NEXT-NEAREST NEIGHBOR FORCES IN DIATOMIC SUPERLATTICES

Results are presented for phonons in a 1D diatomic model superlattice. Each frequency-omega corresponds to two Bloch wavenumbers Q1 and Q2. For alpha < 0.25, where alpha is the ratio of next-nearest to nearest force constants, the results are essentially a perturbation of those found with nearest-neighbour forces only. For alpha > 0.25, however, qualitatively new results are found, and in particular both Q1 and Q2 can be real over some frequency range. Illustrative dispersion curves are presented.