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Science

The Quest for Superfluidity in Fermi Gases

Лев П. Питаевский, S. Stringari

Published Dec 12, 2002• 28 citations
Publisher

Abstract

Studies of ultracold atomic gases have focused on bosons--fundamental particles that have an integer spin and can form Bose-Einstein condensates. In their Perspective, Pitaevskii and Stringari discuss recent studies of the quantum behavior of fermion gases (with noninteger spin) at ultracold conditions. They highlight the report by O9Haraet al., whose realization and characterization of an ultracold Fermi gas sheds light on its quantum properties.

DOI
10.1126/science.1080087

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Studies of ultracold atomic gases have focused on bosons--fundamental particles that have an integer spin and can form Bose-Einstein condensates. In their Perspective,
Pitaevskii and Stringari
discuss recent studies of the quantum behavior of fermion gases (with noninteger spin) at ultracold conditions. They highlight the report by
O9Haraet al., whose realization and characterization of an ultracold Fermi gas sheds light on its quantum properties.
Abstract This volume introduces the basic concepts of Bose–Einstein condensation and superfluidity. It makes special reference to the physics of ultracold atomic gases; an area in which enormous experimental and theoretical progress has been achieved in the last twenty years. Various theoretical approaches to describing the physics of interacting bosons and of interacting Fermi gases, giving rise to bosonic pairs and hence to condensation, are discussed in detail, both in uniform and harmonically trapped configurations. Special focus is given to the comparison between theory and experiment, concerning various equilibrium, dynamic, thermodynamic, and superfluid properties of these novel systems. The volume also includes discussions of ultracold gases in dimensions, quantum mixtures, and long-range dipolar interactions.
The quantum mechanical phenomenon of Bose-Einstein condensation appears throughout physics with manifestations ranging from superconductivity in metals to superfluidity in nuclei and neutron stars. In most cases the condensation emerges due to correlations or pairing of fermionic particles. We explore this quantum behavior in a uniquely controllable and clean system consisting of an ultracold gas of atoms. In particular, by tuning the interactions between the fermionic atoms that make up our ultracold gas we can create condensates consisting of pairs of correlated atoms. These experiments introduce the ability to access the intriguing crossover regime where the quantum behavior of fermions becomes intertwined with that of bosons.
Physicists are getting closer and closer to the holy grail of research into ultracold atomic gases – the observation of superfluidity in a Fermi gas. Research groups in Austria and the US have recently taken a major step towards this goal by creating a Bose–Einstein condensate of bosonic molecules from a gas of fermionic atoms. If the atoms can be persuaded to form weakly bound pairs, rather than molecules, physicists hope to learn more about the basic mechanisms responsible for superconductivity.
The paper gives an overview of the recent developments in the field of ultracold fermionic quantum gases. In particular, results obtained on an ultracold spin-mixture of /sup 6/Li atoms in a series of experiments performed in Innsbruck is presented . Bose-Einstein condensation of strongly bound atom pairs (molecules) , the reversible crossover to a degenerate Fermi gas, measurements of collective excitation modes, and spectroscopic observation of pairing in a strongly interacting Fermi gas of atoms are also discussed. The experimental results provide strong evidence for superfluidity.
The physics of quantum degenerate atomic Fermi gases in uniform as well as in harmonically trapped configurations is reviewed from a theoretical perspective. Emphasis is given to the effect of interactions that play a crucial role, bringing the gas into a superfluid phase at low temperature. In these dilute systems, interactions are characterized by a single parameter, the sss-wave scattering length, whose value can be tuned using an external magnetic field near a broad Feshbach resonance. The BCS limit of ordinary Fermi superfluidity, the Bose-Einstein condensation (BEC) of dimers, and the unitary limit of large scattering length are important regimes exhibited by interacting Fermi gases. In particular, the BEC and the unitary regimes are characterized by a high value of the superfluid critical temperature, on the order of the Fermi temperature. Different physical properties are discussed, including the density profiles and the energy of the ground-state configurations, the momentum distribution, the fraction of condensed pairs, collective oscillations and pair-breaking effects, the expansion of the gas, the main thermodynamic properties, the behavior in the presence of optical lattices, and the signatures of superfluidity, such as the existence of quantized vortices, the quenching of the moment of inertia, and the consequences of spin polarization. Various theoretical approaches are considered, ranging from the mean-field description of the BCS-BEC crossover to nonperturbative methods based on quantum Monte Carlo techniques. A major goal of the review is to compare theoretical predictions with available experimental results.
Ever since the discovery of Bose-Einstein condensation in an atomic Bose gas in 1995, physicists have been searching for its analog, the fermion superfluid in atomic Fermi gases. Fermion superfluids are expected to exhibit quantum phenomena on the macroscopic scale, just as Bose-Einstein condensates do. Making use of a process called Feshbach resonance, which allows the interaction between atoms to be increased strongly, several groups have recently reported evidence on fermion superfluids. In his Perspective, [Ho][1] highlights the report by [ Chin et al .][2], who have directly observed a pairing gap, a key property of a fermion superfluid. Their interpretation is strongly supported by the theoretical calculation of [ Kinnunen et al .][3] Ho argues that the evidence accumulated so far provides an overwhelming case for the realization of fermion superfluids. [1]: http://www.sciencemag.org/cgi/content/full/305/5687/1114 [2]: http://www.sciencemag.org/cgi/content/short/305/5687/1128 [3]: http://www.sciencemag.org/cgi/content/short/305/5687/1131