Bose-Einstein condensation of S=1 nickel spin degrees of freedom in NiCl2-4SC(NH2)2

It has recently been suggested that the organic compound NiCl2-4SC(NH2)(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 similar to 2 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 proportional to T-c(alpha) near the quantum critical point, with an exponent that is consistent with the 3D BEC universal value of alpha=1.5.