The magnetic field behaviour of a Haldane-gap antiferromagnet

Abstract

We investigate the magnetic field behaviour of an antiferromagnetic\nHeisenberg spin-1 chain with the most general single-ion anisotropy. We discuss\nthe regime in which the magnetic field is below the transition value. The\nsplitting of the Haldane triplet is obtained as a function of a field applied\nin an arbitrary orientation by means of a Lancz\\H os exact diagonalization of\nchains of up to 16 spins. Our results are nicely summarized in terms of a\nfirst-order perturbation theory. We explain various level crossings that occur\nby the existence of discrete symmetries. A discussion is given of the electron\nspin resonance and neutron scattering experiments on the compound\nNi(C$_2$H$_8$N$_2$)$_2$NO$_2$ClO$_4$ (NENP).\n