Spin-exchange interactions of spin-one bosons in optical lattices: singlet, nematic and dimerized Phases

We consider insulating phases of cold spin-1 bosonic particles with antiferromagnetic interactions, such as Na-23, in optical lattices. We show that spin-exchange interactions give rise to several distinct phases, which differ in their spin correlations. In two- and three- dimensional lattices insulating phases with odd number of particles per site are always nematic. For insulating states with even number of particles per site there is always a spin-singlet phase and there may also be a first-order transition into the nematic phase. The nematic phase breaks spin rotational symmetry, but preserves time reversal symmetry, and has gapless spin-wave excitations. The spin singlet phase does not break spin symmetry and has a gap to all excitations. In one-dimensional lattices insulating phases with odd number of particles per site always have a regime where translational symmetry is broken and the ground state is dimerized. We discuss signatures of various phases in Bragg scattering and time-of-flight measurements.