Bulletin of the American Physical Society
40th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 54, Number 7
Tuesday–Saturday, May 19–23, 2009; Charlottesville, Virginia
Session K1: Disorder in Ultracold Gases |
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Chair: Klaus Moelmer, Aarhus University Room: Chemistry Hall 402 |
Thursday, May 21, 2009 10:30AM - 11:00AM |
K1.00001: Anderson localization in interacting Bose-Einstein condensates Invited Speaker: We report the observation of exponential localization of a Bose-Einstein condensate (BEC) released into a one-dimensional waveguide in the presence of a controlled disorder created by laser speckle. We operate in a regime allowing Anderson Localization: i) weak disorder such that localization results from many quantum reflections of small amplitude; ii) atomic density small enough that interactions are negligible. We image directly the atomic density profiles vs time, and find that weak disorder can lead to the stopping of the expansion and to the formation of a stationary, exponentially localized wave function, a direct signature of AL. Fitting the exponential wings, we extract the localization length, and compare it to our theoretical calculations. Moreover we show that, in our one-dimensional speckle potential whose noise spectrum has a high spatial frequency cut-off, exponential localization occurs only when the de Broglie wavelengths of the atoms in the expanding BEC are larger than an effective mobility edge corresponding to that cut-off. In the opposite case, we find that the density profiles decay algebraically. The method presented here can be extended to localization of atomic quantum gases in higher dimensions, and with controlled interactions. [Preview Abstract] |
Thursday, May 21, 2009 11:00AM - 11:30AM |
K1.00002: Ultracold atoms in disordered optical lattices Invited Speaker: Fifty years ago, Nobel Prize winner P. W. Anderson predicted that the presence of disorder in a crystalline solid could stop the electronic flow, leading to a localization of the electrons and turning a metal into an insulator. We have use ultracold $^{39}$K atoms in disordered crystals made of laser light in order to quantum-simulate the propagation of electrons in a disordered solid [1]. By tuning the amount of disorder in the lattice we observe the transition from a conducting state to an insulator characterized by absence of diffusion and exponential localization, as originally conjectured by Anderson. The possibility to modify the collisions between the particles via magnetic Feshbach resonances opens to the experimental study of the localization transition in the presence of interactions, which still poses open problems in condensed-matter theory. We will discuss the state of the ongoing research at LENS and the possibility of studying novel quantum phases in different interaction regimes, also in connection with the development of new diagnostic techniques for quantum gases in optical lattices. \newline \newline [1] G. Roati et al., \it{Nature} \bf{453}, 895 (2008). [Preview Abstract] |
Thursday, May 21, 2009 11:30AM - 12:00PM |
K1.00003: Superfluid Transport Through Random Disorder Invited Speaker: Disorder plays an important role in the transport of particles in a variety of contexts, including electronic materials, granular superconductors, and liquid helium in porous media. We use optical speckle to create a disordered potential, and explore its effect on a Bose-Einstein condensate (BEC) of $^7$Li. The BEC presents a highly idealized environment, where most of the relevant parameters, such as the disorder strength and length scale, the interparticle interaction strength, and even the particle velocity may be precisely controlled. We have shown that the scattering length in $^7$Li can be controlled over a range of nearly 7 decades by using a Feshbach resonance with a shallow zero-crossing [S. E. Pollack \emph{et al.}, arXiv:0811.4456]. Of particular interest is the regime of Anderson localization where very weak interactions produce a condensate healing length that is comparable to the disorder length scale. We investigate superfluid transport by exciting dipole oscillations of the condensate through sudden displacement of the harmonic trapping potential. Weak disorder damps the dipole oscillations at a rate dependent upon disorder strength, initial velocity, and atomic interactions. We find a universal behavior in which the damping rate varies with disorder strength scaled to condensate chemical potential, and velocity scaled by the Landau critical velocity. We also study localization by suddenly removing the axial confining potential, allowing the condensate to freely expand in one-dimension in the presence of disorder. [Preview Abstract] |
Thursday, May 21, 2009 12:00PM - 12:30PM |
K1.00004: Disorder and strong correlations in optical lattices Invited Speaker: Remarkable experimental progress has recently allowed the creation of fine-grained optical disorder potentials, where localization effects of ultracold atoms can be clearly observed. Adding an optical lattice gives access to highly tunable quantum many-body systems with disorder. I will focus on the interplay between disorder and strong correlations from a theoretical perspective. Our approach is based on stochastic and dynamical mean-field theories for bosons and fermions. Specifically, I will discuss the following recent results: \newline \newline 1) We are able to describe the highly debated Bose glass phase in three spatial dimensions at any temperature and predict a direct transition between Mott insulator and superfluid [1]. \newline \newline 2) Spin-ordering of fermions (e.g. $^{40}$K) in an optical lattice is of current interest. We provide a complete phase diagram in the presence of disorder, including a new antiferromagnetic metal [2]. \newline \newline 3) We are now able to treat spinful bosons with strong correlations in a unified dynamical mean-field framework. For 2-component bosons (e.g. $^{41}K - ^{87}$Rb) mixtures) this yields a rich phase diagram, including anisotropic spin order and supersolid phases [3]. \newline \newline References: [1] U. Bissbort and W. Hofstetter, preprint arXiv:0804.0007 [2] K. Byczuk, W. Hofstetter, and D. Vollhardt, preprint arXiv: 0810.2958 [3] A. Hubener, M. Snoek, and W. Hofstetter, preprint. [Preview Abstract] |
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