The internal degrees of freedom provided by ultracold atoms provide a route for realizing higher dimensional physics in systems with limited spatial dimensions. Nonspatial degrees of freedom in these systems are dubbed “synthetic dimensions.” This connection is useful from an experimental standpoint but complicated by the fact that interactions alter the condensate ground state. Here we use the Gross-Pitaevskii equation to study the ground-state properties of a spin-1 Bose gas under the combined influence of an optical lattice, spatially varying spin-orbit coupling, and interactions at the mean-field level. The associated phases depend on the sign of the spin-dependent interaction parameter and the strength of the spin-orbit field. We find “charge”- and spin-density-wave phases which are directly related to helical spin order in real space and affect the behavior of edge currents in the synthetic dimension. We determine the resulting phase diagram as a function of the spin-orbit coupling and spin-dependent interaction strength, considering both attractive (ferromagnetic) and repulsive (polar) spin-dependent interactions, and we provide a direct comparison of our results with the noninteracting case. Our findings are applicable to current and future experiments, specifically with Rb-87, Li-7, K-41, and Na-23.

http://journals.aps.org/pra/abstract/10.1103/PhysRevA.94.063613

*Real-space mean-field theory of a spin-1 Bose gas in synthetic dimensions*; M. Hurst, J. H. Wilson, J. H. Pixley, I. B. Spielman, and S. S. Natu; Phys. Rev. A **94** 063613 (2016). doi:10.1103/PhysRevA.94.063613