2 DISTINCT CLASSES OF TUNNELING STATES IN CN--DOPED KBR

We have investigated the low-temperature behaviour of the dielectric constant of the mixed crystal (KBr)1-x(KCN)x for x = 0.01 and x = 0.001. The experiments were performed at microwave frequencies between 150 and 1227 MHz in the temperature range from 0.015 K to 10 K. Our experiments prove the existence of tunnelling states with a very small and narrowly distributed energy splitting near E/k(B) = 10 mK for x = 0.001. E increases with CN- concentration and reaches E/k(B) = 40 mK for x = 0.01. These states occur in addition to those at 1 K, well known from thermal experiments.