EFFECT OF SPIN-1/2 IMPURITY ON SPIN-1 ANTIFERROMAGNETIC HEISENBERG CHAIN

Low-lying excited states as well as the ground state of the spin-1 antiferromagnetic Heisenberg chain with a spin-1/2 impurity are investigated by means of a variational method and a method of numerical diagonalization. It is shown that 1) the impurity spin brings about massive modes in the Haldane gap; 2) when impurity-host coupling is sufficiently weak, the phenomenological Hamiltonian used by Hagiwara et al. (Phys. Rev. Lett. 65 (1990) 3181) in the analysis of ESR experimental results for Ni(C2H8N2)2NO2(ClO4) containing a small amount of spin-1/2 Cu2+ impurities is derived from a more realistic Hamiltonian, as far as the energies of the low-lying states are concerned; 3) the results obtained by the variational method are in semiquantitative agreement with those obtained by numerical diagonalization.