News: 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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Mon January 1, 2007

A Bright, Guided Molecular Beam With Hydrodynamic Enhancement

Molecular beams form the technological basis for a wide range of chemical and physical studies. We report here an advance in producing cold beams of atoms and molecules with high intensity. This method is general in that it uses helium, an inert gas, to cool species, even refractory and radical molecules. Cold molecular oxygen and...
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Mon January 1, 2007

Matter-Wave Interferometry with Phase Fluctuating Bose-Einstein Condensates

A non-interacting zero-temperature Bose-Einstein condensate is the matter-wave analogue to the optical laser, and therefore the ideal atom source for matter-wave interferometry.  However, at finite temperature elongated condensates (e.g. in wave guides) suffer from phase fluctuations.
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