Abstract

The field-induced long-range antiferromagnetic order in the quasi-one dimensional S=1 antiferromagnet is investigated with the exact diagonalization of finite chains and the mean field approximation for the interchain interaction. With increasing external field perpendicular to the principal axis of the single-ion anisotropy, the Haldane gap vanishes at the critical field H c and the quantum phase transition belongs to the Ising universality class. It is revealed that the gap re-opens for H> H c, where the long-range antiferromagnetic order appears in the presence of the interchain interaction. The behavior of the gap is consistent with the recent inelastic neutron scattering experiment on NDMAP.

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