Spin Dynamics in the Spin-1 Large-D Antiferromagnetic Chain System NiCl₂-4SC(NH₂)₂ in high magnetic fields

NiCl₂-4SC(NH₂)₂ (DTN) is a quantum S = 1 chain system with strong easy-pane anisotropy and a new candidate for the field-induced Bose-Einstein condensation of the spin degrees of freedom. ESR studies of magnetic excitations in DTN in fields up to 25 T are presented. Based on analysis of the single-magnon excitation mode in the high- field spin-polarized phase and previous experimental results [Phys. Rev. Lett. 96, 077204 (2006)], a revised set of spin-Hamiltonian parameters is obtained. Our results yield D = 8:9 K, J_c = 2:2 K, and J_a;b = 0:18 K for the anisotropy, intrachain, and interchain exchange interactions, respectively. These values are used to calculate the antiferromagnetic phase boundary, magnetization and the frequency-field dependence of two-magnon bound-state excitations predicted by theory and observed in DTN for the first time. Excellent quantitative agreement between the theory and experimental data is obtained.