Observation of Pair Condensation in the Quasi-2D BEC-BCS Crossover

Ries, M. G.; Wenz, A. N.; Zürn, Gerhard; Bayha, Luca; Boettcher, Igor; Kedar, Dhruv; Murthy, P. A.; Neidig, M.; Lompe, Thomas; Jochim, Selim

doi:10.1103/physrevlett.114.230401

Cited by 167 publications

(258 citation statements)

References 51 publications

(139 reference statements)

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“…These follow earlier work addressing the ground state [11] and the higher temperature regime, away from condensation [12]. These experiments [9,10] show that strong normal state pairing is an essential component of 2D Fermi superfluids, even in the BCS regime. In fact, much of the theory invoked to explain these experiments was based upon true Bose systems.…”

supporting

confidence: 48%

Quasicondensation in Two-Dimensional Fermi Gases

Anderson

Boyack

et al. 2015

Phys. Rev. Lett.

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In this paper we follow the analysis and protocols of recent experiments, combined with simple theory, to arrive at a physical understanding of quasi-condensation in two dimensional Fermi gases. We find that quasi-condensation mirrors Berezinskiȋ-Kosterlitz-Thouless behavior in many ways, including the emergence of a strong zero momentum peak in the pair momentum distribution. Importantly, the disappearance of this quasi-condensate occurs at a reasonably well defined crossover temperature. The resulting phase diagram, pair momentum distribution, and algebraic power law decay are compatible with recent experiments throughout the continuum from BEC to BCS.Understanding two dimensional (2D) fermionic superfluidity has a long history relating to the Mermin- Thus recent reports [9, 10] of a form of pair condensation in 2D fermionic gases are particularly exciting. These follow earlier work addressing the ground state [11] and the higher temperature regime, away from condensation [12]. These experiments [9,10] show that strong normal state pairing is an essential component of 2D Fermi superfluids, even in the BCS regime. In fact, much of the theory invoked to explain these experiments was based upon true Bose systems. A characteristic feature of 2D superfluidity at finite T is the presence of narrow peaks in the momentum distribution of the pairs, without macroscopic occupation of the zero momentum state. Throughout the paper this will be our definition of "quasi-condensation." This quasi-condensation in momentum space is associated with algebraic decay of coherence in real space. Importantly, the BKT-related transition temperature is manifested as a sudden change in slope of a normalized peak momentum distribution for pairs.In this paper we present a theory of a 2D Fermi gas near quantum degeneracy and show how it reproduces rather well the results of these recent experiments [9,10] through an analysis of the phase diagram, the pair momentum distribution and algebraic power laws. Given the ground breaking nature of the experiments, it is important to have an accompanying theoretical study which follows exactly the same protocols without any adjustments or phenomenology. Our approach is to be distinguished from other studies of 2D Fermi gases [4,[13][14][15][16][17][18][19][20][21][22][23]. In particular, those addressing BKT physics [4,[13][14][15][16]20], use existing formulae [24,25] and determine the unknown parameters to obtain T BKT c . By contrast here we reverse the procedure and follow experimental protocols to thereby provide a new formula, involving composite bosons, for the transition temperature associated with quasi-condensation. In the homogeneous case, this is analytically tractable and presented as Eq. (6) below.Importantly, there is a rather abrupt crossover out of a quasi-condensed phase at a fairly well defined temperature T qc . In the BEC regime this matches earlier theoretical estimates of the BKT transition temperature which are based on different theoretical formalisms [4,[13][14][15][16]. We ...

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supporting

confidence: 48%

Quasicondensation in Two-Dimensional Fermi Gases

Anderson

Boyack

et al. 2015

Phys. Rev. Lett.

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“…Proving the existence or non-existence of true phase coherence would lead to a significant advance in the understanding of the physics of 2D Fermi gases. As in previous work [15,17,18] true superfluidity in 2D has not been established here or in experiments. This will require future experimental probes related to coherence features, including interference measurements, or detection of the presence of collective modes in Bragg scattering.…”

Section: T /Tf1mentioning

confidence: 70%

“…The evidence from experiments [17,18] on 2D spinbalanced Fermi gases suggests that the bosonic degrees of freedom (accessed by rapid magnetic field sweeps) exhibit strong |q| → 0 peaks in their momentum distribution, represented by a trap-average, denotedn B (q), of n B (q) = b q 2 /2M B − µ pair appearing in Eq. (2).…”

Section: T /Tf1mentioning

confidence: 99%

Two-dimensional spin-imbalanced Fermi gases at nonzero temperature: Phase separation of a noncondensate

Boyack

Levin

2016

Phys. Rev. A

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We study a trapped two-dimensional spin-imbalanced Fermi gas over a range of temperatures. In the moderate temperature regime, associated with current experiments, we find reasonable semiquantitative agreement with the measured density profiles as functions of varying spin imbalance and interaction strength. Our calculations show that, in contrast to the three-dimensional case, the phase separation which appears as a spin balanced core, can be associated with non-condensed fermion pairs. We present predictions at lower temperatures where a quasi-condensate will first appear, based on the pair momentum distribution and following the protocols of Jochim and collaborators. While these profiles also indicate phase separation, they exhibit distinctive features which may aid in identifying the condensation regime.Ultracold Fermi gases are a valuable resource for learning about highly correlated superfluids. Their utility comes from their tunability [1] which allows the dimensionality, band structure, interaction strength, and spin imbalance to be freely varied. With these various parameters one can, in principle, simulate a number of important condensed matter systems ranging from preformed pair and related effects in the high T c cuprates [2-4] to intrinsic topological superfluids [5][6][7] and other exotic pairing states.In this paper we focus on recent experiments [8,9] of two-dimensional (2D) spin-imbalanced Fermi gases. These address the interesting conflict between the tendency towards enhanced pairing (which is associated with two dimensionality [10]), and spin imbalance which acts to greatly undermine pairing. These imbalance effects are believed [11] to have related effects in studies of color superconductivity and quark-gluon plasmas. In condensed matter systems, lower dimensional imbalanced superfluids are thought to be ideal for observing more exotic phases, such as the elusive LOFF state [12], or algebraic order [13,14].The approach we use has been rather successful in addressing 2D low temperature quasi-condensation [15] in balanced gases. In this paper we present predictions for future very low temperature experiments on spinimbalanced gases. Importantly, our calculations, which find no true long range order, are consistent with the Mermin-Wagner theorem [16]. Following the experiments of Jochim and collaborators [17,18], we show how quasicondensation is reflected in the pair-momentum distribution which has a strong peak at low momentum. This peak, which we study throughout the crossover from BCS to BEC, disappears somewhat abruptly at a fixed temperature, T qc , which denotes quasi-condensation. We find T qc varies only weakly with the polarization.A central finding in this paper is that 2D spinimbalanced systems in a trap exhibit a new form of phase separation involving non-condensed pairs appearing primarily in the center region of the trap. This is to be contrasted with 3D gases [19,20] where the phase separation is associated with a true condensate. In the 2D polarized case, because almost all the ...

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“…A rich area of study subject to ongoing investigation is that of low dimensionality [16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38]. These dilute cold atomic gases have been trapped using anisotropic potentials, resulting in a quasi-2D pancake-shape gas cloud.…”

Section: Introductionmentioning

confidence: 99%

Fermions in Two Dimensions: Scattering and Many-Body Properties

et al. 2017

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Ultracold atomic Fermi gases in two-dimensions (2D) are an increasingly popular topic of research. The interaction strength between spin-up and spindown particles in two-component Fermi gases can be tuned in experiments, allowing for a strongly interacting regime where the gas properties are yet to be fully understood. We have probed this regime for 2D Fermi gases by performing T = 0 ab initio diffusion Monte Carlo (DMC) calculations. The many-body dynamics are largely dependent on the two-body interactions, therefore we start with an in-depth look at scattering theory in 2D. We show the partial-wave expansion and its relation to the scattering length and effective range. Then we discuss our numerical methods for determining these scattering parameters. We close out this discussion by illustrating the details of bound states in 2D. Transitioning to the many-body system, we use variationally optimized wave functions to calculate ground-state properties of the gas over a range of interaction strengths. We show results for the energy per particle and parametrize an equation of state. We then proceed to determine the chemical potential for the strongly interacting gas.

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Observation of Pair Condensation in the Quasi-2D BEC-BCS Crossover

Cited by 167 publications

References 51 publications

Quasicondensation in Two-Dimensional Fermi Gases

Quasicondensation in Two-Dimensional Fermi Gases

Two-dimensional spin-imbalanced Fermi gases at nonzero temperature: Phase separation of a noncondensate

Fermions in Two Dimensions: Scattering and Many-Body Properties

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