Critical behavior of the magnetization in the spin-gapped system $NiCl2–4SC(NH2)2$

We report accurate magnetization measurements on the spin-gap compound $NiCl2–4SC(NH2)2$ around the low portion of the magnetic induced phase ordering. The critical density of the magnetization at the phase boundary is analyzed in terms of a Bose–Einstein condensation (BEC) of bosonic particles, and the boson interaction strength is obtained as $v0=0.61meV$⁠. The detailed analysis of the magnetization data across the transition leads to the conclusion for the preservation of the $U(1)$ symmetry, as required for BEC.