UNDERSTANDING FAR-INFRARED ABSORPTION IN THE S = 1 ANTIFERROMAGNETIC CHAIN COMPOUND NENP

Infrared transmission measurements on the S = 1 antiferromagnetic chai n compound NENP [Ni(C2H8N2)2NO2ClO4] 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 NEN P, as the gap excitations occur at the Brillouin zone boundary. We arg ue that the crystal structure admits terms which can explain the absor ption 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.