We investigate effects of interactions between substitutional defects on theproperties of defect crystals at low temperatures, where defect motion isgoverned by quantum effects. Both, thermal and dynamical properties areconsidered. The influence of interactions on defect motion is described via acollective effect. Our treatment is semiclassical in the sense that we analyzecollective effects in a classical setting, and analyze the influence onquantized defect motion only thereafter. Our theory describes a crossover toglassy behavior at sufficiently high defect concentration. Our approach ismeant to be general. For the sake of definiteness, we evaluate most of ourresults with parameters appropriate for Li-doped KCl crystals.
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