Bulletin of the American Physical Society
39th Annual Meeting of the APS Division of Atomic, Molecular, and Optical Physics
Volume 53, Number 7
Tuesday–Saturday, May 27–31, 2008; State College, Pennsylvania
Session I1: New Physics with Ultracold Atoms |
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Chair: Charles Clark, National Institute of Standards and Technology Room: Nittany Lion Inn Ballroom CDE |
Thursday, May 29, 2008 8:00AM - 8:36AM |
I1.00001: Damped Bloch Oscillations of Bose-Einstein Condensates in Disordered Potential Gradients Invited Speaker: We investigate both experimentally and theoretically disorder induced damping of Bloch oscillations of Bose-Einstein condensates in 1D optical lattices. Particles in periodic potentials subjected to an external force will undergo an oscillatory motion instead of a linear acceleration. A comparison with solid state systems, where scattering at imperfections of the crystal structure leads to a strong damping of these Bloch oscillations, gives rise to the question how the controlled addition of disorder to an optical lattice will affect the dynamics of particles in such systems. In our experiments the disorder is realised by a combination of a spatially inhomogeneous optical potential and a magnetic gradient. We show that this inhomogeneous force causes a broadening of the quasimomentum spectrum, which in turn results in a damping of the centre-of-mass oscillation. Good quantitative agreement of the experimental damping rates and the simulations using the Gross-Pitaevskii equation is obtained. Our results are relevant for high precision experiments on very small forces, which require the observation of a large number of oscillation cycles. Therefore a detailed quantitative understanding of the effect of the disorder and the underlying mechanism of the damping is important for such applications. [Preview Abstract] |
Thursday, May 29, 2008 8:36AM - 9:12AM |
I1.00002: Few-body physics of trapped unequal mass fermions Invited Speaker: The behavior of a two-component dilute Fermi gas exhibits an interesting dependence on the mass ratio between the two species. Our study tackles this system with 3-20 particles, using two independent techniques. First, an essentially exact diagonalization for 3-6 particles determines both the ground state and also the pattern of excited state energies, and our analysis permits an extraction of the dimer-dimer scattering length and effective range. Secondly, the nature of the system ground state is studied as a function of the mass ratio and the number of particles, up to N=20, using fixed-node diffusion Monte Carlo (DMC) techniques. By using two different solution techniques in their overlapping range of applicability from N=3-6, we are able to assess the accuracy of the nodal surface employed in the fixed-node DMC calculation. Physical properties such as the excitation gap will be analyzed over this range of particle number, and the intriguing unitarity limit is also considered. [Preview Abstract] |
Thursday, May 29, 2008 9:12AM - 9:48AM |
I1.00003: Pairing and superfluidity in a gas of strongly interacting fermions Invited Speaker: Ultracold quantum degenerate Fermi gases provide a remarkable opportunity to study superfluidity with the control and precision of atomic physics. We have studied a two-component gas of lithium-6 atoms for various strengths of the interactions and population imbalances and obtained the phase diagram for the superfluid and normal phases. Below a tricritical temperature, the first order phase transition is observed through a discontinuity of the density at the superfluid-to-normal phase boundary. Insight into the nature of fermionic pairing is obtained through RF spectroscopy. In the superfluid system studied previously which consists of atoms in the two lowest hyperfine states (the 1-2 mixture), RF spectra could not be consistently interpreted due to strong final state interactions. We find that a 1-3 mixture is stable and yields clean pair dissociation spectra from which we obtain the size of the superfluid pairs. The value of 1.4/k$_{F}$ is smaller than the interparticle spacing and constitutes the smallest pair size observed in any fermionic superfluids. This work was done in collaboration with Yong-Il Shin, Christian Schunck, and Andre Schirotzek. [Preview Abstract] |
Thursday, May 29, 2008 9:48AM - 10:24AM |
I1.00004: Population dynamics in a sodium spinor condensate Invited Speaker: Spinor condensates provide an accessible way to study the dynamics of a quantum system. A number of interesting studies have been published of spin population dynamics and domain formation in ferromagnetic Rb BECs, and we have begun to examine the antiferromagnetic case of Na. Na atoms are condensed in the F=1 state in a tight optical trap which holds the different spin projections equally well. For Na the spatial wavefunction of the BEC can be treated as ``single mode,'' and there is no phase separation of the spin projections. This allows a description with a single spatial wavefunction and an independent ``spinor'' wavefunction containing the spin variables. The interesting collisions are ones that couple two m=0 atoms to one m=+1 and one m=-1 atom. The linear Zeeman shift cancels and the interesting interactions are simply the quadratic Zeeman shift and the difference between the scattering lengths for the different spin states. Na is antiferromagnetic inasmuch as the m=+1/m=-1 collision has a lower energy state than the collision of two atoms in the same spin state. If the populations are initialized to a non-equilibrium state a collisional exchange takes place, leading to oscillations in the spin populations. The collisional interaction tends to cancel the quadratic Zeeman interaction in an antiferromagnetic system, leading to a divergence in the oscillation period near a critical magnetic field. We observe population oscillations and see signs of this divergence. We measure a phase diagram for the equilibrium populations as a function of magnetic field and magnetization (the difference in population between spin +1 and spin -1) that shows signs of a zero-temperature quantum phase transition. We have begun a series of experiments to observe the population dynamics through Faraday rotation spectroscopy. This is a less-destructive means of observing the system and should allow us to both observe and control the dynamics of the system. [Preview Abstract] |
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