Observation of Direct Interaction between Low-Energy Excitations in Glasses

Abstract

The striking anomalous thermal and acoustic properties of amorphous materials /1,2,3/ at low temperatures have given rise to the hypothesis that two-level systems exist in these materials with an almost constant density of states no /4/. The resonant scattering of phonons by these two-level systems depends on the acoustic intensity: The attenuation becomes smaller at higher powers when both levels are equally populated (acoustic self-induced transparency or saturated attenuation). Consequently the attenuation increases with decreasing power and becomes independent below a critical intensity Ic /5/. For example Ic = 10−7 W/cm2 in Borosilicate glass BK 7 at 740 MHz and 0.5 K. The coupling between these two-level systems and phonons is described by a deformation potential /6/, which is of the order of 1 eV. Such a large value obviously makes possible the existence of a strong direct elastic coupling between these two-level systems and led to the prediction /7/ of a broadened excited state.