Weak Double Layers: Existence, Stability, Evidence

Two more classes of monotonic double layers complementing the class of beam-type double layers are investigated analytically, and their range of existence is explored in the small amplitude limit. One class preferentially exists for hot ions and electron drifts of the order of electron thermal velocity. The second one, instead, assumes hot electrons and needs almost current-free conditions. The first class, called SEADL, is based on the slow electron acoustic branch and exhibits a tuning-fork configuration in the electron phase space. Its density decreases with increasing potential. The second one (SIADL) rests on the slow ion acoustic branch and. consequently, has a tuning-fork pattern in the ion phase space. Its density increases with the potential. Both classes are found to be linearly stable with respect to one-dimensional, but unstable with respect to two-dimensional electrostatic perturbations. A comparison with experiments suggests an identification of the second type with the double layers obtained by Hollenstein

Phonon Dispersion of Transverse Basal Plane Modes in Alkali-Graphite Compounds

ABSTRACTDispersion curves for low-frequency transverse modes propagating in the basal plane have been measured in the compounds KC8, KC24 and RbC24 by means of neutron inelastic scattering. The acoustic branches show at low q a behavior ω2= Aq2+Bq4 characteristic of layered materials. The optical branches are derived qualitatively from graphite-like optical branches hybridized with new alkali-like branches. In stage 2 compounds, the shear constant C44, which can be obtained by extrapolating the acoustic branch towards q=0, is appreciably smaller than in stage-1 compounds or in pure graphite. At low temperatures, it was noted in KC24 that a frequency gap in the acoustic branch opens up near q=0.6 A-1, corresponding to the Brillouin zone boundary of the low temperature alkali superstructure.