Massive triplet excitations in a magnetized anisotropic Haldane spin chain

TL;DR

This study uses inelastic neutron scattering to explore the excitation spectrum of a Haldane spin chain in high magnetic fields, revealing triplet excitations and a re-opened spin gap, with theoretical models compared to experimental results.

Contribution

It provides the first detailed experimental observation of triplet excitations and spin gap re-opening in a magnetized anisotropic Haldane chain, supported by quantitative theoretical analysis.

Findings

Re-opening of the spin gap above the critical magnetic field.

Observation of three distinct long-lived excitation branches.

Quantitative agreement between experimental data and theoretical models.

Abstract

Inelastic neutron scattering experiments on the Haldane-gap quantum antiferromagnet \nd are performed at mK temperatures in magnetic fields of almost twice the critical field $H_c$ applied perpendicular to the spin cahins. Above $H_c$ a re-opening of the spin gap is clearly observed. In the high-field N\'eel-ordered state the spectrum is dominated by three distinct long-lived excitation branches. Several field-theoretical models are tested in a quantitative comparison with the experimental data.