Calculation of Reorientation Rates of OH−Defects in RbBr Using Accurate Phonon Spectra

Abstract

In a recent experiment Kapphan and Lüty measured the reorientation rate of O${\mathrm{H}}^{-}$ defects in RbBr in the temperature range 1-15°K. In this paper we present a theoretical calculation of the reorientation rate in which we treat the defect-lattice coupling and the lattice frequency spectrum as accurately as possible. The coupling is assumed to contain a strain-dipole component and an electric-dipole component; the magnitudes of the two components and the "bare" tunneling matrix element provide three parameters which are adjusted to produce the best agreement with experiment. The lattice spectrum is calculated using shell-model parameters chosen to agree with recent neutron-diffraction data on phonons in RbBr. The relaxation rate is calculated to all orders in the number of phonons. The temperature dependence of the renormalized tunneling matrix element is included. Calculated reorientation rates are in excellent agreement with experiment, but there is no unique choice of the parameters that produces a best fit. The results indicate that at low temperatures the rate is controlled by strain-dipole single-phonon processes and that at high temperatures electric-dipole multiphonon processes are dominant. Plausibility arguments are used to choose a "most probable" set of parameters that is consistent with the reorientation data, and this choice is in satisfactory agreement with values of the parameters obtained from the measured external electric-dipole moment and stress-splitting factor.