NONLINEAR ACOUSTIC AND MICROWAVE-ABSORPTION IN GLASSES

A theory of weakly nonlinear low-temperature relaxational absorption o f acoustic and electromagnetic waves in dielectric and metallic glasse s is developed. Based upon the model of two-level tunneling systems we show that the nonlinear contribution to the absorption can be anomalo usly large. This is the case at low enough frequencies, omega tau(0)(T )much less than 1, where tau(0)(T) is the minimal relaxation time for two-level systems with the interlevel splitting similar to k(B)T. In d ielectric glasses, the lowest-order nonlinear contribution is proporti onal to the wave's intensity. It is negative and exhibits anomalous fr equency and temperature dependences, Delta<Gamma/>Gamma(0) proportiona l to[omega tau(0)(T)]T--1/2(-2). In metallic glasses, the nonlinear co ntribution is also negative, and it is proportional to the square root of the wave's intensity and independent of the frequency. Numerical e stimates show that the predicted nonlinear contribution can be measure d experimentally.