Polarized-neutron study of longitudinal Haldane-gap excitations in Nd$_2$BaNiO$_5$
S. Raymond, T. Yokoo, A. Zheludev, S. E. Nagler, A. Wildes, J., Akimitsu
TL;DR
This study uses polarized neutron scattering to observe the first longitudinal Haldane-gap excitation in Nd2BaNiO5, revealing limitations of existing models for describing longitudinal spin dynamics.
Contribution
It reports the first observation of a longitudinal Haldane-gap mode in Nd2BaNiO5, challenging current theoretical models of spin excitations in mixed-spin chains.
Findings
First observation of longitudinal Haldane-gap excitation
Existing models fail to explain longitudinal spin gap results
Highlights need for revised theoretical understanding
Abstract
Polarized and unpolarized inelastic neutron scattering is used to study Haldane-gap excitations in the mixed-spin linear-chain antiferromagnet Nd2BaNiO5. The longitudinal mode, polarized along the direction of ordered moments, is observed for the first time. The model of isolated Haldane chains in a static staggered exchange field, that is known to work very well for static properties and transverse spin excitations in R2BaNiO5 compounds, fails to explain new results for the longitudinal spin gap.
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