Understanding Far-Infrared Absorption in the S=1 Antiferromagnetic Chain Compound NENP

TL;DR

This study investigates the infrared absorption features of the S=1 antiferromagnetic chain compound NENP under magnetic fields, revealing violations of expected selection rules and proposing structural and magnetic explanations for observed phenomena.

Contribution

The paper introduces a new understanding of the absorption lines in NENP by considering crystal structure effects and staggered magnetic fields, challenging previous Hamiltonian assumptions.

Findings

Infrared absorption line observed at the Haldane gap shifts with magnetic field.

Violations of wave-vector selection rules explained by crystal structure terms.

Staggered g-tensors produce a finite gap at all applied fields.

Abstract

Infrared transmission measurements on the $S=1$ antiferromagnetic chain compound NENP 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. In addition, in an applied field, staggered orientations of the g-tensors produce a staggered magnetic field. This can explain the observation of a finite gap at all applied fields.