ELEMENTARY EXCITATIONS OF AN ANISOTROPIC SPIN-1 CHAIN

The neutron-scattering intensity and energy eigenvalues of the elementary excitations of the S = 1 antiferromagnetic Heisenberg chain with single-ion anisotropy [H = J SIGMA(i=1)N S(i).S(i+1) + D SIGMA(S(i)z)2] are calculated with a numerical diagonalization method and a projector Monte Carlo method. We set D/J = 0.2, 0.0, -0.2 and N = 14,16,18,32. For D5 not-equal 0, the elementary excitations in the subspace of S(z) = +/-1 and in the subspace of S(z) = 0 are different. The S(z) = +/-1 branch is lower than the S(z) = 0 branch near Q = pi; however, it becomes slightly higher in other regions. The crossing of levels occurs at Q congruent-to +/- 0.7pi. We find that the low-lying excited state has a weight of more than 90% in the dynamical structure factor, except for Q congruent-to 0. Our results explain recent experimental results on Ni(C2H8N2)2NO2ClO4.