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MesoFermi Report Summary

Project ID: 335431
Funded under: FP7-IDEAS-ERC
Country: Germany

Mid-Term Report Summary - MESOFERMI (Mesoscopic Fermi Gases)

The ERC funded research project MesoFermi, Mesoscopic Fermi Gases, has realized and studied strongly interacting superfluid two-dimensional Fermi gases in the the first reporting period. To freeze out all motion along the third direction, a cloud consisting some tens of thousands of atoms of Lithium 6 is transferred into a single layer of a standing wave potential exerting an extremely strong vertical confining force onto the atoms. Below a temperature of 150nK, the atoms are expected to become superfluid.

In a first experiment we have been able to measure the speed of sound in a such 2D superfluids. To our knowledge this constitutes the only measurement of first sound in a 2D superfluid system so far: in superfluid Helium films first sound does not exist. First sound is a key quantity determining the dynamics in a system. Moreover, it probes the thermodynamic properties of a system. To create the sound wave, a small attractive potential is used to form a local density excess. Its sudden release leads to a circularly symmetric outgoing density wave. In a complementary mesurement, we determine the speed of sound from the pressure equation of state, which we extract from in-situ density profiles. The results shed light on the thermodynamics of 2D superfluids.

In a second research endeavor we are investigating the interplay between superfluidity and coherence: In 3D systems, bosonic atoms or Cooper pairs condense into a macroscopic wavefunction exhibiting true long range coherence. Meanwhile, 2D superfluids show a strikingly different behavior: True long-range coherence is precluded by thermal fluctuations, nevertheless Berezinskii-Kosterlitz-Thouless (BKT) theory predicts that 2D systems can still become superfluid. However, the first order correlation function g1(r) decays algebraically with distance with a temperature-dependent scaling exponent. We measure the coherence of a strongly interacting 2D gases locally and observe this algebraic, scale-free decay. We are able to determine the scaling exponent and reconstruct the superfluid density. This opens the field of local studies of superfluid density and flow.

Very recently, we have been able to realize a homogeneous ultracold 2D Fermi gas in a box for the first time. In the future, we will study the properties and dynamics of such homogeneous 2D superfluids before going on to realize mesoscopic systems such as double wells, plaquettes and small lattices embedded into a larger reservoir. The vision of this project is to shed light on superfluid states in 2D and bring together the field of ultracold atoms and mesocopic physics.

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