Bose-Einstein Condensation ofS=1Nickel Spin Degrees of Freedom inNiCl2−4SC(NH2)2

It has recently been suggested that the organic compound ${\mathrm{NiCl}}_2\mathrm{\text{−}}4\mathrm{SC}({\mathrm{NH}}_2{)}_2$ (DTN) undergoes field-induced Bose-Einstein condensation (BEC) of the Ni spin degrees of freedom. The Ni $S=1$ spins exhibit three-dimensional $XY$ antiferromagnetism above a critical field $H_{c1}\sim 2\mathrm{T}$. The spin fluid can be described as a gas of hard-core bosons where the field-induced antiferromagnetic transition corresponds to Bose-Einstein condensation. We have determined the spin Hamiltonian of DTN using inelastic neutron diffraction measurements, and we have studied the high-field phase diagram by means of specific heat and magnetocaloric effect measurements. Our results show that the field-temperature phase boundary approaches a power-law $H-H_{c1}\propto T_c^{\alpha }$ near the quantum critical point, with an exponent that is consistent with the 3D BEC universal value of $\alpha =1.5$.