This mechanism in a static dipolar field is absent in a two-level system, where homogenization must occur during the time spent in the upper level. Lifetime broadening of 2A cannot homogenize the phonon frequencies. See P. W. Anderson, Phys. Rev. U4, 1002 (1959. 7 P. G. Klemens, J. Appl. Phys. 38, 4573 (1967). 8 The change in effective width of the phonons with the metastable population (inset to Fig. 2) would slightly reduce the calculated power. However, this could easily be offset by residual effects of spatial diffusion, which predicts a third power (Ref. 4). 9 This is one order of magnitude smaller than the observed width of the R lines. The calculated ratio of the R lines. The calculated ratio of the broadening due to random crystal fields between 2A*~E and the R lines is ~ 0.1 (V. M. Hoi and H. W. de Wijn, unpublished calculations), in agreement with our findings. 10 At the highest excited-state populations reached in our experiments the interruption rate of a phonon o/T Lh «10 4 /10" 6 = 10 10 Hz is comparable with the resonance linewidth of 0.02 cm" 1 , so that the reactive coupling (Ref. 4) between the phonons and the spins is beginning to play a part. In our case the effects are small, but in more concentrated ruby the coupling may be observable.Analysis of the diffuse x-ray scattering in the one-dimensional ionic conductor K u 54 -Mg 0<77 Ti 7# 23O16 (hollandite) yields a detailed microscopic picture of the cationic shortrange order. This order is characterized by large shifts of some ions off their crystallographic sites, evidencing that in a superionic conductor the ion-ion interaction may be stronger than the periodic potential of the host crystal.The rapidly growing interest in electrochemical devices based on solid electrolytes has stimulated an intensive search for new materials with high ionic conductivities, the so-called superionic conductors. 12 The mobile ions in a superionic conductor typically reside on a fractionally occupied sub lattice with open pathways between adjacent sites. The number of sites available to the mobile ions is often not much larger than the number of occupied sites, and diffusion in such systems is no longer a single-particle process as described by the traditional random-walk theory. The path probability method by Sato and Kikuchi 3 is a major and significant step towards a correct description of diffusion in concentrated systems. More recently, Monte Carlo calculations have been performed on simple model systems 4 which go beyond the nearest-neighbor interaction of the path probability method.Experimental and theoretical studies of the dynamics of ionic motion in superionic conductors 5 have shown that interaction among the mobile ions leads to structure in a (GO) in the frequency range of a typical jump rate and affects the parameters describing the high-frequency behavior. The structure of the static short-range order among the mobile species gives in principle the most direct information about ion-ion interactions; but, unfortunately, the complex diffuse x-ray s...
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