title: Bose-Einstein condensation of S = 1 nickel spin degrees of freedom in NiCl2-4SC(NH2)2.
abstract: 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.
authors.affiliation: National High Magnetic Field Laboratory, Los Alamos, New Mexico, USA.
authors.forename: V S
authors.initials: VS
authors.lastname: Zapf
authors.forename: D
authors.initials: D
authors.lastname: Zocco
authors.forename: B R
authors.initials: BR
authors.lastname: Hansen
authors.forename: M
authors.initials: M
authors.lastname: Jaime
authors.forename: N
authors.initials: N
authors.lastname: Harrison
authors.forename: C D
authors.initials: CD
authors.lastname: Batista
authors.forename: M
authors.initials: M
authors.lastname: Kenzelmann
authors.forename: C
authors.initials: C
authors.lastname: Niedermayer
authors.forename: A
authors.initials: A
authors.lastname: Lacerda
authors.forename: A
authors.initials: A
authors.lastname: Paduan-Filho