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.