Magnon wave function and impurity effects in S=1 antiferromagnetic chains:A large-n approach

A large-n approximation to the S = 1 antiferromagnetic chain, using the sym metric tensor representation and its conjugate, is developed to order 1/n i n order to calculate the magnon wave function and to study the effect of mo difying the exchange coupling from J to J' on a single link. It is shown th at a magnon bound state exists below the Haldane gap for arbitrarily small negative J' - J but only above a certain critical value of J' - J for posit ive values. In the former case the binding energy vanishes as (J - J')(2).