1991 年 60 巻 4 号 p. 1347-1354
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′→∞, 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.
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