Phase Coherence and Superfluid-Insulator Transition in a Disordered Bose-Einstein Condensate
by: Yong P. Chen, J. Hitchcock, D. Dries, M. Junker, C. Welford, R. G. Hulet
Abstract: We have studied the effects of a disordered optical potential on the transport and phase coherence of a Bose-Einstein condensate (BEC) of 7Li atoms. At moderate disorder strengths (V_D), we observe inhibited transport and damping of dipole excitations, while in time-of-flight images, random but reproducible interference patterns are observed. The interference reflects phase coherence in the disordered BEC and is interpreted as speckle for matter waves. At higher V_D, the interference contrast diminishes as the BEC fragments into multiple pieces with little phase coherence.
arXiv:0710.5187v1 [cond-mat.other]
Non-equilibrium coherence dynamics in one-dimensional Bose gases
Authors: S. Hofferberth1,2, I. Lesanovsky3, B. Fischer1, T. Schumm2 & J. Schmiedmayer1,2
Abstract: Low-dimensional systems provide beautiful examples of many-body quantum physics1. For one-dimensional (1D) systems2, the Luttinger liquid approach3 provides insight into universal properties. Much is known of the equilibrium state, both in the weakly4, 5, 6, 7 and strongly8, 9 interacting regimes. However, it remains a challenge to probe the dynamics by which this equilibrium state is reached10. Here we present a direct experimental study of the coherence dynamics in both isolated and coupled degenerate 1D Bose gases. Dynamic splitting is used to create two 1D systems in a phase coherent state11. The time evolution of the coherence is revealed through local phase shifts of the subsequently observed interference patterns. Completely isolated 1D Bose gases are observed to exhibit universal sub-exponential coherence decay, in excellent agreement with recent predictions12. For two coupled 1D Bose gases, the coherence factor is observed to approach a non-zero equilibrium value, as predicted by a Bogoliubov approach13. This coupled-system decay to finite coherence is the matter wave equivalent of phase-locking two lasers by injection. The non-equilibrium dynamics of superfluids has an important role in a wide range of physical systems, such as superconductors, quantum Hall systems, superfluid helium and spin systems14, 15, 16. Our experiments studying coherence dynamics show that 1D Bose gases are ideally suited for investigating this class of phenomena.
http://www.nature.com/nature/journal/v449/n7160/full/nature06149.html
Nature 449, 324-327 (20 September 2007)
Tonks–Girardeau gas of ultracold atoms in an optical lattice
Authors: Belén Paredes, Artur Widera, Valentin Murg, Olaf Mandel, Simon Fölling, Ignacio Cirac, Gora V. Shlyapnikov, Theodor W. Hänsch and Immanuel Bloch
Abstract: Strongly correlated quantum systems are among the most intriguing and fundamental systems in physics. One such example is the Tonks–Girardeau gas1, 2, proposed about 40 years ago, but until now lacking experimental realization; in such a gas, the repulsive interactions between bosonic particles confined to one dimension dominate the physics of the system. In order to minimize their mutual repulsion, the bosons are prevented from occupying the same position in space. This mimics the Pauli exclusion principle for fermions, causing the bosonic particles to exhibit fermionic properties1, 2. However, such bosons do not exhibit completely ideal fermionic (or bosonic) quantum behaviour; for example, this is reflected in their characteristic momentum distribution3. Here we report the preparation of a Tonks–Girardeau gas of ultracold rubidium atoms held in a two-dimensional optical lattice formed by two orthogonal standing waves. The addition of a third, shallower lattice potential along the long axis of the quantum gases allows us to enter the Tonks–Girardeau regime by increasing the atoms’ effective mass and thereby enhancing the role of interactions. We make a theoretical prediction of the momentum distribution based on an approach in which trapped bosons acquire fermionic properties, finding that it agrees closely with the measured distribution.
Nature 429, 277-281 (20 May 2004)
http://www.nature.com/nature/journal/v429/n6989/abs/nature02530.html
Phase Coherence of an Atomic Mott Insulator
Authors: Fabrice Gerbier, Artur Widera, Simon Fölling, Olaf Mandel, Tatjana Gericke, and Immanuel Bloch
Abstract: We investigate the phase coherence properties of ultracold Bose gases in optical lattices, with special emphasis on the Mott insulating phase. We show that phase coherence on short length scales persists even deep in the insulating phase, preserving a finite visibility of the interference pattern observed after free expansion. This behavior can be attributed to a coherent admixture of particle-hole pairs to the perfect Mott state for small but finite tunneling. In addition, small but reproducible kinks are seen in the visibility, in a broad range of atom numbers. We interpret them as signatures for density redistribution in the shell structure of the trapped Mott insulator.
Phys. Rev. Lett. 95, 050404 (2005)
http://link.aps.org/abstract/PRL/v95/e050404
Formation of Spatial Shell Structure in the Superfluid to Mott Insulator Transition
Authors: Simon Fölling, Artur Widera, Torben Müller, Fabrice Gerbier, and Immanuel Bloch
Abstract: We report on the direct observation of the transition from a compressible superfluid to an incompressible Mott insulator by recording the in-trap density distribution of a Bosonic quantum gas in an optical lattice. Using spatially selective microwave transitions and spin-changing collisions, we are able to locally modify the spin state of the trapped quantum gas and record the spatial distribution of lattice sites with different filling factors. As the system evolves from a superfluid to a Mott insulator, we observe the formation of a distinct shell structure, in good agreement with theory.
http://link.aps.org/abstract/PRL/v97/e060403
Phys. Rev. Lett. 97, 060403 (2006)
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