Influence of Random Internal Fields on the Tunneling of OH Defects in NaCl Crystals

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.Comment: 3 pages 3 figures, REVTeX, submitted to the proceedings of the PHONONS 2001 conferenc