Understanding far-infrared absorption in theS=1 antiferromagnetic chain compound NENP

Infrared transmission measurements on the S=1 antiferromagnetic chain compound NENP [Ni(C₂{H}₈N₂{)}₂NO₂{ClO}₄$] in applied magnetic fields show a sharp absorption line at the field-shifted Haldane gap. This violates a wave-vector selection rule of the Hamiltonian normally used for NENP, as the gap excitations occur at the Brillouin zone boundary. We argue that the crystal structure admits terms which can explain the absorption lines. The observation of a finite gap at all applied fields can be explained by the presence of an induced staggered magnetic field, originally proposed by Chiba et al.