NONLINEAR ACOUSTIC RESPONSE OF AMORPHOUS METALS IN THE TUNNELING MODEL

The nonlinear nature of the acoustic response of metallic glasses at v ery low temperatures, which has defied a satisfactory theoretical desc ription until now, is explained in terms of the tunneling model by the intrinsically nonlinear response of two-level systems to an external force. Recent advances in the systematic theoretical treatment of this nonlinear response including the effect of Ohmic dissipation are used in a detailed model, with special emphasis on vibrating-reed experime nts. The apparent contradiction between the assumptions of the tunneli ng model and experimental results is naturally reconciled when a valid theory is substituted for the linear response approximation. Our resu lts are in quantitative agreement with experimental data for both soun d velocity and acoustic absorption.