PLASMON DISPERSION IN STRONGLY CORRELATED SUPERLATTICES

The dielectric response function of a strongly correlated superlattice is calculated in the quasilocalized charge (QLC) approximation. The r esulting QLC static local-field correction, which contains both intral ayer and interlayer pair-correlational effects, is identical to the co rrelational part of the third-frequency-moment sum-rule coefficient. T his approximation treats the interlayer and intralayer couplings on an equal footing. The resulting dispersion relation is first analyzed to determine the effect of intralayer coupling on the out-of-phase acous tic-mode dispersion; in this approximation the interlayer coupling is suppressed and the mutual interaction of the layers is taken into acco unt only through the average random-phase approximation (RPA) field. I n the resulting mode dispersion, the onset of a finite-k (k being the in-plane wave number) reentrant low-frequency excitation developing (w ith decreasing d/a) into a dynamical instability is indicated (a being the in-plane Wigner-Seitz radius and d the distance between adjacent lattice planes). This dynamical instability parallels a static structu ral instability reported earlier both for a bilayer electron system an d a superlattice and presumably indicates a structural change in the e lectron liquid. If one takes account of interlayer correlations beyond the RPA, the acoustic excitation spectrum is dramatically modified by the appearance of an energy gap which also has a stabilizing effect o n the instability. We extend a previous energy gap study at k=0 [G. Ka lman, Y. Ren, and K. I. Golden, Phys Rev. B 50, 2031 (1994)] to a calc ulation of the dispersion of the gapped acoustic excitation spectrum i n the long-wavelength domain.