Spin-1 bosons with coupled ground states in optical lattices

TL;DR

This paper theoretically investigates the superfluid--Mott-insulator phase transition of spin-1 bosons in optical lattices, revealing how coupling ground states and laser polarization angle influence phase behavior and collective excitations.

Contribution

It introduces a model for coupled ground states of spin-1 bosons in optical lattices, analyzing phase diagrams and excitation spectra with control over atomic component populations.

Findings

Superfluid--Mott-insulator transition is second order for ferromagnetic interactions.

Transition can be first order with coexistence in antiferromagnetic case.

Laser polarization angle controls atomic component populations and asymmetry.

Abstract

The superfluid--Mott-insulator phase transition of ultracold spin-1 bosons with ferromagnetic and antiferromagnetic interactions in an optical lattice is theoretically investigated. Two counterpropagating linearly polarized laser beams with the angle $\theta$ between the polarization vectors (lin-$\theta$-lin configuration), driving an $F_g=1$ to $F_e=1$ internal atomic transition, create the optical lattice and at the same time couple atomic ground states with magnetic quantum numbers $m=\pm 1$. Due to the coupling the system can be described as a two-component one. At $\theta=0$ the system has a continuous isospin symmetry, which can be spontaneously broken, thereby fixing the number of particles in the atomic components. The phase diagram of the system and the spectrum of collective excitations, which are density waves and isospin waves, are worked out. In the case of ferromagnetic interactions, the superfluid--Mott-insulator phase transition is always second order, but in the case of antiferromagnetic interactions for some values of system parameters it is first order and the superfluid and Mott phases can coexist. Varying the angle $\theta$ one can control the populations of atomic components and continuously turn on and tune their asymmetry.