Evidence for the Importance of Interactions between Active Defects in Glasses

We report new evidence obtained between $500\mu \mathrm{K}$ and $150\mathrm{mK}$ for the importance of interactions between tunneling two level systems in structural glasses. After the application of a large dc electric field we observed logarithmic relaxations in ${\epsilon }^{\prime }\left(\omega \right)$ and ${\epsilon }^{\prime \prime }\left(\omega \right)$ for up to ${10}^4\mathrm{s}$. The frequency dependence of the relaxation in ${\epsilon }^{\prime }\left(\omega \right)$ observed at higher temperatures vanishes for temperatures well below the minimum of ${\epsilon }^{\prime }(\omega ,T)$ and we find ${\epsilon }^{\prime \prime }\left(\omega \right)\propto T$ below the high temperature plateau, both qualitatively in agreement with models involving interactions.