The nature of three-magnon excitations in general spin-S quantum spin chains is studied using the recursion method. The asymptotic behaviour of the recurrence coefficients can be used to identify the presence of bound states in the spectrum. Special features of integrable models are easily identified.
The spectra of both two- and three-magnon excitations in Heisenberg spin chains with next-nearest-neighbour (NNN) interactions are studied using scaling methods and the recursion method respectively. Both two-spin and three-spin couplings are considered for general spin S. In the three-magnon case, the asymptotic behaviour of the recursion coefficients 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-state spectra. Our results for the three-magnon bound states of S = 1 chains differ appreciably from those obtained in previous studies based upon an integral equation approach.
In this thesis we examine the nature of three magnon excitations in alternating spin/bond ferromagnetic chains. We study a Hamiltonian that describes a chain (composed of two non-identical one-dimensional uniform and homogeneous sublattices) with alternating spin magnitudes, S and Sspprime, and alternating nearest neighbor interactions, Jsb1 and Jsb2. The Recursion Method is used to locate the bound states and their relationship to the three-magnon continuum. The specific cases studied are the S = Sspprime = 1/2, S = 2Sspprime = 1, and S = Sspprime = 1 alternating spin chains. We consider not only the effects of spin alternation but also of varying strengths. For each case, we studied different bond sets, more exactly, Jsb1 = 1 and Jsb2 equal to 0, 1/4, 1/2 and 1.
The nature of multimagnon excitations in alternating spin/bond ferromagnetic chains are studied using a combination of scaling methods and the recursion method. Two-magnon excitations for a Heisenberg chain with either alternating spin magnitudes, S and ${S}^{\ensuremath{'}},$ or alternating nearest-neighbor couplings, J1 and J2 can be studied using real-space rescaling techniques. These results are then used to investigate three-magnon excitations using the recursion method.