Signatures of superfluidity in atomic Fermi gases

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Doctoral thesis (article-based)
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Helsinki University of Technology Laboratory of Computational Engineering publications. Report B, 34
After the experimental realization of Bose-Einstein condensation in dilute gases of alkali atoms, experimentalists started to trap the fermionic isotopes. The degenerate state for fermions was reported in 1999. The main objective of these experiments is to obtain superfluidity of fermionic gases. When there are attractive interactions between the fermions, the Fermi sea becomes unstable with respect to the formation of atomic Cooper pairs and the system becomes a superfluid. It turns out that the existing experimental cooling techniques allow minimum temperatures for fermions of the order of the Fermi temperature. Using Feshbach resonances induced by magnetic fields enhances the effective interactions between the atoms leading to superfluid transition temperatures of the order of the Fermi energy. This is a completely new regime of fermionic superfluidity far from the BCS superconductors, 3He and even high-Tc superconductors. The achievement of superfluidity on gases of fermionic alkalis is currently being pursued in many experimental groups. In this thesis, different signatures of the superfluid transition have been considered. The use of almost on-resonant laser light for coupling between the different internal states of the atoms as a method for probing superfluidity has been analyzed. Coupling between the paired states has been proposed as a way to directly detect the Cooper pair size. The Josephson effect, related to the phases of two coupled superfluids, is shown to present an asymmetry when the internal states of the atoms forming the pairs are coupled with different detunings. Vortices, intimately related to superfluidity, have also been considered. The single vortex solution of the Ginzburg-Landau equation for the superfluid order parameter has been numerically computed and a new vortex core size reflecting the trapping geometry has been obtained. Bloch oscillations have been analyzed for fermionic atoms both in the degenerate regime and in the superfluid regime. Superfluidity is found to supress the amplitude of the Bloch oscillations.
superfluidity, Fermi gases, quantum gases, Bose-Einstein condensation, vortices
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  • Bruun G. M., Törmä P., Rodríguez M. and Zoller P., 2001. Laser probing of Cooper-paired trapped atoms. Physical Review A 64, pages 033609 : 1-10. [article1.pdf] © 2001 American Physical Society. By permission.
  • Rodríguez M. and Törmä P., 2002. Laser-induced collective excitations in a two-component Fermi gas. Physical Review A 66, pages 033601 : 1-6. [article2.pdf] © 2002 American Physical Society. By permission.
  • Paraoanu Gh.-S., Rodríguez M. and Törmä P., 2001. Cooper pair coherence in a superfluid Fermi gas of atoms. Journal of Physics B: Atomic, Molecular and Optical Physics 34, No. 23, pages 4763-4773. [article3.pdf] © 2001 Institute of Physics Publishing Ltd. By permission.
  • Paraoanu Gh.-S., Rodríguez M. and Törmä P., 2002. Josephson effect in superfluid atomic Fermi gases. Physical Review A 66, pages 041603 (R) : 1-4. [article4.pdf] © 2002 American Physical Society. By permission.
  • Rodríguez M., Paraoanu G.-S. and Törmä P., 2001. Vortices in trapped superfluid Fermi gases. Physical Review Letters 87, No. 10, pages 100402 : 1-4. [article5.pdf] © 2001 American Physical Society. By permission.
  • Rodríguez M. and Törmä P., Bloch oscillations in Fermi gases. Physical Review A, submitted for publication. [article6.pdf] © 2003 by authors and © 2003 American Physical Society. By permission.
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