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 dc electric field below 1 K. When compared with the analogous dielectric response of the same material, the acoustic data show, within experimental precision, identical dependence on the perturbing field, but stronger temperature dependence. These data are difficult 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.