COLLECTIVE LOW-ENERGY EXCITATIONS OF 2-LEVEL TUNNELING DEFECTS IN MIXED-CRYSTALS

The dynamics of strongly interacting two-level tunnelling defects in mixed crystals as KCl:Li is investigated in the framework of Mori's projection method. Applying a mode-coupling scheme on the memory functions and performing the average over the random defect configuration in a mean-field type approximation, the ensueing self-consistent equations can be solved analytically. At low concentration this solution coincides with the exact dynamics of the pair model, whereas for higher concentration the motional spectrum shows several novel features. In particular we find a strong relaxation peak and a characteristic two-peak structure for the density of states; there is an additional feature at energies far below the bare tunnelling amplitude epsilon which accounts well for several experiments indicating the existence of such low-energy states.