Observation of two-magnon bound states in the spin-1 anisotropic Heisenberg antiferromagnetic chain system NiCl₂-4SC(NH₂)₂

Results of systematic tunable-frequency ESR studies of the spin dynamics in NiCl₂-4SC(NH₂)₂ (known as DTN), a gapped S = 1 chain system with easy-plane anisotropy dominating over the exchange coupling (large-D chain), are presented. We have obtained direct evidence for two-magnon bound states, predicted for S = 1 large-D spin chains in the fully spin-polarized (FSP) phase. The frequency–field dependence of the corresponding excitations was calculated using the set of parameters obtained earlier [S.A. Zvyagin, et al., Phys. Rev. Lett. 98 (2007) 047205]. Very good agreement between the calculations and the experiment was obtained. It is argued that the observation of transitions from the ground to two-magnon bound states might indicate a more complex picture of magnetic interactions in DTN, involving a finite in-plane anisotropy.