THE SPIN-ONE HEISENBERG-BIQUADRATIC QUANTUM SPIN CHAIN TREATED BY THECOUPLED-CLUSTER METHOD

The coupled-cluster method is used to obtain the ground-state energy o f the isotropic Heisenberg-biquadratic quantum spin-one chain as a fun ction of the ratio of the magnitudes of the two terms in the Hamiltoni an. Two different model states are used which are expected to be valid in different regimes. In both cases we use simple approximation schem es to obtain numerical results for the ground-state energy which are c ompared with results of exact diagonalizations of short chains. For bo th cases we are able to incorporate some of the long-range correlation s explicitly, using the so-called full-SUB2 approximation schemes, and this leads to evidence of phase changes at certain points. These are discussed in the light of known and conjectured phase transitions in t his system.