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 ac oustic plasmon is converted into an optic-like mode where the k = 0 lo ng-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 measure ments on GaAs/AlGaAs samples in parameter domains where the d/a ratio (d is the separation distance between adjacent layers, a the interpart icle spacing within a layer) is of order unit and where r(s) > 2.