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978-3-8439-1998-2, Reihe Physik
Kai H. Morgener
Microscopy of 2D Fermi Gases: Exploring Excitations and Thermodynamics
190 Seiten, Dissertation Universität Hamburg (2014), Softcover, B5
This thesis presents experiments on three-dimensional (3D) and two-dimensional (2D) ultracold fermionic lithium-6 gases providing local access to microscopic quantum many-body physics. A broad magnetic Feshbach resonance is used to tune the interparticle interaction strength freely to address the entire crossover between the Bose-Einstein-Condensate (BEC) and Bardeen-Cooper-Schrieffer (BCS) regime.
We map out the critical velocity in the crossover from BEC to BCS superfluidity by moving a small attractive potential through the 3D cloud. We compare the results with theoretical predictions and achieve quantitative understanding in the BEC regime by performing numerical simulations. Of particular interest is the regime of strong correlations, where no theoretical predictions exist. In the BEC regime, the critical velocity should be closely related to the speed of sound, according to the Landau criterion and Bogoliubov theory. We measure the sound velocity by exciting a density wave and tracking its propagation.
The focus of this thesis is on our first experiments on general properties of quasi-2D Fermi gases. We realize strong vertical confinement by generating a 1D optical lattice by intersecting two blue-detuned laser beams under a steep angle. The large resulting lattice spacing enables us to prepare a single planar quantum gas deeply in the 2D regime. The first measurements of the speed of sound in quasi-2D gases in the BEC-BCS crossover are presented. In addition, we present preliminary results on the pressure equation of state, which is extracted from in-situ density profiles. Since the sound velocity is directly connected to the equation of state, the results provide a crosscheck of the speed of sound. Moreover, we benchmark the derived sound from available equation of state predictions, find very good agreement with recent numerical calculations, and disprove a sophisticated mean field approach.
These studies are carried out with a novel apparatus which has been set up in the scope of this work. An all-optical cooling scheme and optical transport is employed to provide us with ultracold atomic clouds inside a separate small vacuum cell with optimal optical access. Above and below this cell, two high numerical aperture microscope objectives are placed to image and probe the ultracold atomic clouds in-situ on length scales comparable to the intrinsic length scales.