COHERENT TUNNELING OF LITHIUM DEFECT PAIRS IN KCL CRYSTALS

The tunneling of two lithium ion impurities on next-nearest neighbor s ites in potassium chloride are investigated both experimentally and th eoretically. The strong dipolar interaction leads to coherent tunnelin g motion of the two defect ions between degenerate off-center position s. Comparing data of rotary echo experiments for impurity pairs Li-7-L i-7, Li-6-Li-6, and Li-7-Li-6 with theory permits a thorough investiga tion of the isotope effect and of the effect of the interaction on the tunnel states. Our findings confirm the tunneling model with (111) of f-center states to be valid even for strongly interacting impurities. Using degenerate perturbation theory in terms of two-par-tide states, we obtain essentially exact expressions for the tunneling spectrum and the dynamical susceptibility which agree well with the measured data.