Phase diagram of two-component bosons on an optical lattice

Abstract

We present a theoretical analysis of the phase diagram of two-component bosonson an optical lattice. A new formalism is developed which treats the effectivespin interactions in the Mott and superfluid phases on the same footing. Usingthis new approach we chart the phase boundaries of the broken spin symmetrystates up to the Mott to superfluid transition and beyond. Near the transitionpoint, the magnitude of spin exchange can be very large, which facilitates theexperimental realization of spin-ordered states. We find that spin and quantumfluctuations have a dramatic effect on the transition, making it first order inextended regions of the phase diagram. When each species is at integer filling, anadditional phase transition may occur, from a spin-ordered insulator to a Mottinsulator with no broken symmetries. We determine the phase boundaries inthis regime and show that this is essentially a Mott transition in the spin sector.