HALDANE GAP IN THE SPIN-1/2 DOUBLE CHAIN HEISENBERG-ANTIFERROMAGNET - NUMERICAL DIAGONALIZATION AND PROJECTOR MONTE-CARLO STUDY

The properties of the spin-1/2 double chain Heisenberg antiferromagnet are studied by means of the numerical diagonalization and the projector Monte Carlo simulation. The model consists of the two spin 1/2 antiferromagnetic quantum Heisenberg chains coupled with each other via the ferromagnetic interchain coupling J'. It is shown that the first excited state has a gap for large enough J'. This gap continues to the Haldane gap in the limit J' --> infinity, where spins on the different chains form triplets on each site. On the other hand, the gap decreases rapidly between J' = 1.0 and 0.6 and is undetectable for J' < 0.6 within the present accuracy.