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Sound propagation and quantum-limited damping in an ultracold two-dimensional Fermi gas

Abstract : Strongly correlated two-dimensional (2D) systems are a fascinating field of study. The reduced dimensionality should in principle impede phenomena such as Bose-Einstein condensation or superfluidity. Yet, evidence suggests that superfluidity and superconductivity are especially robust in 2D: In almost all known high Tc-supercondcutors, strongly correlated 2D structures and higher-partial-wave coupling seem to play a crucial role. In this thesis, we use ultracold homogeneous gases of lithium 6 with tunable interactions to perform analog quantum simulation of these captivating systems. As the main result of this thesis, I present the first measurements of the speed and attenuation of sound waves in ultracold 2D Fermi gases, which we use to probe the thermodynamic and transport properties of the gas. From the speed of sound, we extract the compressibility equation of state and compare it both to an independent static measurement and to quantum Monte Carlo calculations and find reasonable agreement between the three. The damping of the sound waves, which is determined by the shear and bulk viscosities as well as the thermal conductivity of the gas, exhibits a minimum in the strongly correlated regime. Here, the sound diffusivity approaches a universal quantum bound ~/m and the strongly correlated 2D Fermi gas thus realizes a nearly perfect fluid. In addition, I report on further related measurements performed in the course of this thesis, which were led by my coworkers N. Luick and L. Sobirey. We show that the 2D Fermi gas is phase coherent by realizing a Josephson junction in the homogeneous gas and observing Josephson oscillations between two reservoirs separated by a thin barrier. When the barrier height is reduced to zero, these oscillations transform smoothly into sound waves. By dragging a lattice though the homogeneous system at variable velocities, we observe a critical velocity for the creation of excitations, proving that the system is superfluid. Here, the sound velocity determines the critical velocity for a large range of interaction strengths. Finally, I present the characterization of a novel d-wave Feshbach resonance in ultracold potassium 40 via measurements of the inelastic loss rate L⁽²⁾ and via the dynamics of spin populations, led by my coworker T. Reimann. The experimental results are compared to theoretical predictions and we observe good agreement between the theoretical and experimental loss rates. The evolution of the spin populations is found to be consistent with the expected behavior for the theoretically predicted exit channel.
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Markus Bohlen. Sound propagation and quantum-limited damping in an ultracold two-dimensional Fermi gas. Quantum Physics [quant-ph]. Université Paris sciences et lettres, 2020. English. ⟨NNT : 2020UPSLE068⟩. ⟨tel-03651235⟩

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