CORRELATIONS DESTROY ACOUSTIC PLASMONS IN SUPERLATTICES

It is demonstrated that interlayer correlations bring about important qualitative changes in the collective mode structure of superlattices, as compared to the random-phase-approximation predictions. The RPA acoustic plasmon is converted into an optic-like mode where the k = 0 long-wavelength frequency is a sensitive function of the strength of the interlayer correlations and of q, the wave number along the direction perpendicular to the layers. The result should motivate Raman measurements on GaAs/AlGaAs samples in parameter domains where the d/a ratio (d is the separation distance between adjacent layers, a the interparticle spacing within a layer) is of order unit and where r(s) > 2.