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Physics World

Fermi gas goes superfluid

Published Jul 1, 2005• 0 citations
Publisher

Abstract

Physicists have found conclusive evidence for superfluidity-flow without resistrance in an ultracold Fermi gas. Although indirect evidence for superfluidityu in Fermi gases has been searching for definitive evidence in the form of quantized vortices in a rotating gas.Now Wolfgang Ketterle and colleagues at the Massachusetts Institute of Technology (MIT) have observed these vortices in a gas of lithium-6 atoms (Nature 435 1047). The results could shed new light on systems as dicerse as high-temperature superconductors and the quark-gluon plasma.

DOI
10.1088/2058-7058/18/7/1

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Physicists have found conclusive evidence for superfluidity-flow without resistrance in an ultracold Fermi gas. Although indirect evidence for superfluidityu in Fermi gases has been searching for definitive evidence in the form of quantized vortices in a rotating gas.Now Wolfgang Ketterle and colleagues at the Massachusetts Institute of Technology (MIT) have observed these vortices in a gas of lithium-6 atoms (Nature 435 1047). The results could shed new light on systems as dicerse as high-temperature superconductors and the quark-gluon plasma.
In our recent experiments at MIT, superfluidity has been observed in a strongly interacting atomic Fermi gas. These systems constitute a novel form of matter with model character: One can vary the temperature, density and dimensionality, the number of “spin up” versus “spin down” fermions, and, most remarkably, the interactions can be precisely controlled over an enormous range. This allows to study the crossover of a Bose-Einstein condensate of tightly bound molecules to a Bardeen-Cooper-Schrieffer superfluid of long-range Cooper pairs. Superfluidity in this crossover regime is demonstrated by setting the gas under rotation and observing ordered lattices of quantized vortices [1]. Thanks to its strong interactions, the gas is a high-temperature superfluid: Scaled to the density of electrons in a metal, superfluidity would occur already far above room temperature. A new regime is entered when the number of spin up versus spin down atoms is imbalanced. In this case, not every spin up atom can find a spin down partner. The ground state of such a system has been under debate for over 40 years. We observe the breakdown of superfluidity at a critical imbalance, the Chandrasekhar-Clogston limit [2]. The superfluid gives way to an intriguing, strongly-interacting Fermi gas with unequal spin populations [3, 4].
Summary form only given. Superfluidity is amongst the most impressive macroscopic manifestations of quantum behavior. Frictionless flow has been observed in several low-temperature quantum systems, such as liquid helium [1, 2], or more recently ultracold gases [3-5]. Several hallmarks of superfluidity such as critical velocity, quantized vortices, and second sound have been observed [6-11], in both liquid helium and ultracold gases, and in both Bose and Fermi systems.Here, we use the high level of tunability offered by ultracold gases to control their interactions and obtain a double Bose-Fermi superfluid of fermionic 6Li and bosonic ,Li. We have typically Nf = 3.5 . 105 fermionic atoms and Nb = 4 . 104 bosonic atoms at a temperature of about 100 nK, which is below the critical temperature for superfluidity for both the Bose and the Fermi cloud [13].
We study the expansion of a rotating, superfluid Fermi gas. The presence and absence of vortices in the rotating gas are used to distinguish the superfluid and normal parts of the expanding cloud. We find that the superfluid pairs survive during the expansion until the density decreases below a critical value. Our observation of superfluid flow in the expanding gas at 1/kFa=0 extends the range where fermionic superfluidity has been studied to densities of 1.2x10(11) cm(-3), about an order of magnitude lower than any previous study.