Spin dynamics in the BEC candidate DTN studied by high-field ESR

NiCl₂-4SC(NH₂)₂ (DTN) is a quantum S=1 chain system with strong easy-pane anisotropy and a new candidate for the Bose-Einstein condensation of the spin degrees of freedom. Employing high-field electron spin resonance technique, we were able to study frequency-field dependence of magnetic excitations in DTN in fields up to 25 T [1] and at temperatures down to 0.45 K [2]. As a result of our studies, we were able to accurately estimate parameters of the spin-Hamiltonian for this compound. The values were 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 with experimental data has been obtained. In addition, the excitation spectrum in DTN in the field-induced ordered phase has been studied. Two gapped modes have been observed and their frequency-field dependencies. We show that our observations can be consistently interpreted within the four-sublattice antiferromagnet model, involving a finite interaction between two tetragonal sublattices and leaving the axial symmetry intact, which is crucial for the interpretation of the field-induced ordering in DTN in terms of the BEC of magnons. Supported in part by NHMFL (through NSF and DOE) and DFG.