It has recently been suggested that the organic compound
NiCl2-4SC(NH2)2 (DTN) exhibits Bose-Einstein Condensation (BEC) of the
Ni spin degrees of freedom for fields applied along the tetragonal c-axis. The
Ni spins exhibit 3D XY-type antiferromagnetic order above a field-induced
quantum critical point at Hc1∼2 T. The Ni spin fluid can be
characterized as a system of effective bosons with a hard-core repulsive
interaction in which the antiferromagnetic state corresponds to a Bose-Einstein
condensate (BEC) of the phase coherent S=1 Ni spin system. We have
investigated the the high-field phase diagram and the occurrence of BEC in DTN
by means of specific heat and magnetocaloric effect measurements to dilution
refrigerator temperatures. Our results indicate that a key prediction of BEC is
satisfied; the magnetic field-temperature quantum phase transition line
Hc(T)−Hc1∝Tα approaches a power-law at low temperatures,
with an exponent α=1.47±0.06 at the quantum critical point,
consistent with the BEC theory prediction of α=1.5.Comment: 4 pages, 4 figure