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) approximately 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 variant T(alpha)(c) near the quantum critical point, with an exponent that is consistent with the 3D BEC universal value of alpha = 1.5.

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Members of the RBiPt (R =Ce-Lu with the exceptions of Pm and Eu) series have been grown as single crystals. Magnetic susceptibility and electrical resistance have been measured on all members of the series, and specific heat measurements have been performed on rcprcscntatives. Tue high temperature resistance uniformly changes from that of a small-gap semiconductor or semimetal seen in NdBiPt to that of a heavy-fermion meta! seen in YbBiPt, which shows a linear coefficient of specific heat at low temperatures of 8 J/K. 2 mole.Further, the lighter rare earth members s how an unusually sharp increase in their resistance associated with antiferromagnetic ordering at low temperatures.

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Results are presented of anisotropic temperature and field-dependent magnetization M (H,T) and resistivity (H,T) measurements on high quality single crystals of the light rare-earth diantimonides RSb 2 , RϭLa-Nd, Sm. All of these, excepting LaSb 2 , magnetically order at low temperatures, and for CeSb 2 and NdSb 2 several magnetically ordered phases were observed in low-field magnetization and zero-field resistivity measurements. For RϭCe-Sm strong anisotropies, associated with crystalline electric field ͑CEF͒ splitting of the R 3ϩ ion, were found in M (T) measurements both below and above magnetic ordering temperatures. Furthermore, for RϭCe-Nd metamagnetic transitions were observed in M (H) and (H) for Hʈ(ab) in the magnetically ordered state. In addition, above 15 kG de Haas-van Alphen oscillations are observed for SmSb 2 and Shubnikov-de Haas quantum oscillations are observed above ϳ120 kG for NdSb 2 and SmSb 2 . The zero-field in-plane resistivity ab of all of the compounds is metallic (d/dTϾ0), with residual resistance ratios ranging from 40 to 750. The c-axis resistivity is also metallic, but appears to be considerably larger than the in-plane value, consistent with the diantimonides being quasi-two-dimensional materials. The magnetoresistance of all members of the series is large, approximately linear in H at moderate fields, and is also dependent on the relative orientation of the applied magnetic fields to the crystallographic axes. The extreme case of SmSb 2 has ͓(55 kG)Ϫ(0)]/(0)Ͼ50 000% at Tϭ2 K and Hʈc. ͓S0163-1829͑98͒03621-2͔