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Collective excitations, pressure, and compressibility in multilayer systems.

We present a simple method to calculate the spectrum of elementary collective excitations in semiconductor superlattices. It is based on a variational many-body calculation with the hypernetted-chain approximation and the transformation of the structure functions and other quantities depending on the lattice layer indices into Bloch sums. In this method, we can calculate the excitation spectrum for electron and electron-hole superlattices and other more complicated structures. We also calculate the pressure and compressibility in a system of arbitrarily many electron (or hole) gas layers.