Spin excitations under fields in an anisotropic bond-alternating quantum S=1 chain: contrast with Haldane spin chains

TL;DR

This study investigates the spin excitations in an anisotropic bond-alternating S=1 chain under magnetic fields, revealing distinct behaviors compared to Haldane chains, including the interaction with magnon continua and changes in excitation modes.

Contribution

The paper provides experimental insights into the field-dependent spin excitations in a bond-alternating S=1 chain, contrasting it with Haldane chains and highlighting the effects of anisotropy and bond alternation.

Findings

Zeeman splitting observed below Hc with unusual mode suppression

Interaction with a low-lying two magnon continuum in dimerized chains

Only one excited mode observed above Hc, unlike Haldane chains

Abstract

Inelastic neutron scattering experiments on the S=1 quasi-one-dimensional bond-alternating antiferromagnet Ni(C9D24N4)(NO2)ClO4 have been performed under magnetic fields below and above a critical field Hc at which the energy gap closes. Normal field dependece of Zeeman splitting of the excited triplet modes below Hc has been observed, but the highest mode is unusually small and smears out with increasing field. This can be explained by an interaction with a low-lying two magnon continuum at q=pi that is present in dimerized chains but absent in uniform ones. Above Hc, we find only one excited mode, in stark contrast with three massive excitations previously observed in the structurally similar Haldane-gap material NDMAP [A. Zheludev et al., Phys. Rev. B 68, 134438 (2003)].