Excitation hierarchy in the BEC candidate DTN

NiCl2-4SC(NH2)2 (known as DTN) is a quantum S = 1 chain system with strong easyplane anisotropy that is regarded as a new candidate for the Bose-Einstein condensation (BEC) of spin degrees of freedom, with critical fields B_c1 = 2.1 T and B_c2 = 12.6 T. Tuneable-frequency Electron Spin Resonance (ESR) studies of magnetic excitations in DTN in fields up to 25 T are presented. Based on analysis of the single-magnon excitation mode observed in the high-field spin-polarized phase1 and previous experimental results2, 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 theory3 and observed in DTN for the first time. Furthermore, we present a systematic study of the low-energy excitation spectrum of DTN in the fieldinduced magnetically ordered phase (which is regarded as the magnon BEC phase) at temperatures down to 0.45 K. We argue that two gapped modes observed in the experiment4 can be consistently interpreted within a four-sublattice antiferromagnet model with a weak isotropic corner-center interaction of magnetic ions in the body-centered tetragonal lattice with unbroken axial symmetry. The latter is of particular importance, being a necessary prerequisite for the interpreting of the antiferromagnetic ordering in DTN in terms of the BEC scenario. The physically simplest scenario would correspond to the isotropic "corner-center" exchange. Since the exchange interaction within each sublattice is antiferromagnetic, the ground state in DTN in the ordered state (B_c1 < B < B_c2) is predicted to be frustrated and thus infinitely degenerated.