We report measurements of the two-magnon states in a dimerized antiferromagnetic chain material, copper nitrate [Cu(NO3)(2).2.5D(2)O]. Using inelastic neutron scattering we have measured the one- and two-magnon excitation spectra in a large single crystal. The data are in excellent agreement with a perturbative expansion of the alternating Heisenberg Hamiltonian from the strongly dimerized limit. The expansion predicts a two-magnon bound state for qsimilar to(2n+1)pid which is consistent with the neutron scattering data.
Tennant, D. A., Broholm, C., Reich, D. H., Nagler, S. E., Granroth, G. E., Barnes, T., Damle, K., Xu, G., Chen, Y., & Sales, B. C. (2003). Neutron scattering study of two-magnon states in the quantum magnet copper nitrate. Physical Review B Condensed Matter, 67(5), 054414. https://doi.org/10.1103/PhysRevB.67.054414