HALDANE GAP IN THE SPIN-1/2 DOUBLE CHAIN ANISOTROPIC HEISENBERG-ANTIFERROMAGNET

The double chain Heisenberg antiferromagnet with anisotropy is studied using the mapping onto the coupled anisotropic nonlinear sigma-model. This model consists of two anisotropic antiferromagnetic quantum Heisenberg chains with half-integer spin coupled with each other via the ferromagnetic interchain coupling J'. For small J', our analysis suggests that J' is an irrelevant parameter and consequently the two chains behave independently and no gap is generated. On the other hand, for large enough J', the two chains behave as a single Heisenberg chain with integer spin which has a gap in the excitation spectrum. The expected behavior in the isotropic limit is also discussed.