The dissipative dynamics of a pair of paraelectric tunnelling impurities

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Abstract
Paraelectric impurities contribute significantly to the low-temperature properties of alkali halide crystals. Even at very low density the dipolar interaction of adjacent defect ions may lead to deviations from the behaviour expected for isolated impurities; in a certain range of concentration it is sufficient to consider pairs of coupled defects. Applying a projection operator method, previous work on this pair model is extended to the case of finite asymmetry and weak coupling to acoustic phonons. After performing the ensemble averaging, the specific heat and the zero-frequency susceptibility are calculated and compared with experimental data on KCl:Li and KCl:CN. The isotope effect on the Rabi frequency and relaxation rate is discussed.