Stability of superflow in supersolid phases of lattice bosons with dipole-dipole interaction

We investigate the stability of superflow of bosons with isotropic dipole-dipole interactions in a two-dimensional optical lattice. We perform linear stability analyses for the dipolar Bose-Hubbard model in the hardcore boson limit, and show that the superflow can exist in a supersolid phase unless the velocity exceeds a certain critical value and that the critical value is remarkably smaller than that in the standard superfluid phase. Additionally, it is found that there exists a parameter range in which the SS phases are stabilized by a finite superflow. We also discuss the influence of quantum fluctuations on these results within the cluster mean-field approximation. Recently, the physics of systems with strong dipole-dipole interactions has received considerable interest because of the successful experimental preparation of ultracold dipolar atoms [1] and polar molecules [2]. The long-range nature and anisotropy of the dipole-dipole interaction greatly enrich the variety of phenomena which can be observed in experiments with cold atoms and molecules. Recent theoretical studies predicted that various quantum phases, such as fermionic p-wave superfluids (SF) [3] and Haldane-Bose insulators [4], can be realized by controlling appropriately the strength of the dipole-dipole interaction and the orientation of the dipole moments. Furthermore, the systems of ultracold dipolar gases have the potential to answer the long-standing question of whether a solid can exhibit superfluidity [5–8]. The coexisting phase of solid order and superfluidity is usually called the “supersolid (SS)” phase. Recent quantum Monte Carlo (QMC) simulations for a two-dimensional Bose-Hubbard system with the isotropic long-range interactions have shown that an off-diagonal long-range order with a finite SF stiffness coexists with a crystalline long-range order in a wide range of parameters [7]. Hence, it is highly likely that the SS phases will be experimentally observed in the context of dipolar Bose gases loaded into optical lattices in the near future. In order to prove the existence of SS phases in experiments, one has to verify that both the crystalline order and superfluidity are simultaneously present. While the existence of the crystalline order can be identified by using the Bragg scattering techniques in experiments of ultracold gases, it is diffecult to measure directly the SF fraction. Instead, the superfluidity of the gases can be revealed by measuring the critical velocity above which superflow breaks down. In the experimental demonstrations of superfluidity of weaklyand strongly-interacting Bose gases [9, 10], and fermionic SF across the BEC-BCS crossover [11], the critical velocity of the superflow has been measured by using a moving optical lattice. In this work, we investigate the stability of superflow of dipolar Bose gases loaded into a two-dimensional moving optical International Conference on Strongly Correlated Electron Systems (SCES 2010) IOP Publishing Journal of Physics: Conference Series 273 (2011) 012020 doi:10.1088/1742-6596/273/1/012020 Published under licence by IOP Publishing Ltd 1 lattice, and propose that the superfluidity of the SS phases can be also identified by measuring the critical velocity of superflow. Assuming that the dipole moments are polarized to the direction perpendicular to the lattice plane, we consider here the Bose Hubbard model with isotropic dipole-dipole interactions [5],