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Journal of Physics Condensed Matter

Multi-magnon excitations in Heisenberg spin-Schains with next-nearest-neighbour interactions

S L M Cyr, B. W. Southern, David A. Lavis

Published Jun 24, 1996• 4 citations
Publisher

Abstract

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.

DOI
10.1088/0953-8984/8/26/010

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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.
A study of two-magnon bound states in the next-nearest-neighbour (NNN) ferromagnetic Heisenberg chain begun by Majumdar in 1969 is extended by finding the bound states and continuum resonances for all pair wave-vectors for eta =J 2 /J 1 in the ferromagnetic range of eta =-1/4 to infinity. An exact Green function formalism is combined with the exact analytic results recently obtained by the authors for the lattice Green functions of the system to afford a definitive analysis. The Heisenberg case exhibits a modified nearest-neighbour (NN) type bound state throughout the range of eta studied and for total pair wave-vector (K=k 1 +k 2 ) throughout the zone, in contrast to Majumdar who concluded that this was only the case for positive eta . The continuum also possesses a rich structure which is displayed via a shaded (grey-level) intensity plot in the ( omega , K) plane. An unexpected result is the presence of a second (anti-)bound state just above the continuum for negative eta . The evolution of the bound state and resonance structure is also examined for the transition to dominant NNN interactions. A general formulation is given which includes Ising and uniaxial anisotropies as well as biquadratic exchange for arbitrary range, dimension and Bravais lattice.
The problem of three spin deviations from a fully aligned state is studied for the Heisenberg model with next-nearest-neighbour interactions for the case of spin 1. The method used is a straightforward generalization of the equation-of-motion method of Fukuda and Wortis, taking care of the unphysical states. The resulting integral equation is solved in one dimension and the dependence of the bound states on the next-nearest-neighbour interaction discussed. Numerical calculations have also been done for closed chains containing up to 40 spins. By using C N invariance of the Hamiltonian, the dimensionality of the space is substantially reduced. The results of the finite-chain calculation agree well with solutions obtained from the equation-of-motion method.
The bound state problems associated with two magnons in a linear chain of spin 1/2 with nearest and next nearest neighbor exchange integrals, J and α J , respectively, are studied. The Hamiltonian is rewritten in terms of the Fermi operators in place of spin operators, and an integral equation to determine the bound state energies is obtained. In the cases when the total wave number K of two magnons is π or π/2, analytical solutions are easily obtained. A part of the present results disagrees with Majumdar's ones. Computer studies on the bound states are performed for a finite chain composed of spins up to twenty. They are consistent with the analytical results.
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 SspprimeS\\sp\\primeSspprime, and alternating nearest neighbor interactions, Jsb1J\\sb1Jsb1 and Jsb2.J\\sb2.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 = SspprimeS\\sp\\primeSspprime = 1/2, S = 2Sspprime2S\\sp\\prime2Sspprime = 1, and S = SspprimeS\\sp\\primeSspprime = 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, Jsb1J\\sb1Jsb1 = 1 and Jsb2J\\sb2Jsb2 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, SSS and ${S}^{\ensuremath{'}},$ or alternating nearest-neighbor couplings, J1{J}_{1}J1​ and J2{J}_{2}J2​ can be studied using real-space rescaling techniques. These results are then used to investigate three-magnon excitations using the recursion method.