BOND OPERATOR-THEORY OF ISOTROPIC SPIN-1 ANTIFERROMAGNETIC CHAINS

The bond operator representation of quantum S=1/2 spins with the singl et state being projected out is applied to investigate the isotropic s pin-1 Heisenberg antiferromagnetic chain H=Sigma(i)[S-i.Si+1-beta(S-i. Si+1)(2)]. Tn the approximation of Bose condensation, the ground-state properties of the system and their beta dependences are studied. The results at Heisenberg point beta=0 and at valence-bond-solid (VBS) poi nt beta=-1/3 agree quite well with the analytical, numerical, and expe rimental studies. The excitation spectrum is nonsymmetric with respect to k=pi/2 and shows a minimum at k=pi. The beta dependences of the gr ound-state energy and the excitation gap show the similarities between the Heisenberg point and VBS point. The related properties of the spi n ladders are also discussed.