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Critical velocity for superfluid flow across the BEC-BCS crossover
The recent realization of theBEC-BCS crossover in ultracold atomic gases allows one to study how bosonicsuperfluidity transforms into fermionic superfluidity. Many quantities, such as the speed of soundand the transition temperature, vary monotonously through the crossover. In contrast, the critical velocity forsuperfluid flow has been predicted to show a pronounced maximum [1]. This maximum occurs at the transition from abosonic region where excitation of sound limits superfluid flow to afermionic region where pair breaking dominates.
By crossing two tightly focused laser beams, weexposed only the central region to a 1D moving optical lattice and couldobserve the response of the superfluid at a well-defined density. In this way, critical velocities wereobtained throughout the BEC-BCS crossover [2].
In good agreement with theoretical predictionswe found a pronounced peak of the critical velocity at unitarity which confirmsthat superfluidity is most robust for resonant atomic interactions. Thedependence of the critical velocity on lattice depth and on the inhomogeneousdensity profile was carefully studied.
Pictures:
(left) Onset ofdissipation for superfluid fermions in a moving optical lattice. Shown is thenumber of fermion pairs which remained in the condensate after being subjectedto a moving optical lattice at variable velocity. The abrupt onset of dissipation occurred at the criticalvelocity.
(right) Critical velocities throughout the BEC-BCS crossover. A pronounced maximumwas found at resonance. Data are shown for a lattice with a depth of 0.2 EF deep lattice. Thesolid line is a guide to the eye.
1. R.Combescot, M.Y. Kagan, and S. Stringari, Collective mode of homogeneoussuperfluid Fermi gases in the BEC-BCS crossover, Physical Review A (Atomic,Molecular, and Optical Physics) 74, 042717 (2006).
2. D.E.Miller, J.K. Chin, C.A. Stan, Y. Liu, W. Setiawan, C. Sanner, and W. Ketterle,Critical Velocity for Superfluid Flow across the BEC-BCS Crossover, Phys. Rev.Lett. 99, 070402 (2007).