News: Research Highlights

Tue January 1, 2008

Crystallization of Strongly Interacting Photons in a Nonlinear Optical Fibre

D. E. Chang, V. Gritsev, G. Morigi, V. Vuletic, M. D. Lukin, and E. Demler, Crystallization of Strongly Interacting Photons in a Nonlinear Optical Fibre, Nature Physics 4, 884-889 (2008).
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Research Highlights
Tue January 1, 2008

Theoretical Program

We have extended our studies of collisional processes in gases at ultracold and cold temperatures in researches that are responsive to experiments. We have calculated the 2Sg+ and  2Su+ potential curves of the molecular ion Yb2+ and we have obtained the cross-sections for charge exchange of its several isotopes of Yb 2 + in the parent...
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Tue January 1, 2008

Determination of the equation of state of a polarized Fermi gas at unitarity

Y. Shin, Determination of the equation of state of a polarized Fermi gas at unitarity, Phys. Rev. A 77, 041603 (2008).
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Tue January 1, 2008

Co-trapping of Cold NH Molecules with N Atoms

A frontier in physics with cold polar molecules is the study of paths toward collisional cooling. Here we co-trap an very low polarizability atom along with a very “atom-like” polar molecule. Earlier work with He colliding with NH found that the inelastic processes were highly suppressed due to the small of the NH ground state...
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Mon January 1, 2007

Characterization of topological states on a lattice with Chern number

M. Hafezi, A. S. Sorensen, M. D. Lukin, and E. Demler, Characterization of topological states on a lattice with Chern number, EPL 81, 10005 (2007).
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Research Highlights
Mon January 1, 2007

Spin-Exchange Collisions of Submerged Shell Atoms Below 1 Kelvin

Submerged shell atoms
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Mon January 1, 2007

Phase Sensitive Recombination of Two Bose-Einstein Condensates on an Atom Chip

Most
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Mon January 1, 2007

Tomographic RF Spectroscopy of a Trapped Fermi Gas at Unitarity

Experiments on
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Research Highlights
Mon January 1, 2007

Critical velocity for superfluid flow across the BEC-BCS crossover

One key property of a superfluid is the existence of a threshold (critical) velocity for dissipationless flow. Measuring this critical velocity not only unambiguously illustrates the superfluidity of the system, but furthermore provides important insight about the excitation mechanisms that eventually cause the breakdown of superfluidity.

 

In this paper we report on the measurement of critical velocities in an ultracold superfluid Fermi gas.  The experimental signature was very clear, an abrupt onset of dissipation, when the velocity of the moving lattice was increased past a critical value.

We determined critical velocities for variable atom-atom interactions magnetically tuned through the BEC-BCS crossover.  Most quantities describing the system vary monotonically across the BEC-BCS crossover, whereas the critical velocity shows a pronounced peak on resonance, which was clearly confirmed by our experiment.  The maximum in the value of the critical velocity is caused by the change of the dissipation mechanism (pair breaking vs. phonon excitation)  On the BCS side, the pairs become fragile and break more easily, whereas on the BEC side, the pairs are stable, but now the lower velocity of sound determines the critical velocity for the onset of dissipation.

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Research Highlights
Mon January 1, 2007

Determination of the Superfluid Gap in Atomic Fermi Gases by Quasiparticle Spectroscopy

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News type:
Research Highlights