Evidence for the importance of interactions between active defects in glasses

We report new evidence obtained between 500 mu K and 150 mK for the importance of interactions between tunneling two level systems in structural glasses. After the application of a large de electric field we observed logarithmic relaxations in epsilon'(omega) and epsilon ''(omega) for up to 10(4) s. The frequency dependence of the relaxation in epsilon'(omega) observed at higher temperatures vanishes for temperatures well below the minimum of epsilon'(omega,T) and we find epsilon ''(omega) proportional to T below the high temperature plateau, both qualitatively in agreement with models involving interactions.