Macroscopic Spin Tunneling and Quantum Critical Behavior of a Condensate in a Double-Well Potential

In a previous work [H. Pu, W. Zhang, andP. Meystre, Phys. Rev. Lett. 87, 140405 (2001)], we have shown that a spinor condensate confined in a periodic or double-well potential exhibits ferromagnetic behavior due to the magnetic dipole-dipole interactions between different wells, and in the absence of external magnetic field, the ground state has a twofold degeneracy. In this work, we demonstrate the possibility of observing macroscopic quantum spin tunneling between these two degenerate states and show how the tunneling rate critically depends on the strength of the transverse field.