  Alkali halide crystals doped with certain impurity ions show a low
temperature behaviour, which differs significantly from that of pure crystals.
The origin of these characteristic differences are tunneling centers formed by
atomic or molecular impurity ions. We have investigated the dielectric
susceptibility of hydroxyl ions in NaCl crystals at very low concentrations
(below 30 ppm), where interactions are believed to be negligible. We find that
the temperature dependence of the susceptibility is noticeably different from
what one would expect for isolated defects in a symmetric environment. We
propose that the origin of these deviations are random internal strains arising
from imperfections of the host crystal. We will present the experimental data
and a theoretical model which allows a quantitative understanding on a
microscopic basis.

Disordered Systems and Neural Networks