PERTURBATIVE APPROXIMATION SCHEME FOR ISOLATED IMPURITY BONDS IN THE 2-DIMENSIONAL SPIN-1 2 HEISENBERG-ANTIFERROMAGNET/

An isolated impurity bond of coupling-K replacing a link J in the spin -1/2 Heisenberg antiferromagnet on a square lattice is considered. A s imple perturbative approximation scheme is deviced, in which the two-s ite cluster formed by the spins linked by the impurity bond is embedde d into the linearized spin wave approximation for the antiferromagnet. The resolvent for the local states of the cluster is calculated in th ree successive levels of approximation. The ground state wave function , the magnetization at the impurity link, and the transverse correlati on across the impurity link as a function of the coupling K are discus sed.