Novel dielectric relaxation in Li+ doped KCl

We report measurements of the dielectric loss of lithium defects in potassium chloride (KCl:Li), Experiments were performed at audio frequencies and in the temperature range between 8 mK and 30 K on single crystals doped with different compositions of Li-6 and Li-7. Our results clearly show that the mean relaxation rate exhibits a rather surprising isotope dependence, because the tunneling states, associated with the heavier isotope Li-7 relax much faster than the ones due to Li-6 defects. Such an unusual behavior can be understood in the framework of a novel relaxation mechanism, based on the collective motion of the Li+ defects.