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

The dynamics of strongly interacting two-level tunnelling defects in m ixed crystals as KCl:Li is investigated in the framework of Mori's pro jection method. Applying a mode-coupling scheme on the memory function s 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 coinci des with the exact dynamics of the pair model, whereas for higher conc entration the motional spectrum shows several novel features. In parti cular we find a strong relaxation peak and a characteristic two-peak s tructure for the density of states; there is an additional feature at energies far below the bare tunnelling amplitude epsilon which account s well for several experiments indicating the existence of such low-en ergy states.