Magnon wave function and impurity effects inS=1antiferromagnetic chains: A large-napproach

Abstract

A large-n approximation to the S=1 antiferromagnetic chain, using the symmetric tensor representation and its conjugate, is developed to order 1/n in order to calculate the magnon wave-function and to study the effect of modifying the exchange coupling from J to J' on a single link. It is shown that a magnon boundstate exists below the Haldane gap for arbitrarily small negative J'-J but only above a certain critical value of J'-J for positive values. In the former case the binding energy vanishes as the square of (J-J').