Dynamics of the dielectric absorption in the disordered solid RbCl:OH at low temperatures

The dielectric dissipation in two crystals of RbCl containing 100- and 9-ppm OH^- dipolar impurities was measured at 930 MHz for temperatures between 15 and 500 mK. The power and temperature dependence of ε^'' are similar to those observed in glasses and indicate saturable resonant absorption of a broad distribution of two-level systems at low energies. From a study of the dynamics of the saturation, the tunneling energy of 7 mK is determined, along with the longitudinal and transverse relaxation rates, both of which scale linearly with temperature.