Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Q16: Focus Session: Dipolar Gases and Ultra-Cold Molecules |
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Sponsoring Units: DAMOP Chair: Dan Stamper-Kurn, University of California, Berkeley Room: 317 |
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q16.00001: Novel quantum magnets: Dipolar quantum gases Invited Speaker: Experimental creation of chromium condensate with large magnetic dipole moment and ultracold polar molecules with large electric dipole moment has generated significant interest in dipolar quantum gases. The long-range and anisotropic nature of the dipolar interaction potential greatly enriches the properties of the system, many of which are not present in their non-dipolar counterparts. In this talk, I will present our recent studies on dipolar BEC as well as dipolar fermions, with the emphasis on the latter which is relatively less studied. From a semi-classical calculation based upon the phase space representation of the Fermi gas, I will show how the dipolar interaction deforms the Fermi surface and manifests itself in collective excitations and time-of-flight expansion dynamics of the cloud. Finally, I will describe a self-consistent Hartree-Fock-Bogoliubov theory that also takes dipolar induced fermionic superfluid pairing into account. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:27PM |
Q16.00002: Ultracold Heteronuclear Fermi-Fermi Molecules Invited Speaker: Spin mixtures of quantum-degenerate fermionic gases exhibit long lifetimes in the strongly-interacting regime near a Feshbach resonance. This has opened the door for numerous key experiments like the creation of Fermi-Fermi molecules, the realization of molecular BEC, the observation of a pairing gap and of superfluidity in a fermionic gas in the BEC-BCS cross-over region near a Feshbach resonance. We present the production of $^{6}$Li-$^{40}$K heteronuclear molecules based on our experimental platform for the production of a two-species mixture of quantum-degenerate Fermi gases. Our production scheme for quantum-degenerate fermionic $^{6}$Li and $^{40}$K and bosonic $^{87}$Rb gases is based on multiple species magneto-optical trapping [1] and sympathetic cooling of the fermions by rubidium. We demonstrated catalytic cooling of lithium by potassium, overcoming the small lithium rubidium cross section. We achieved to simultaneously enter quantum degeneracy for all three species [2] with lowest temperatures of 0.25 and 0.35 times the Fermi temperature for lithium and potassium at about 260 nK. The highest atom numbers achieved are 1.8x10$^{5}$ for lithium as well as potassium, and about 1x10$^{5}$ for rubidium. We studied two s-wave Feshbach resonances between lithium and potassium [3] at 155 G and 168 G. By magnetic field sweeps we created about 4$\cdot $10$^{4 6}$Li-$^{40}$K molecules at conversion efficiencies of up to 50 {\%} [4]. With a Stern-Gerlach purification technique we are able to image molecules and atoms spatially separated from each other. We discuss the lifetime of the molecule-atom mixture close to resonance. \\[4pt] References: \\[0pt] [1] M.Taglieber, A.-C.Voigt, F.Henkel, S.Fray, T.W.H\"{a}nsch, and K. Dieckmann, Phys. Rev. A 73, 011402(R) (2006)\\[0pt] [2] M. Taglieber, A.-C. Voigt, T. Aoki, T. W. H\"{a}nsch, and K. Dieckmann, Phys. Rev. Lett., 100, 010401, (2008)\\[0pt] [3] E.Wille, et al., PRL, 100,053201,(2008)\\[0pt] [4] A.-C. Voigt, M. Taglieber, L. Costa, T. Aoki, W. Wieser, T.W. Haensch, and K. Dieckmann, accepted for publication in Phys. Rev. Lett. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q16.00003: Quantum phases of a two-dimensional dipolar Fermi gas Georg Bruun, Edward Taylor We examine the superfluid and collapse instabilities of a quasi two-dimensional gas of dipolar fermions aligned by an orientable external field. It is shown that the interplay between the anisotropy of the dipole-dipole interaction, the geometry of the system, and the $p$-wave symmetry of the superfluid order parameter means that the effective interaction for pairing can be made very large without the system collapsing. This leads to a broad region in the phase diagram where the system forms a stable superfluid. Analyzing the superfluid transition at finite temperatures, we calculate the Berezinskii--Kosterlitz--Thouless temperature as a function of the dipole angle. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q16.00004: Fermi surface distortions in a neutral Fermi fluid with dipolar interactions Benjamin M. Fregoso, Kai Sun, Eduardo Fradkin, Benjamin Lev We show that the Fermi surface of a neutral fluid of fermions with aligned dipole moments by an external field is elongated along the direction of the aligning field. The distortion of the Fermi surface can be expressed as a linear combination of Legendre polynomials $P_l(\hat{k})$ with $l=even$, with $l=2$ being the leading term. The possible existence of a phase transition to a spontaneous biaxial phase is discussed. The zero- sound collective modes of the system are found to be strongly anisotropic. We discuss the possible use of light scattering experiments to detect spatial anisotropies in dipolar gases. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q16.00005: Devil's staircase and supersolidity in one-dimensional dipolar Bose gases S.L. Sondhi, F.J. Burnell, Meera M. Parish, N.R. Cooper The classical ground states of particles in a convex repulsive potential are known to have a phase portrait displaying a complete devil's staircase structure. We consider a single- component gas of dipolar bosons confined in a one-dimensional optical lattice, where the dipoles are aligned such that the long-ranged dipolar interactions are maximally repulsive. Introducing a kinetic term tunes the system away from the classical limit and results in a phase diagram with a Mott- Hubbard lobe for each rational filling fraction. Tuning the on- site interaction away from convexity yields alternative commensurate states with double occupancies which can form a staircase of their own, as well as one dimensional ``supersolids'' which simultaneously exhibit discrete broken symmetries and superfluidity. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q16.00006: Roton softening and supersolidity in Rb spinor condensates Robert Cherng, Eugene Demler Superfluids with a tendency towards periodic crystalline order have both a phonon and roton like spectrum of collective modes. The softening of the roton spectrum provides one route to a supersolid. We show that roton softening occurs in $^{87}$Rb spinor condensates once dipolar interactions and spin dynamics are taken into account. By including the effects of a quasi-two-dimensional geometry and rapid Larmor precession, we show a dynamical instability develops in the collective mode spectrum at finite wavevectors. We construct phase diagrams showing a variety of instabilities as a function of the direction of the magnetic field and strength of the quadratic Zeeman shift. Our results provide a possible explanation of current experiments in the Berkeley group Phys. Rev. Lett. 100:170403 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q16.00007: Hexatic, Wigner crystal and superfluid phases of dipolar bosons Kaushik Mitra, Carl Williams, Carlos Sa de Melo The finite temperature phase diagram of two-dimensional dipolar bosons versus dipolar interaction is discussed for different values of short range repulsions. We identify the stable phases as superfluid, dipolar Wigner crystal (DWC), dipolar hexatic liquid crystal (DHLC), and normal fluid. In particular, we show that the DWC exists at low temperatures for large dipolar interactions, but it melts into a DHLC at higher temperatures, where translational lattice order is destroyed, but orientational order is preserved. Upon further increase in temperature the DHLC phase melts into the normal fluid, where both orientational and translational lattice order are absent. We also find that the supersolid phase has always higher energy than the superfluid or Wigner crystal phases at low temperatures, but the supersolid is metastable, having an energy minimum that may be accessed through thermal quenching. Lastly, we calculate the static structure factor for each of the stable phases and show that each phase can be identified uniquely in an optical Bragg scattering experiment. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q16.00008: Dipole moments of ultra-cold polar molecules: a quantum Monte Carlo study Michal Bajdich, Shi Guo, Lubos Mitas, Peter J. Reynolds Recently, there has been a great deal of interest in the production of ultra-cold heteronuclear molecules having large electric dipole moments [1]. This is of interest both for fundamental reasons as well as for applications such as qubits for quantum computing [2]. In this work, we calculate the dipole moment of a potentially implementable two-atom alkaline-alkaline-earth molecule, LiSr. We use correlated wave-function methods including both the quantum chemical configuration interaction (CI) method, and a stochastic quantum Monte Carlo (QMC), to calculate the potential energy surface and dipole moment. We study the dipole moment with increasing accuracy of correlated wave-functions. We then variationally re-optimize the wave-functions, which then serve as the representation of the Fermion nodes in the fixed-node QMC. To treat the Sr atom we employ two types of effective core potentials (ECPs), large core ECPs have only $s$-states in the valence space, while the small core ECP's valence space includes also the highest $s$ and $p$ semi-core sub-shells. We find significant sensitivity of thedipole moment on both the size of the valence space and on the accuracy of the Fermion nodes. [1] B. Damski, {\it et. al} Phys. Rev. Lett. 90, 110401 (2003). [2] D. DeMille, Phys. Rev. Lett. 88, 067901 (2002). [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q16.00009: Using an optical lattice to preform KRb molecules and enhance the efficiency of ultracold polar molecule formation James Freericks, Maciej Maska, Romuald Lemanski, Thomas Hanna, Paul Julienne We will discuss recent computational work that employs both direct quantum Monte Carlo simulation and inhomogeneous dynamical mean-field theory to study the efficiency of preforming KRb pairs in an optical lattice. We will describe how to optimize the efficiency by adjusting the lattice depth and the interspecies interaction (via the Feshbach resonance) with parameters specific for fermionic $^{40}$K and bosonic $^{87}$Rb (since the ground-state dipolar molecule has already been formed from those atoms in free space). We work with a deep enough lattice that the K atoms are mobile, but the Rb atoms are localized, so the system is described by the spinless Falicov-Kimball model on a two-dimensional lattice. We also calculate the entropy and estimate the temperature that one can achieve by cooling the atoms and adiabatically turning on the lattice. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q16.00010: The bound states of ultracold KRb molecules Paul Julienne, Thomas Hanna Recently ultracold vibrational ground state $^{40}$K$^{87}$Rb polar molecules have been made using magnetoassociation of two cold atoms to a weakly bound Feshbach molecule, followed by a two-color optical STIRAP process to transfer molecules to the molecular ground state [1]. We have used accurate potential energy curves for the singlet and triplet states of the KRb molecule [2] with coupled channels calculations to calculate all of the bound states of the $^{40}$K$^{87}$Rb molecule as a function of magnetic field from the cold atom collision threshold to the {\$}v=0{\$} ground state. We have also developed approximate models for understanding the changing properties of the molecular bound states as binding energy increases. Some overall conclusions from these calculations will be presented. [1] K.-K. Ni, S. Ospelkaus, M. H. G. de Miranda, A. Peer, B. Neyenhuis, J. J. Zirbel, S. Kotochigova, P. S. Julienne, D. S. Jin, and J. Ye, Science, 2008, 322, 231--235. [2] A. Pashov, O. Docenko, M. Tamanis, R. Ferber, H. Kn\"{o}ckel, and E. Tiemann, Phys. Rev. A, 2007, 76, 022511. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q16.00011: Multichannel quantum defect theory model of Feshbach resonances Thomas Hanna, Eite Tiesinga, Paul Julienne Multichannel quantum defect theory (MQDT) has a large number of applications in atomic physics, including the properties of collisions near threshold. The key concept is that the short range physics can be accounted for very simply and then matched to the asymptotic long range interaction. We have developed a model of Feshbach resonances based on the ideas of MQDT. This model allows calculation of the magnetic fields at which resonances occur, as well as properties such as the resonance width and background scattering length. Apart from known atomic properties, only three input parameters are required: the singlet and triplet scattering lengths, and the coefficient of the long range van der Waals potential. Analytic reference functions defined by the potential [1] are used to calculate the long range properties, which are linked to the short range physics through a frame transformation. We apply our theory to $^{6}$Li-$^{40}$K scattering, and obtain good agreement with experimental data and full coupled channels calculations, but with far less computational effort. This makes MQDT a useful tool for investigating collisions of new combinations of species. [1] B. Gao \textit{et al.}, Phys. Rev. A \textbf{72}, 042719 (2005). [Preview Abstract] |
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