Interaction driven exotic quantum phases in spin-orbit coupled spin$-1$ bosons

We study the interplay between large-spin, spin-orbit coupling, and superfluidity for bosons in a two dimensional optical lattice, focusing on the spin-1 spin-orbit coupled system recently realized at the Joint Quantum Institute [Campbell et. al., arXiv:1501.05984]. We find a rich quantum phase diagram, where, in addition to the conventional phases ---superfluid and insulator--- contained in the spin-$1$ Bose-Hubbard model, there are new lattice symmetry breaking phases. For weak interactions, the interplay between two length scales, the lattice momentum and the spin-orbit wave-vector induce a phase transition from a uniform superfluid to a phase where bosons simultaneously condense at the center and edge of the Brillouin zone at a non-zero spin-orbit strength. This state is characterized by spin density wave order, which arises from the spin-$1$ nature of the system. Interactions suppress spin density wave order, and favor a superfluid \textit{only} at the Brillouin zone edge. This state has spatially oscillating mean field order parameters, but a homogeneous density. We show that the spin density wave superfluid phase survives in a two dimensional harmonic trap, and thus establish that our results are directly applicable to experiments on $^{87}$Rb, $^7$Li, and $^{41}$K.

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