Europhys. Lett, 33 (7), pp. 533-538 (1996)
Cross-over to incoherent tunnelling of substitutional defects in alkali halide crystals
A. Würger , R. Weis , M. Gaukler and C. Enss
Institut Laue-Langevin - B.P. 156X, 38042 Grenoble, France
Institut für Angewandte Physik, Universität Heidelberg
Albert-Ueberle-Str. 3-5, 69120 Heidelberg, Germany
(received 22 September 1995; accepted in final form 11 January 1996)
PACS. 61.72Yx - Interaction between different crystal defects; gettering effect.
PACS. 77.22Gm - Dielectric loss and relaxation.
Abstract:The dielectric susceptibility of substitutional defects in alkali halides shows a surprising dependence on concentration c. In the dilute case c<100 ppm, the susceptibility is proportional to the number of defects. It reaches a maximum at a concentration where the average dipole-dipole interaction is equal to the bare tunnelling amplitude of a single defect and for still higher concentrations c>1000 ppm it strongly decreases. The data for various defect systems are in quantitative agreement with a parameter-free master curve derived from a continued fraction expansion for the relevant correlation function. With increasing defect concentration a universal cross-over from coherent tunnelling to incoherent relaxation is driven by a novel relaxation mechanism, based on the dipolar interaction.
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