MULTI-MAGNON EXCITATIONS IN HEISENBERG SPIN-S CHAINS WITH NEXT-NEAREST-NEIGHBOR INTERACTIONS

The spectra of both two- and three-magnon excitations in Heisenberg sp in chains with next-nearest-neighbour (NNN) interactions are studied u sing scaling methods and the recursion method respectively. Both two-s pin and three-spin couplings are considered for general spin S. In the three-magnon case, the asymptotic behaviour of the recursion coeffici ents can be used to directly identify the presence of bound states in the spectrum. Both integrable and non-integrable models can be studied , and the integrable models display special features in the bound-stat e spectra. Our results for the three-magnon bound states of S = 1 chai ns differ appreciably from those obtained in previous studies based up on an integral equation approach.