Low-temperature excitation spectrum in the BEC candidate DTN: an ESR study

NiCl₂-4SC(NH₂)₂ (known as DTN) is a quantum S = 1 system with an easy-plane anisotropy dominating over the exchange interaction and exhibiting a field-induced low-temperature ordering with critical fields B_c1=2.1 T and B_c2=12.6 T. A systematic study of the low-energy excitation spectrum of DTN in the field-induced magnetically ordered phase at temperatures down to 0.45 K and frequencies down to 25 GHz is presented. It is argued that two gapped modes observed in the experiment 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.1 The latter is of particular importance, being a necessary prerequisite for the interpretion of the antiferromagnetic ordering in DTN in terms of the Bose-Einstein condensation of the spin degrees of freedom