Acoustic plasmons inHg3−δAsF6in the long-wavelength limit

In mercury chain compounds such as ${\mathrm{Hg}}_{3\mathrm{-}\mathrm{\delta }}$ ${\mathrm{AsF}}_6$, the Hg atoms form two mutually perpendicular arrays of parallel chains (the a and b chains). Mohan has recently studied the random-phase-approximation coupled dynamic electronic response functions of a system in which the electrons are restricted to move in channels of such a coupled array. Besides the usual kind of bulk plasmonlike mode which propagates in the a-b plane, Mohan predicted a new acoustic plasmon in which the charge fluctuations in the two arrays are out of phase with each other. We give a simplified analysis in the long-wavelength limit (based on neglecting the effects of the chain lattice geometry) which allows a more transparent discussion of the properties of this acoustic plasmon.