Theory of Haldane-gap antiferromagnets in applied fields

As first predicted by Haldane, and later observed in neutron-scattering experiments, one-dimensional Heisenberg antiferromagnets of integer spin have an excitation gap. Recently, the highly one-dimensional antiferromagnet Ni(${\mathrm{C}}_2$ ${\mathrm{H}}_8$ ${\mathrm{N}}_2$ ${)}_2$ ${\mathrm{NO}}_2$(${\mathrm{ClO}}_4$) has been studied in an applied magnetic field. We discuss the susceptibility, high-field magnetization, and field-dependent neutron scattering using general arguments and field-theory methods.