Topological phase transition for a quantum rotor with cold bosonic atoms

We investigate the properties of a Bose-Einstein condensate (BEC) of F=1 rubidium 87 atoms in a 2D spin-dependent optical lattice potential generated by intersecting laser beams with a superposition of polarizations. The Rb atoms behave as a quantum rotor (QR) with rotor angular momentum given by the sum of the atomic motion rotational angular momentum around the lattice minima and the atomic hyperfine spin $F$. Properties of the QR are strongly affected when an external magnetic field perpendicular to the plane of QR motion is applied. We observe a topological phase transition as the strength of the external transverse magnetic field is varied. We determine the spin texture of the QR BEC. At such a quantum phase transition the symmetry of the ground state changes. This transition is a result of the interplay between the Zeeman and rotational-kinetic energies in different eigenstates of the QR as the external magnetic field is increased.