Phase diagram of spin-1 antiferromagnetic Bose-Einstein condensates

We study experimentally the equilibrium phase diagram of a spin-1 Bose-Einstein condensate with antiferromagnetic interactions, in a regime where spin and spatial degrees of freedom are decoupled. For a given total magnetization m(z), we observe for low magnetic fields an antiferromagnetic phase where atoms condense in the m = +/-1 Zeeman states, and occupation of the m = 0 state is suppressed. Conversely, for large enough magnetic fields, a phase transition to a broken-axisymmetry phase takes place: The m = 0 component becomes populated and rises sharply above a critical field B-c (m(z)). This behavior results from the competition between antiferromagnetic spin-dependent interactions (dominant at low fields) and the quadratic Zeeman energy (dominant at large fields). We compare the measured B-c as well as the global shape of the phase diagram with mean-field theory, and find good quantitative agreement. DOI: 10.1103/PhysRevA.86.061601