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arXiv:1109.6417 · New Journal of Physics

Density distribution of a trapped two-dimensional strongly interacting Fermi gas

A A Orel, Paul Dyke, Marion Delehaye, Chris J. Vale, Hui Hu

Published Nov 23, 2011• 44 citations
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Abstract

We calculated and measured the density distribution and cloud size of a trapped two-dimensional (2D) 6 Li Fermi gas near a Feshbach resonance at low temperatures. Density distributions and cloud sizes were calculated for a wide range of interaction parameters using a local density approximation (LDA) and a zero-temperature equation of state obtained from quantum Monte Carlo simulations reported by Bertaina and Giorgini (2011 Phys. Rev. Lett. 106 110403). We found that LDA predictions agree well with experimental measurements across a Feshbach resonance. Theoretical results for Tan's contact parameter in a trapped gas are reported here along with predictions for the static structure factor at large momentum which could be measured in future Bragg spectroscopy experiments on 2D Fermi gases.

DOI
10.1088/1367-2630/13/11/113032

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We calculated and measured the density distribution and cloud size of a trapped two-dimensional (2D) 6 Li Fermi gas near a Feshbach resonance at low temperatures. Density distributions and cloud sizes were calculated for a wide range of interaction parameters using a local density approximation (LDA) and a zero-temperature equation of state obtained from quantum Monte Carlo simulations reported by Bertaina and Giorgini (2011 Phys. Rev. Lett. 106 110403). We found that LDA predictions agree well with experimental measurements across a Feshbach resonance. Theoretical results for Tan's contact parameter in a trapped gas are reported here along with predictions for the static structure factor at large momentum which could be measured in future Bragg spectroscopy experiments on 2D Fermi gases.
We investigate theoretically the transition from two to three dimensions in a weakly interacting ultracold Fermi gas. This transition was recently observed by Dyke et al. [Phys. Rev. Lett. 106, 105304 (2011)] in a cloud of fermionic 6{}^{6}6Li atoms trapped in an oblate harmonic potential. We calculate the density distribution and cloud size of the Fermi cloud by numerically solving three-dimensional Hartree-Fock mean-field equations. Local density approximation is used to ease the calculations in the weakly confined radial direction. Our theoretical results are shown to be in good agreement with the experimental measurements.
Dynamics of strongly interacting Fermi gases, consisting of a 50-50 mixture of two different fermionic species, is investigated. For the equation of state we consider a Padé [2/2] approximations, which gives the weak-coupling perturbative formula (up to 4th order) in the low density regime, the unitary-limit Monte Carlo result in the high density regime, and reproduces the 4-fermion prediction for dimer-dimer scattering length in the BEC region. We use a time-dependent LDA to derive various properties of the Fermi gas under a harmonic confinement and compare them with the data of very recent experiments of 6Li atoms across a Feshbach resonance.
Using a scattering length parametrization of the crossover from a BCS state to a Bose-Einstein condensate as well as the local density approximation for the density profile, we calculate the momentum distribution of a harmonically trapped atomic Fermi gas at zero temperature. Various interaction regimes are considered, including the BCS phase, the unitarity limit, and the molecular regime. We show that the relevant parameter which characterizes the crossover is given by the dimensionless combination N1/6a/aho,{N}^{1/6}{a/a}_{\mathrm{ho}},N1/6a/aho​, where N is the number of atoms, a is the scattering length, and aho{a}_{\mathrm{ho}}aho​ is the oscillator length. The width of the momentum distribution is shown to depend in a crucial way on the value and sign of this parameter. Our predictions can be relevant for experiments on ultracold atomic Fermi gases near a Feshbach resonance.
We present a comprehensive experimental study of Tan's universal contact parameter in a two-component ultracold Fermi gas, using Bragg spectroscopy. The contact uniquely parameterizes a number of universal properties of Fermi gases in the strongly interacting regime. It is linked to the spin-antiparallel component of the static structure factor S ↑↓ ( k ) at high momenta, which can readily be obtained via Bragg scattering. Contact depends upon the relative interaction strength 1/( k F a ) and temperature T / T F , where k F is the Fermi wave vector, a is the s-wave scattering length and T F is the Fermi temperature. We present measurements of both of these dependencies in a cloud of 6 Li atoms and compare our findings to theoretical predictions. We also compare Bragg spectroscopic methods based on measuring the energy and momentum transferred to the cloud and examine the conditions under which the energy transfer method provides improved accuracy. Our measurements of the dynamic structure factor and contact are found to be in good agreement with theoretical predictions based on the quantum virial expansion.
We analyze the results of a recent experiment with bosonic rubidium atoms harmonically confined in a quasi-two-dimensional (2D) geometry. In this experiment a well-defined critical point was identified, which separates the high-temperature normal state characterized by a single component density distribution, and the low-temperature state characterized by a bimodal density distribution and the emergence of high-contrast interference between independent 2D clouds. We first show that this transition cannot be explained in terms of conventional Bose–Einstein condensation of the trapped ideal Bose gas. Using the local density approximation (LDA), we then combine the mean-field (MF) Hartree–Fock theory with the prediction for the Berezinskii–Kosterlitz–Thouless (BKT) transition in an infinite uniform system. If the gas is treated as a strictly 2D system, the MF predictions for the spatial density profiles significantly deviate from those of a recent quantum Monte Carlo (QMC) analysis. However, when the residual thermal excitation of the strongly confined degree of freedom is taken into account, excellent agreement is reached between the MF and the QMC approaches. For the interaction strength corresponding to the experiment, we predict a strong correction to the critical atom number with respect to the ideal gas theory (factor ∼2). Quantitative agreement between theory and experiment is reached concerning the critical atom number if the predicted density profiles are used for temperature calibration.
We report on the experimental realization of homogeneous two-dimensional (2D) Fermi gases trapped in a box potential. In contrast to harmonically trapped gases, these homogeneous 2D systems are ideally suited to probe local as well as nonlocal properties of strongly interacting many-body systems. As a first benchmark experiment, we use a local probe to measure the density of a noninteracting 2D Fermi gas as a function of the chemical potential and find excellent agreement with the corresponding equation of state. We then perform matter wave focusing to extract the momentum distribution of the system and directly observe Pauli blocking in a near unity occupation of momentum states. Finally, we measure the momentum distribution of an interacting homogeneous 2D gas in the crossover between attractively interacting fermions and bosonic dimers.