Dielectric constant of glasses: Evidence for dipole-dipole interactions

The 1 kHz real part chi(') of the dielectric constant of a structural glass (a-SiO2+xC1+yHz) was measured at low temperature T. Reducing the sample thickness h below 100 nm weakens the slope \partial derivativechi(')/partial derivativeT\ for Tless than or similar to0.1 K, for all measuring fields E. This contrasts with the predictions of the two-level system (TLS) model but is in agreement with the recently proposed delocalization of excitations derived from a field-induced TLS-TLS interaction mechanism. For small h this interaction is screened, which explains the h effects on chi('). Hence, interactions must play a key role in standard thick samples as soon as Tless than or similar to0.1 K.