Excitation spectrum of an antiferromagnetic model

Abstract

Short chains of up to 12 spins are studied exactly to obtain the excitation spectrum of an antiferromagnetic hamiltonian H=1/2J Sigma i=1N sigma i, sigma i+1+1/4J Sigma i=1N sigma i, sigma i+2 (J>0, even, N+1 identical to 1, N+2 identical to 2). As well as antiferromagnetic spin waves of total spin S=1, there are S=0 bound magnon states lying partly below the spin wave branch. The susceptibility shows the usual peak at low temperatures, and the zeros of the partition function lie on the negative real axis in the complex mu =exp(-mHe/kBT) plane.