Ground-State Pressure of Quasi-2D Fermi and Bose Gases

Makhalov, Vasiliy; Martiyanov, Kirill; Turlapov, Andrey

doi:10.1103/physrevlett.112.045301

Cited by 138 publications

(244 citation statements)

References 34 publications

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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.…”

supporting

confidence: 80%

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: 80%

Quasicondensation in Two-Dimensional Fermi Gases

Anderson

Boyack

et al. 2015

Phys. Rev. Lett.

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“…Reducing the dimensionality [9] led to the observation of a Berezinskii-Kosterlitz-Thouless (BKT) type phase transition to a superfluid phase in weakly interacting 2D Bose gases [10,11]. Tuning the strength of interactions in a three-dimensional two-component Fermi gas made it possible to explore the crossover between a molecular Bose-Einstein Condensate (BEC) and a BCS superfluid [12][13][14][15].Recently, efforts have been made to combine reduced dimensionality with the tunability of interactions and to experimentally explore ultracold 2D Fermi gases [16][17][18][19][20][21]. However, the phase transition to a condensed phase has not yet been observed.…”

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confidence: 99%

“…Recently, efforts have been made to combine reduced dimensionality with the tunability of interactions and to experimentally explore ultracold 2D Fermi gases [16][17][18][19][20][21]. However, the phase transition to a condensed phase has not yet been observed.…”

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confidence: 99%

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

et al. 2015

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The condensation of fermion pairs lies at the heart of superfluidity. However, for strongly correlated systems with reduced dimensionality the mechanisms of pairing and condensation are still not fully understood. In our experiment we use ultracold atoms as a generic model system to study the phase transition from a normal to a condensed phase in a strongly interacting quasi-two-dimensional Fermi gas. Using a novel method, we obtain the in situ pair momentum distribution of the strongly interacting system and observe the emergence of a low-momentum condensate at low temperatures. By tuning temperature and interaction strength we map out the phase diagram of the quasi-2D BEC-BCS crossover.The characteristics of quantum many-body systems are strongly affected by their dimensionality and the strength of interparticle correlations. In particular, strongly correlated two-dimensional fermionic systems have been of interest because of their connection to high-T c superconductivity. Although they have been the subject of intense theoretical studies [1][2][3][4][5][6][7][8], a complete theoretical framework has not yet been established.Ultracold quantum gases are an ideal realization for exploring strongly interacting 2D Fermi gases, as they offer the possibility of independently tuning the dimensionality and the strength of interparticle interactions. Reducing the dimensionality [9] led to the observation of a Berezinskii-Kosterlitz-Thouless (BKT) type phase transition to a superfluid phase in weakly interacting 2D Bose gases [10,11]. Tuning the strength of interactions in a three-dimensional two-component Fermi gas made it possible to explore the crossover between a molecular Bose-Einstein Condensate (BEC) and a BCS superfluid [12][13][14][15].Recently, efforts have been made to combine reduced dimensionality with the tunability of interactions and to experimentally explore ultracold 2D Fermi gases [16][17][18][19][20][21]. However, the phase transition to a condensed phase has not yet been observed. Here, we report on the condensation of pairs of fermions in the quasi-2D BEC-BCS crossover.The BEC-BCS crossover smoothly links a bosonic superfluid of tightly bound diatomic molecules to a fermionic superfluid of Cooper pairs in 2D as well as 3D systems. However, changing the dimensionality leads to some inherent differences. In two dimensions, there is a two-body bound state for all values of the interparticle interaction. Furthermore, because of the enhanced role of * To whom correspondence should be addressed. E-mail: mries@physi.uni-heidelberg.de † These authors contributed equally to this work. ‡ Present address: MIT-Harvard Center for Ultracold Atoms, MIT, Cambridge, MA 02139, USA.fluctuations in 2D, true long-range order is forbidden for homogeneous systems at finite temperature [22,23]. Still, a low temperature superfluid phase with quasi-long-range order can emerge due to the BKT mechanism [24,25]. In a 2D gas with contact interactions, the interactions can be described by the 2D scattering length a 2D . Using th...

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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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Ground-State Pressure of Quasi-2D Fermi and Bose Gases

Cited by 138 publications

References 34 publications

Quasicondensation in Two-Dimensional Fermi Gases

Quasicondensation in Two-Dimensional Fermi Gases

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

Fermions in Two Dimensions: Scattering and Many-Body Properties

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