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arXiv:2205.11094

Controllable Production of Degenerate Fermi Gases of $^6$Li Atoms in the 2D-3D Crossover

Hongwei Gong, Haotian Liu, Bolong Jiao, Haoyi Zhang, Qinxuan Peng, Peng, Shuai, Tangqian Shu, Yu, Hang, Yan Zhu, Jiaming Li, Le Luo

Published May 23, 2022• 0 citations
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Abstract

The many-body physics in the dimensional crossover regime attracts much attention in cold atom experiments, but yet to explore systematically. One of the technical difficulties existed in the experiments is the lack of the experimental technique to quantitatively tune the atom occupation ratio of the different lattice bands. In this letter, we report such techniques in a process of transferring a 3D Fermi gas into a 1D optical lattice, where the capability of tuning the occupation of the energy band is realized by varying the trapping potentials of the optical dipole trap (ODT) and the lattice, respectively. We could tune a Fermi gas with the occupation in the lowest band from unity to 50$\%$ quantitatively. This provides a route to experimentally study the dependence of many-body interaction on the dimensionality in a Fermi gas.

DOI
10.48550/arxiv.2205.11094

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Controllable Production of Degenerate Fermi Gases of 6^66Li Atoms in the 2D-3D Crossover
The many-body physics in the dimensional crossover regime attracts much attention in cold atom experiments, but yet to explore systematically. One of the technical difficulties existed in the experiments is the lack of the experimental technique to quantitatively tune the atom occupation ratio of the different lattice bands. In this letter, we report such techniques in a process of transferring a 3D Fermi gas into a 1D optical lattice, where the capability of tuning the occupation of the energy band is realized by varying the trapping potentials of the optical dipole trap (ODT) and the lattice, respectively. We could tune a Fermi gas with the occupation in the lowest band from unity to 50%\%% quantitatively. This provides a route to experimentally study the dependence of many-body interaction on the dimensionality in a Fermi gas.
Many-body physics in dimensional crossover systems has attracted much attention, but is yet to explore intensively. Cold atoms provide an excellent experimental platform to address the questions in this area, but technical difficulties still exist. When an optical lattice is applied to tune the dimensionality of trapped cold atoms, it is usually difficult to precisely tune the occupation ratio of the atom in the different lattice bands. In this paper we report the method to tune the occupation ratio of the energy band when transferring a three-dimensional Fermi gas into a one-dimensional optical lattice, where we could tune the occupation ratio in the lowest band from unity to 5050%50 quantitatively by jointly varying the trapping potentials of the optical dipole trap and the one-dimensional lattice. This method provides a route to study the dependence of the many-body interaction on the dimensionality by continuously changing the dimensionality of a Fermi gas in the crossover from two dimensions to three dimensions.
We have characterized the one-dimensional (1D) to three-dimensional (3D) crossover of a two-component spin-imbalanced Fermi gas of ^{6}Li atoms in a 2D optical lattice by varying the lattice tunneling and the interactions. The gas phase separates, and we detect the phase boundaries using in situ imaging of the inhomogeneous density profiles. The locations of the phases are inverted in 1D as compared to 3D, thus providing a clear signature of the crossover. By scaling the tunneling rate t with respect to the pair binding energy ε_{B}, we observe a collapse of the data to a universal crossover point at a scaled tunneling value of t[over ˜]_{c}=0.025(7).
Ultracold atoms have become an essential tool in studying condensed matter phenomena. The advantage of atomic physics experiments is that they provide an easily tunable system. This experiment uses the lowest two ground state hyperfine levels of fermionic lithium. Having two different states creates a pseudo-spin-1/2 system and allows us to emulate electronic systems, such as superconductors and crystal lattices. In our experiment, we can control the ratio between these two states resulting in either a spin-balanced or a spin-imbalanced gas. Imposing an imbalance is analogous to applying a magnetic field to a superconductor which causes the electrons in the material to align to the field (thus breaking the electron pairs which cause superconductivity). This motivates us to understand the phases created when a spin-imbalance is created and the effect of changing the atomic interactions. In a 3D system, we find where superfluidity is suppressed throughout the BEC to BCS crossover. Using phase separation as a guide, we probe the dimensional crossover between 1D and 3D. The phase separation in 1D is inverted from that in 3D, which provides a unique characteristic to distinguish between the dimensions. By varying the tunneling between tubes and the atomic interactions in a 2D optical lattice, we control whether the system is 1D, 3D, or in between. Using the properties of a 3D gas as a guide, we directly observe when the gas has crossed over from being dominated by 1D-like behavior to 3D. In this way, we have found a universal value for the dimensional crossover. The 1D-3D crossover paves the way to search for the exotic FFLO (Fulde-Ferrell-Larkin-Ovchinnikov) superconductor. While most superconductors do not coexist with magnetism, the FFLO phase requires large magnetic fields to support its pairing mechanism. Additionally, this phase is more likely to be found in lower dimensional systems. However, at low dimensions, the effect of temperature fluctuations on the phase is destabilizing, but these temperature effects are reduced with higher dimensionality. Thus, the quasi-1D regime is the optimal region of parameter space to find this phase. The search for direct evidence of FFLO continues in this regime.
We report the experimental production of degenerate Fermi gases of 6Li atoms in an optical dipole trap. The gray-molasses technique is carried out to decrease the atomic temperature to 57 μK, which facilitates the efficient loading of cold atoms into the optical dipole trap. The Fermi degeneracy is achieved by evaporative cooling of a two-spin mixture of 6Li atoms on the Feshbach resonance. The degenerate atom number per spin is 3.5 × 104, and the reduced temperature T/T F is as low as 0.1, where T F is the Fermi temperature of the non-interacting Fermi gas. We also observe the anisotropic expansion of the atom cloud in the strongly interacting regime.
Quasi-2D degenerate Fermi gases provide a rich physical environment to explore since many effects that are present do not exist in 3D. This thesis describes the first experiments carried out in a quasi-2D Fermi gas across the broad 6Li Feshbach resonance at 834 G. A description of the apparatus used to form molecular Bose-Einstein condensates and degenerate Fermi gases in the two lowest hyperfine states of 6Li, |F = 1/2,mf = 1/2i and |F = 1/2,mf = −1/2i, is presented. An anisotropic 2D optical potential is formed by an elliptical Gaussian beam produced by tightly focusing a circular Gaussian beam in one direction with a cylindrical lens. The trapping frequencies are ∼3 kHz in the axial direction and ∼50 Hz in the radial direction giving an aspect ratio of ∼60. For a non-interacting Fermi gas this allows ∼1700 atoms to populate the lowest transverse harmonic oscillator state and to be considered as quasi-2D. We present the transition from a three dimensional gas to a two dimensional gas at ∼2000 atoms agreeing within experimental uncertainty with the non-interacting atom number. The observations are made through measuring the radial and axial widths of the gas at three different magnetic fields, 810 G, 834 G, and 991 G. The widths σ are then plotted against the atom number N. The curve, log σ(N) = n logN+a, is then fit to the data in the 3D regime and in the 2D regime. The expansion of a non-interacting Fermi gas in the 2D regime changes from a scaling of N1/4 to N1/6 in the 3D regime and for a Bose gas it changes to N1/5. A value for n is extracted in each of these regimes. For each magnetic field we then obtain in the 2D regime: n = 0.27(0.04) at 810 G, n = 0.23(0.02) at 834 G and n = 0.26(0.01) at 991 G. In the 3D regime we obtain: n = 0.195(0.01) at 810 G, n = 0.154(0.01) at 834 G and n = 0.175(0.01 at 991 G. We also observe a change in the aspect ratio as we approach the 2D regime which is not present in our 3D single beam optical dipole trap and is absent in any 3D system. These first results indicate that we have a quasi-2D Fermi gas. The main experimental results are the first indications of a confinement induced resonance in a 6Li Fermi gas through the measurement of the cloud width in the tightly confined direction after a fixed expansion time. We observe a significant deviation in the transverse width of the 2D interacting Fermi gas from the 3D case due to the increased tight confinement. The confinement induced resonance should appear when the 3D scattering length approaches the transverse harmonic oscillator length, az = p~/mωz, where m is the mass and ωz is the tight transverse trapping frequency. For our experimental parameters this leads to a 3D scattering length of 1.03 μm, where the observation is at 819(1) G. In the weakly confined direction there is no such increase in the width due to the free motion in the xy plane. Instead the cloud width across the Feshbach resonance increases monotonically, similar to that of a 3D gas and as described theoretically for a quasi-2D Fermi gas by Lin et al. [1].
This thesis presents the experimental study on the two-spin component, strongly interacting 6 Li Fermi gases in 3D and 1D traps. The interaction strength is tuned from the molecular BEC regime to the BCS regime using a Feshbach resonance. The trap dimension can be tuned from 3D to 1D with the implementation of optical lattice. The evaporation of imbalanced Fermi gases in 3D trap is studied. The anisotropic and fast evaporation is the cause of the deformation observed in the 2006 Rice experiment. In a balanced Fermi system, the fraction of correlated states is measured as a function of interaction and temperature. At unitarity, the fraction of correlated states is ∼85% and exists above T c . The one-body-like photoexcitation rate can be related to the contact quantity. Lastly, the spin-imbalance in a one-dimensional Fermi gas is studied. The 1D phase diagram is mapped out. The result agrees well with the 1D theory, in which the partially polarized regime is predicted to be a FFLO phase, an exotic superfluid with pairs carrying finite center-of-mass momentum proposed almost 50 years ago.