Dielectric echo experiments on (KBr)1−x(KCN)x

The coherent dynamics of atomic tunneling states in (KBr${)}_{1\mathrm{-}\mathit{x}}$(KCN${)}_{\mathit{x}}$ crystals in the concentration range 0.001≥x≥0.18 have been investigated at low temperatures with two types of dielectric echo experiments at 800 MHz. In rotary echoes we find a distribution of the Rabi frequency which is very narrow in comparison with corresponding results from structural glasses. This shows that the distribution of tunnel splittings in (KBr${)}_{1\mathrm{-}\mathit{x}}$(KCN${)}_{\mathit{x}}$ differs significantly from that in amorphous materials. At least from this point of view, it is doubtful whether (KBr${)}_{1\mathrm{-}\mathit{x}}$(KCN${)}_{\mathit{x}}$ can be considered a model system for the low-temperature properties of the tunneling states in glasses. In addition we have investigated the temperature dependence of the phase memory and of the energy relaxation time of the tunneling states by studying the decay of the spontaneous and the stimulated echo in two- and three-pulse echo experiments.