Bound states and field-polarized Haldane modes in a quantum spin ladder

TL;DR

This study uses neutron spectroscopy to explore multiparticle excitations in a quantum spin ladder, revealing bound states and field-polarized modes that deepen understanding of one-dimensional quantum magnetism.

Contribution

It provides the first experimental observation of singlet and triplet excitations in a fully spin-polarized ladder, aligning with theoretical models of Haldane chain modes.

Findings

Observation of two-magnon bound states at zero field

Detection of novel singlet and triplet excitations under high magnetic fields

Complete quantitative agreement between experiment and theory

Abstract

The challenge of one-dimensional systems is to understand their physics beyond the level of known elementary excitations. By high-resolution neutron spectroscopy in a quantum spin ladder material, we probe the leading multiparticle excitation by characterizing the two-magnon bound state at zero field. By applying high magnetic fields, we create and select the singlet (longitudinal) and triplet (transverse) excitations of the fully spin-polarized ladder, which have not been observed previously and are close analogs of the modes anticipated in a polarized Haldane chain. Theoretical modelling of the dynamical response demonstrates our complete quantitative understanding of these states.