Superfluidity of Spin-Polarized6Li

Abstract

We study the prospects for observing superfluidity in a spin-polarized atomic gas of 6 Li atoms, using state-of-the-art interatomic potentials. We determine the spinodal line and show that a BCS transition to the superfluid state can indeed occur in the (meta)stable region of the phase diagram if the densities are sufficiently low. We also discuss the stability of the gas due to exchange and dipolar relaxation and conclude that the prospects for observing superfluidity in a magnetically trapped atomic 6 Li gas are particularly promising for magnetic bias fields larger than 10 T. PACS numbers: 03.75.Fi, 32.80.Pj, 42.50.Vk Ultracold atomic gases have received much attention in recent years, because of their novel properties. For instance, these gases are well suited for high-precision measurements of single-atom properties and for the observation of collisional and optical phenomena that reflect the (Bose or Fermi) statistics of the constituent particles. Moreover, a large variety of experimental techniques are available to manipulate the atomic gas samples by means of electromagnetic fields, which offers the exciting possibility to achieve the required conditions for quantum degeneracy and to study macroscopic quantum effects in their purest form.At present, most experimental attempts towards quantum degeneracy have been performed with bosonic gases and have been aimed at the achievement of Bose-Einstein condensation. In particular, most of the earlier experiments used atomic hydrogen [1,2]. These experiments provided crucial ingredients for the recent attempts with alkali vapors, for which the experimental advances towards the degeneracy regime were so rapid that Bose-Einstein condensation has actually been reported now for the isotopes 87 Rb [3] and 7 Li [4].In view of these exciting developments it seems timely to investigate theoretically also the properties of spinpolarized atomic 6 Li, since 6 Li is a stable fermionic isotope of lithium that can be trapped and cooled in much the same way as its bosonic counterpart. Therefore, magnetically trapped 6 Li promises to be an ideal system to study degeneracy effects in a weakly interacting Fermi gas, thus providing valuable complementary information on the workings of quantum mechanics at the macroscopic level. Moreover, using a combination of theoretical analysis and experimental results [5][6][7], accurate knowledge of the interparticle (singlet and triplet) potential curves of lithium have recently been obtained, which lead to the prediction of a large and negative s-wave scattering length a of 24.6 3 10 3 a 0 (a 0 is the Bohr radius) for a spin-polarized 6 Li gas. This is important for two reasons: First, the fact that the scattering length is negative implies that at the low temperatures of interest [L ¿ r V , where L ͑2ph 2 ͞mk B T͒ 1͞2 is the thermal de Broglie wavelength of the atoms and r V is the range of the interaction] the effective interaction between the lithium atoms is attractive, and we expect a BCS-like phase transition to a superfluid state ...

Langevin equations for interacting fermions and Cooper-like pairing in trapped one-dimensional fermions

“…Observation of Cooper-like momentum correlations would, therefore, be a direct manifestation of both the quantum and the fermionic nature of the trapped atoms. Experimentally this may be achieved by first trapping 6 Li atoms in a cigar-shaped trap and then turning the trap off, thus allowing the atoms to propagate freely. The Cooper-like momentum correlations should then manifest themselves as correlations in arrival times, assuming that atom detectors are placed at opposite sides of the trap along the trap axis.…”

Laser cooling of trapped Fermi gases far below the Fermi temperature

“…In the context of these experiments it has been recently proposed [5] that a wellknown effect in superconductivity (see e.g. [6]), the BCS transition, could be observable in atomic Fermi gases.…”

Collisional relaxation in a fermionic gas

“…For simplicity we have considered here a non-interacting Fermi gas, but it is clear that this method can be extended to study the effects of interactions onto the fermionic excitation spectrum. In particular we plan to address the consequences of Cooper pairing of the fermions [5,23,24,25] in a subsequent paper.…”

“…The most striking example is the Bose-Einstein condensation of alkali vapors [1,2,3] and more recently, of an atomic hydrogen gas [4]. These techniques can also be applied to fermionic samples, in which spectacular phenomena such as Cooper pairing of atoms [5], or inhibition of the spontaneous emission of an excited atom [6] have been predicted. In this article, we present a simple and powerful tool to study the degeneracy parameters of such a fermionic sample, which we consider ideal for simplicity.…”