Coherent echoes in glasses and crystals with point defects

Dielectric echo experiments have proven to be a very suitable tool for the investigation of the dynamics of tunneling states in solids. In particular, rotary echoes have been used to determine the distribution of the tunnel splitting of amorphous solids and crystalline defect systems like (KBr)(1-x)(KCN)(x), and to investigate the tunneling motion of pairs of strongly coupled Li+ ions in KCl. Recently progress has been made in understanding the decay of spontaneous echoes in glasses which is caused by spectral diffusion processes. The expected non-exponential decay of the echo amplitude has been observed for several glasses. While the temperature dependence of the phase memory time is in good agreement with theoretical predictions, the cross-over from the non-exponential to the exponential decay regime occurs at much shorter times than expected.