EVIDENCE FOR GROWTH OF COLLECTIVE EXCITATIONS IN GLASSES AT LOW-TEMPERATURES

We present new data on the nonequilibrium acoustic response of glasses to an applied de electric field below 1 K. When compared with the ana logous dielectric response of the same material, the acoustic data sho w, within experimental precision, identical dependence on the perturbi ng field, but stronger temperature dependence. These data are difficul t to reconcile with simple generalizations of the dipole gap model of two-level system (TLS) dielectric response, unless we assume that as T is decreased interaction-based TLS collective effects increase.