THE MAGNETIC-FIELD BEHAVIOR OF A HALDANE-GAP ANTIFERROMAGNET

We investigate the magnetic field behaviour of an antiferromagnetic He isenberg spin-1 chain with the most general single-ion anisotropy. We discuss the regime in which the magnetic field is below the transition value. The splitting of the Haldane triplet is obtained as a function of a field applied in an arbitrary orientation by means of a Lanczos exact diagonalization of chains of up to 16 spins. Our results am well summarized in terms of a first-order perturbation theory. We explain various level crossings that occur by the existence of discrete symmet ries, A discussion is given of the electron-spin-resonance and neutron -scattering experiments on the compound Ni(C2H8N2)2NO2ClO4 (NENP).