Bond operator theory of isotropic spin-1 antiferromagnetic chains

The bond operator representation of quantum S=1/2 spins with the singlet state being projected out is applied to investigate the isotropic spin-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) point beta=-1/3 agree quite well with the analytical, numerical, and experimental studies. The excitation spectrum is nonsymmetric with respect to k=pi/2 and shows a minimum at k=pi. The beta dependences of the ground-state energy and the excitation gap show the similarities between the Heisenberg point and VBS point. The related properties of the spin ladders are also discussed.