Low-temperature properties of a dipolar glass: A study ofKBr1−x(CN)xusing dielectric echoes

Dielectric echoes have been used to study the low-temperature dynamics of the model orientational glass ${\mathrm{KBr}}_{1\mathrm{-}\mathit{x}}$(CN${)}_{\mathit{x}}$ The echoes were observed at 0.79 GHz at temperatures between 10 and 60 mK for five single crystals with 0.00034≤x≤0.70. The following properties of the ${\mathrm{CN}}^{\mathrm{-}}$ tunneling centers were measured: relaxation times (${\mathit{T}}_1$), dephasing times (${\mathit{T}}_2$), electric-dipole moments, and densities of states. Relative dielectric constants were also measured from 10 mK to room temperature. The results suggest that the nature of the tunneling centers, as characterized by their electric and elastic dipole moments, does not change significantly over the concentration range studied, but that the distribution in relaxation rates changes dramatically. We interpret this result in terms of variations in the symmetry of the double-well potentials characterizing these excitations.