Two-dimensional Condensation of Polar Molecules with Field-induced Dipoles

We theoretically investigate the ground-state structures of a two-dimensional condensation composed of ultracold polar molecules, in which the condensed particles are subjected to an effective vector potential induced by the Raman coupling between two rotational levels of the molecule. With all dipoles aligned by a DC field in the axial direction and the two counter-propagating Raman beams in the raidal direction, the effective dipole moment induced by the light-matter coupling, which is much larger than the intrinsic dipole moment of the molecule, predominates the interaction between molecules. Based on the previous studies, the effective interaction features not only the standard long-range dipolar form but also a spatial dependence on the relative phase between two coupled rotational states. In the mean-field approximation, the ground state is found to possess four phases: plane-wave phase, zero-momentum phase and two of Stoner-type phase. The first two phases appear when the system is in the coupling-dominant regime, while the last two are in the interaction-dominant regime. Numerical results obtained by solving the Gross-Pitaevskii equation agree with the variational analysis. Dynamical stability of the Stoner-type phases is examined via calculating the Bogoliubov spectrum.