Neutron scattering study of two-magnon states in the quantum magnet copper nitrate

We report measurements of the two-magnon states in a dimerized antiferromagnetic chain material, copper nitrate $[\mathrm{Cu}({\mathrm{NO}}_{3}{)}_{2}\ensuremath{\cdot}2.5{\mathrm{D}}_{2}\mathrm{O}].$ 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 $q\ensuremath{\sim}(2n+1)\ensuremath{\pi}d$ which is consistent with the neutron scattering data.