Plasmons in semiconducting superlattices with a complex unit cell

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Abstract
We discuss the theory of the intrasubband longitudinal electronic excitations in type-I semiconducting superlattices in which the repeat unit is comprised of two electronic layers. The spectrum of a semi-infinite superlattice of this new type consists of both bulk- and surface-plasmon branches. For each value of k, the wave vector along the layering direction, there are two bulk plasmons. As k is varied at a given in-plane component of the wave vector the frequencies of these excitations form two separated continua. The surface plasmons correspond to collective excitations which are localized near the interface with an insulator. Their dispersion relation lies outside of the bulk-plasmon continua. These modes have the peculiarity of being intrinsically free of Landau damping. We find that in these systems acoustic surface plasmons always exist if the distance between the first two layers is more than half the superlattice spacing.