We examine the magnetic field dependent excitations of the dimerized spin-1/2 chain, copper nitrate, with antiferromagnetic intra-dimer exchange J1=0.44 meV and exchange alternation α=J2/J1=0.24. Magnetic excitations in three distinct regimes of magnetization are probed through inelastic neutron scattering at low temperatures. At low and high fields there are three and two long-lived magnon-like modes, respectively. The number of modes and the anti-phase relationship between the wave-vector dependent energy and intensity of magnon scattering reflect the distinct ground states: A singlet ground state at low fields μ0H<μ0Hc1=2.8~T and an Sz=1/2 product state at high fields μ0H>μ0Hc2=4.2~T. In the intermediate field regime, a continuum of scattering for ℏω≈J1 is indicative of a strongly correlated gapless quantum state without coherent magnons.
@article{arxiv.1406.7596,
title = {Magnons and continua in a magnetized and dimerized spin-1/2 chain},
author = {M. B. Stone and Y. Chen and D. H. Reich and C. Broholm and G. Xu and J. R. D. Copley and J. C. Cook},
journal= {arXiv preprint arXiv:1406.7596},
year = {2015}
}