Session C4: Dipolar Gases

Dynamics of reactive ultracold alkali polar molecules

Recently, ultracold polar molecules of KRb have been created. These molecules are chemically reactive and their lifetime in a trap is limited [1]. However, their lifetime increases when they are loaded into a 1D optical lattice in the presence of an electric field [2]. These results naturally raise the question of manipulating ultracold collisions of other species of alkali dimer molecules, with an eye toward both novel stereochemistry, as well as suppressing unwanted reactions, to enable condensed matter applications. In this talk, we report on a comparative study between the bi-alkali polar molecules of LiNa, LiK, LiRb, LiCs which have been predicted to be reactive [3]. We compute the isotropic C$_6$ coefficients of these systems and we predict the elastic and reactive rate coefficients when an electric field is applied in a 1D optical lattice. We will discuss the efficacy of evaporative cooling for each species.\\[4pt] [1] Ni et al., Science 322, 231 (2008); Ospelkaus et al., Science 327, 853 (2010); Ni et al., Nature 464, 1324 (2010). [2] Qu\'em\'ener et al., Phys. Rev. A 81, 060701(R) (2010) ; Qu\'em\'ener et al., Phys. Rev. A 83, 012705 (2011); de Miranda et al., arXiv:1010.3731, to appear in Nature Physics. [3] Zuchowski et al., Phys. Rev. A 81, 060703(R) (2010).   

Universal ultracold collision rates for polar molecules of two alkali-metal atoms

Highly reactive ultracold polar molecules have universal near-threshold reaction rate constants that depend only on the long-range interaction potential, as shown for the KRb molecule [1-3]. We extend these calculations to estimate universal reaction rate constants for the bosonic and fermionic isotopes of the reactive species LiNa, LiK, LiRb, LiCs, and also the universal vibrational quenching rate constants for vibrationally excited states of the non-reactive species NaK, NaRb, NaCs, KCs, and RbCs. We also examine the variation with electric field of the universal collision rates for these species in quasi-2D geometry in a one-dimensional optical lattice. For many of these species an electric field of a few kV/cm perpendicular to the lattice plane should be effective in shielding the molecules from destructive collisions even at relatively modest lattice confinement strength. \\[4pt] [1] Z. Idziaszek and P. S. Julienne, Phys. Rev. Lett. 104, 113202 (2010). \\[0pt] [2] A. Micheli, {\it et al.}, Phys. Rev. Lett. 105, 073202 (2010). \\[0pt] [3] S. Ospelkaus, {\it et al.}, Science 327, 853 (2010).   

Cold dipolar heteromolecular collisions

Research into cold molecules has been progressing at a rapid pace with many exciting topics and techniques being developed. The long range dipole-dipole interaction between cold polar molecules allows us to exquisitely control their collisional dynamics using an external electric field. We combine Stark deceleration, magnetic trapping and buffer gas cooling to make the first experimental observation of cold collisions between two different neutral cold molecule species. The trapping of the collision target, OH, increases the interaction time by a factor of $10^5$, enabling us to make an absolute measurement of the total trap loss cross section between OH and ND$_{3}$ at mean collision energy 3.6 $cm^{-1}$ (5 $K$) . Due to the dipole-dipole interaction between the two species, the application of an external polarizing electric field increases the measured total trap loss cross section.   

Chemical pathways in ultracold reactions of SrF molecules

We present a theoretical investigation of the chemical reaction SrF + SrF $\rightarrow$ products, focusing on reactions at ultralow temperatures. We find that bond swapping, SrF + SrF $\rightarrow$ Sr$_2$ + F$_2$, is energetically forbidden at these temperatures. Rather, the only energetically allowed reaction is SrF + SrF $\rightarrow$ SrF$_2$ + Sr, and even then only singlet states of the SrF$_2$ trimer can form. A calculation along a reduced reaction path demonstrates that this abstraction reaction is barrierless, and proceeds by one SrF molecule ``handing off'' a fluorine atom to the other molecule.   

Bound states of interacting polar molecules in an optical lattice

We discuss the long-range bound states of a pair of ground state polar molecules confined in a cylindrically symmetric optical lattice cell. We have solved the full two-dimensional eigenvalue problem including van der Waals and anisotropic dipolar interactions. The dipole-dipole interaction and lattice confinement are tunable, and with a large s-wave scattering length of the van der Waals potential it is possible to have coincidence of the three corresponding length scales. We study the bimolecular states, varying the z confinement from quasi-2D to quasi-1D geometry. In a quasi-2D geometry, trap states are adiabatically converted to long-range bound states by increasing the electric field to more strongly align the dipoles along the axis of symmetry. In addition to confinement induced resonances, the electric field thereby provides opportunities for controlling collisional properties. Shallow bound states of the van der Waals potential are also strongly affected by the dipole moment and confinement.   

Anisotropic superfluidity in a dipolar Bose Gas

We study the superfluid character of a dipolar Bose-Einstein condensate (DBEC) in a quasi-two dimensional (q2D) geometry. In particular, we allow for the dipole polarization to have some non-zero projection into the plane of the condensate so that the effective interaction is anisotropic in this plane, yielding an anisotropic dispersion for propagation of quasiparticles. By performing direct numerical simulations of a probe moving through the DBEC, we observe the sudden onset of drag or creation of vortex-antivortex pairs at critical velocities that depend strongly on the direction of the probe's motion. This anisotropy emerges because of the anisotropic manifestation of a roton-like mode in the system.   

Density wave patterns for fermionic dipolar molecules on a square optical lattice: Mean-field-theory analysis

We model a system of ultracold fermionic dipolar molecules on a two-dimensional square lattice. Assuming that the molecules are in their nondegenerate hyperfine ground state, and that the dipole moment is polarized perpendicular to the plane (as in the recent experiments on $^{40}$K-$^{87}$Rb molecules), we approximate these molecules as spinless fermions with long range repulsive dipolar interactions. We use mean field theory to obtain the phase diagram as a function of the filling, the strength of interaction and the temperature. We find a number of ordered density wave phases in the system, as well as phase separation between these phases. A Monte Carlo analysis shows that the higher-period phases are usually suppressed in the exact solution.   

Functional RG approach to dipolar fermions in a 2D optical lattice

Recent trapped atom experiments are able to generate an ultra-cold gas of hetero-nuclear molecules with a sufficiently large dipole moment to allow for the occurrence of rich many-body physics leading to exotic quantum phases. A key role is played by the anisotropic and long range nature of the dipole-dipole interaction. We study a system of fermionic dipolar molecules in a 2D optical lattice. Previous studies for homogeneous configurations have revealed the possibility of s-wave CDW and the p-wave BCS phase. However, much remains to be understood. In our study we take an unbiased approach by following the functional RG technique. This method allows us to look at the flow of various channels as one approaches the Fermi surface in the RG sense. We find an intriguing interplay between the s-wave CDW and the p-wave superfluid phases.   

Modification of roton instability due to the presence of a second dipolar Bose-Einstein condensate

We study the behavior of two coupled purely dipolar Bose-Einstein condensates, each located in a cylindrically symmetric pancake-shaped external confining potential, as the separation b between the traps along the tight confining direction is varied. The solutions of the coupled Gross-Pitaevskii and Bogoliubov-de Gennes equations, which account for the full dynamics, show that the system behavior is modified by the presence of the second dipolar BEC. For sufficiently small b, the presence of the second dipolar BEC destabilizes the system dramatically. In this regime, the coupled system collapses through a mode that is notably different from the radial roton mode that induces the collapse of the uncoupled system. Finally, we comment on the shortcomings of an approach that neglects the dynamics in the z-direction, which is assumed to be a good approximation for highly pancake-shaped dipolar BECs in the literature.