Low-temperature behavior of alkali halides doped with hydroxyl

The low-temperature dielectric dispersions of LiF, NaCl, KCl, and KBr have been measured for different concentrations of OH^?. With decreasing tunnel splitting of the OH^? levels, and with increasing dipolar interaction, a behavior is observed below ～1 K similar to that found in amorphous materials and attributed to two-level states (TLS). Differences between OH^? and TLS are noted, the main difference being that the broad spectrum of energy levels associated with the OH^? in alkali halides arises from dipolar interactions rather than an atomically disordered environment. It is argued that an electric dipole analog of the magnetic spin-glass state does not occur in the OH^? systems.