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International Journal of Modern Physics B

SUPERFLUID REGIMES IN DEGENERATE ATOMIC FERMI GASES

Gora V. Shlyapnikov

Published Jul 30, 2006• 0 citations
Publisher

Abstract

We give a brief overview of recent studies of quantum degenerate regimes in ultracold Fermi gases. The attention is focused on the regime of Bose-Einstein condensation of weakly bound molecules of fermionic atoms, formed at a large positive scattering length for the interspecies atom-atom interaction. We analyze remarkable collisional stability of these molecules and draw prospects for future studies.

DOI
10.1142/s0217979206035242

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Quantum kinetic theory for ultracold dipolar Fermi gases

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Interactions between particles play an important role in quantum degenerate gases.\nIn fact, the system’s behavior is usually largely determined by the strength, range, and\nsymmetry of the interactions, even if very weak. For more than two decades after the\nfirst experimental realization of a Bose-Einstein condensate (BEC) in 1995, investigations of ultracold atomic gases have mainly considered contact-type interactions, which\nmodel the short-range van der Waals interactions between the atoms in terms of a single\nparameter, the s-wave scattering length. However, some atomic or molecules species\npossess a magnetic or electric dipole moment and additionally interact among each\nother via the dipole-dipole interaction (DDI), which is long-ranged and anisotropic.\nSignatures of the DDI have been first observed in a BEC of chromium atoms in 2005.\nIn the last several years, a new class of atoms from the lanthanide series, e.g., dysprosium and erbium, with stronger dipolar properties, have come into play. This permits\na more full-fledged study of dipolar effects in BECs. Furthermore, ultracold molecules\nand highly excited Rydberg atoms are other promising examples for futures studies of\nsystems where dipolar interaction will be even more dominant, but they still need experimental efforts to achieve the desired quantum regime. Whereas the weak DDI in a\nchromium BEC is well described with the Gross-Pitaevskii equation, stronger dipolar\ninteractions certainly necessitate a beyond-mean-field description within a Bogoliubov\ntheory.\nIn parallel to research on bosons, cold fermions also trigger great interest because\nthey are actually often found in nature, e.g., within the electron gas of metals, as\nneutrons of heavy stars, or as quarks in plasmas as constituent elements of composite particles. Contrary to Bose systems, quantum degeneracy in experiments is much\nmore difficult to achieve in Fermi systems, since s-wave scattering is absent between\nidentical fermions, due to the Pauli exclusion principle. It was first achieved in 1999\nfor alkali atoms using a technique based on the sympathetic cooling with a second spin\nstate, another species, or a different isotope. For highly magnetic atoms, quantum degeneracy was achieved for dysprosium in 2012 using the standard sympathetic cooling\ntechnique, while in 2014 erbium was cooled deep into the degenerate regime using a\ndirect evaporative cooling mediated by dipolar collisions. This direct cooling scheme\nivallows producing very dense Fermi gas down to 10% of the Fermi temperature.\nFurthermore, a novel kind of strongly dipolar quantum gases became available in\nthe last several years. These are weakly bound polar molecules produced from atoms\nwith large magnetic dipole moments, such as erbium. Only very recently a quantum\ndegenerate dipolar Fermi gas of potassium-rubidium molecules has been experimentally realized. Unlike previously considered magnetic systems, heteronuclear molecules\nsuch as potassium-rubidium possess large electric dipole moments. The next natural\nstep is to polarize this system in a preferential direction by an external electric field,\nsuch that the DDI dominates the behavior of the system, which would open up the\nrealm for experimental investigation of strong dipolar Fermi gases...

•Jun 27, 2019•0 citations
We give a brief overview of recent studies of quantum degenerate regimes in ultracold Fermi gases. The attention is focused on the regime of Bose-Einstein condensation of weakly bound molecules of fermionic atoms, formed at a large positive scattering length for the interspecies atom-atom interaction. We analyze remarkable collisional stability of these molecules and draw prospects for future studies.
The use of Feshbach resonances for tuning the interparticle interaction in ultracold Fermi gases has led to remarkable developments, in particular to the creation and Bose-Einstein condensation of weakly bound diatomic molecules of fermionic atoms. These are the largest diatomic molecules obtained so far, with a size of the order of thousands of angstroms. They represent novel composite bosons, which exhibit features of Fermi statistics at short intermolecular distances. Being highly excited, these molecules are remarkably stable with respect to collisional relaxation, which is a consequence of the Pauli exclusion principle for identical fermionic atoms. The purpose of this review is to introduce theoretical approaches and describe the physics of molecular regimes in two-component Fermi gases and Fermi-Fermi mixtures, focusing attention on quantum statistical effects.
The use of Feshbach resonances for tuning the interparticle interaction in ultracold Fermi gases has led to remarkable developments, in particular to the creation and Bose-Einstein condensation of weakly bound diatomic molecules of fermionic atoms.These are the largest diatomic molecules obtained so far, with a size of the order of thousands of angstroms.They represent novel composite bosons, which exhibit features of Fermi statistics at short intermolecular distances.Being highly excited, these molecules are remarkably stable with respect to collisional relaxation, which is a consequence of the Pauli exclusion principle for identical fermionic atoms.The purpose of this review is to introduce theoretical approaches and describe the physics of molecular regimes in two-component Fermi gases and Fermi-Fermi mixtures, focusing attention on quantum statistical effects.
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.
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].
Interactions between particles play an important role in quantum degenerate gases.\nIn fact, the system’s behavior is usually largely determined by the strength, range, and\nsymmetry of the interactions, even if very weak. For more than two decades after the\nfirst experimental realization of a Bose-Einstein condensate (BEC) in 1995, investigations of ultracold atomic gases have mainly considered contact-type interactions, which\nmodel the short-range van der Waals interactions between the atoms in terms of a single\nparameter, the s-wave scattering length. However, some atomic or molecules species\npossess a magnetic or electric dipole moment and additionally interact among each\nother via the dipole-dipole interaction (DDI), which is long-ranged and anisotropic.\nSignatures of the DDI have been first observed in a BEC of chromium atoms in 2005.\nIn the last several years, a new class of atoms from the lanthanide series, e.g., dysprosium and erbium, with stronger dipolar properties, have come into play. This permits\na more full-fledged study of dipolar effects in BECs. Furthermore, ultracold molecules\nand highly excited Rydberg atoms are other promising examples for futures studies of\nsystems where dipolar interaction will be even more dominant, but they still need experimental efforts to achieve the desired quantum regime. Whereas the weak DDI in a\nchromium BEC is well described with the Gross-Pitaevskii equation, stronger dipolar\ninteractions certainly necessitate a beyond-mean-field description within a Bogoliubov\ntheory.\nIn parallel to research on bosons, cold fermions also trigger great interest because\nthey are actually often found in nature, e.g., within the electron gas of metals, as\nneutrons of heavy stars, or as quarks in plasmas as constituent elements of composite particles. Contrary to Bose systems, quantum degeneracy in experiments is much\nmore difficult to achieve in Fermi systems, since s-wave scattering is absent between\nidentical fermions, due to the Pauli exclusion principle. It was first achieved in 1999\nfor alkali atoms using a technique based on the sympathetic cooling with a second spin\nstate, another species, or a different isotope. For highly magnetic atoms, quantum degeneracy was achieved for dysprosium in 2012 using the standard sympathetic cooling\ntechnique, while in 2014 erbium was cooled deep into the degenerate regime using a\ndirect evaporative cooling mediated by dipolar collisions. This direct cooling scheme\nivallows producing very dense Fermi gas down to 10% of the Fermi temperature.\nFurthermore, a novel kind of strongly dipolar quantum gases became available in\nthe last several years. These are weakly bound polar molecules produced from atoms\nwith large magnetic dipole moments, such as erbium. Only very recently a quantum\ndegenerate dipolar Fermi gas of potassium-rubidium molecules has been experimentally realized. Unlike previously considered magnetic systems, heteronuclear molecules\nsuch as potassium-rubidium possess large electric dipole moments. The next natural\nstep is to polarize this system in a preferential direction by an external electric field,\nsuch that the DDI dominates the behavior of the system, which would open up the\nrealm for experimental investigation of strong dipolar Fermi gases...