Dipol Fermi Gases

• Applicant: Privatdozent Dr. Axel Pelster, since 8/2016
• Subject Area: Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: from 2010 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 187762277

• Final Report Year: 2018

Final Report Abstract

The quest for quantum degenerate Fermi gases interacting through the anisotropic and longrange dipole-dipole interaction (DDI) represents at present an exciting and fast developing branch within the cold-atoms research program. The challenge is here to identify many-body dipolar effects in Fermi gases as they energetically compete with the large kinetic energy at and below the Fermi surface (FS). Recently the group of Francesca Ferlaino in Innsbruck observed in a dipolar Fermi gas experiment with erbium atoms that the corresponding FS is deformed from a sphere to an ellipsoid due to the DDI. Moreover, it was suggested that, when the dipoles are rotated by means of an external field, the Fermi surface follows their rotation, thereby keeping the major axis of the momentum-space ellipsoid parallel to the dipoles. In order to understand such experimental results, this DFG project focused upon working out a mean-field theory which describes both static and dynamic non-superfluid properties for dipolar Fermi gases. To this end we considered the most general geometry of a confinement in an elongated triaxial trap with an arbitrary orientation of the dipoles relative to the trap. In particular, we managed to show how the ellipsoidal FS deformation can be reconstructed, assuming ballistic expansion, from the experimentally measurable real-space aspect ratio after a free expansion. In a common publication with the Innsbruck group we compared our theoretical results with new experimental data measured with an erbium Fermi gas for various trap parameters and dipole orientations. The observed remarkable agreement demonstrates the ability of our mean-field theory to capture the full angular dependence of the FS deformation. Moreover, for systems with even higher dipole moment, our theory predicts an additional unexpected effect: the FS does not simply follow rigidly the orientation of the dipoles but softens showing a change in the aspect ratio depending on the dipoles’ orientation relative to the trap geometry, as well as on the trap anisotropy itself. With this our theory paths the way for analyzing future experiments with quantum degenerate heteronuclear fermionic molecules. But our theory provides also the basis for understanding and interpreting phenomena in future experiments with magnetic dipolar fermionic atoms, where the investigated physics depends on the underlying structure of the Fermi surface, such as fermionic pairing and superfluidity.

Publications

• Ground State of an Ultracold Fermi Gas of Tilted Dipoles
  V. Veljić, A.R.P. Lima, L. Chomaz, S. Baier, M.J. Mark, F. Ferlaino, A. Pelster, and A. Balaž
  
• Collective Motion of Polarized Dipolar Fermi Gases in the Hydrodynamic Regime; Physical Review A 81, 021606(R)/1-4 (2010)
  A.R.P. Lima and A. Pelster
  
• Dipolar Fermi Gases in Anisotropic Traps; Physical Review A 81, 063629/1-15 (2010)
  A.R.P. Lima and A. Pelster
  
• Low-Lying Excitation Modes of Trapped Dipolar Fermi Gases: From Collisionless to Hydrodynamic Regime; Physical Review A 96, 043608/1-16 (2017)
  F. Wächtler, A.R.P. Lima, and A. Pelster
  
• Time-of-Flight Expansion of Trapped Dipolar Fermi Gases: From the Collisionless to the Hydrodynamic Regime; Physical Review A 95, 053635/1-17 (2017)
  V. Veljić, A. Balaž, and A. Pelster