Nonlinear acoustic and microwave absorption in disordered semiconductors

Nonlinear hopping absorption of ultrasound and electromagnetic waves in amo rphous and doped semiconductors is considered. It is shown that even at low amplitudes of the electric (or acoustic) field the nonlinear corrections t o the relaxational absorption appear anomalously large. The physical reason for such behavior is that the nonlinear contribution is dominated by a sma ll group of close impurity pairs having one electron per pair. Since the gr oup is small, it is strongly influenced by the field. An external magnetic field strongly influences the absorption by changing the overlap between th e pair components' wave functions. It is important that the influence is su bstantially different for the linear and nonlinear contributions. This prop erty provides an additional tool to extract nonlinear effects.