiconductors are reviewed. Short-wavelength sound is assumed: the ultrasonic wavelength is taken to be far shorter than the mean free path of the conduction electrons. The case in which the interaction of the electrons with the sound wave can be described by classical mechanics is studied. The basic purpose of the review is to discuss the theory for nonlinear absorption of sound, which gives the absorption coefficient as a function of the sound intensity. The nonlinearity results from an effect of the field of the sound wave on the motion of ``resonant'' electrons, i.e., particles which are moving in phase with the sound and which determine its absorption. This effect leads to a substantial distortion of the (quasi-)momentum distribution of this relatively small group of particles. For this reason, this nonlinearity is called the ``momentum'' nonlinearity. The particular parameters which are the measure of this nonlinearity in different situations are identified. The particular features of the momentum nonlinearity in the presence of a magnetic field are discussed. An extremely unusual variation of the absorption coefficient with the magnetic field which occurs in very weak fields (of the order of a fraction of an oersted) is described. Weak fields at this level do not affect the linear absorption. This unusual variation is due to a suppression of the nonlinearity by the magnetic field, which provides an additional mechanism for removal of particles from the resonant group.