In this work, we demonstrate field-induced Bose-Einstein condensation (BEC) in the organic compound ${\mathrm{NiCl}}_2\mathrm{\text{−}}4\mathrm{SC}({\mathrm{NH}}_2{)}_2$ using ac susceptibility measurements down to 1 mK. The Ni $S=1$ spins exhibit 3D $XY$ antiferromagnetism between a lower critical field $H_{c1}\sim 2\mathrm{T}$ and a upper critical field $H_{c2}\sim 12\mathrm{T}$. The results show a power-law temperature dependence of the phase transition line $H_{c1}(T)-H_{c1}(0)=a{T}^{\alpha }$ with $\alpha =1.47\pm 0.10$ and $H_{c1}(0)=2.053\mathrm{T}$, consistent with the 3D BEC universality class. Near $H_{c2}$, a kink was found in the phase boundary at approximately 150 mK.

• Received 19 July 2008

DOI:https://doi.org/10.1103/PhysRevLett.101.187205