title: Direct measurement of the Bose-Einstein condensation universality class in NiCl2-4SC(NH2)2 at ultralow temperatures.
abstract: In this work, we demonstrate field-induced Bose-Einstein condensation (BEC) in the organic compound NiCl2-4SC(NH2)_{2} using ac susceptibility measurements down to 1 mK. The Ni S=1 spins exhibit 3D XY antiferromagnetism between a lower critical field H_{c1} approximately 2 T and a upper critical field H_{c2} approximately 12 T. The results show a power-law temperature dependence of the phase transition line H_{c1}(T)-H_{c1}(0)=aT;{alpha} with alpha=1.47+/-0.10 and H_{c1}(0)=2.053 T, consistent with the 3D BEC universality class. Near H_{c2}, a kink was found in the phase boundary at approximately 150 mK.
authors.affiliation: Department of Physics, University of Florida, and National High Magnetic Field Laboratory, Gainesville, Florida 32611-8440, USA.
authors.forename: L
authors.initials: L
authors.lastname: Yin
authors.forename: J S
authors.initials: JS
authors.lastname: Xia
authors.forename: V S
authors.initials: VS
authors.lastname: Zapf
authors.forename: N S
authors.initials: NS
authors.lastname: Sullivan
authors.forename: A
authors.initials: A
authors.lastname: Paduan-Filho