Quasi-two-dimensional Fermi gases at finite temperatures

Fischer, Andrea M.; Parish, Meera M.

doi:10.1103/physrevb.90.214503

Cited by 18 publications

(31 citation statements)

References 40 publications

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“…We provide a brief theoretical account of pairing in these two limits in [19]. The crucial difference between the two cases lies in the occupation of the quasiparticle branch which occurs preferentially at k ∼ 0 in the BEC [24] and at k ∼ k F in BCS [25] regimes. While the corresponding RF spectra, shown in Fig.…”

Section: Thementioning

confidence: 99%

“…3 C and D. We fit these local spectra with a combined fit function that includes a symmetric Gaussian (for the quasiparticle peak) and an asymmetric threshold function (for the paired peak) that is convolved with a Gaussian to account for spectral broadening arising from finite RF frequency resolution and final state effects [28]. The choice of fit function has a systematic effect on the quantitative results presented here, which cannot be eliminated at this point since a reliable theoretical prediction of the shape of the spectral function only exists in the weakly coupled BEC [24] and BCS [25] limits. We present a detailed account of our data analysis in [19].…”

Section: Thementioning

confidence: 99%

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High-temperature pairing in a strongly interacting two-dimensional Fermi gas

Murthy

Neidig

Klemt

et al. 2018

Science

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The nature of the normal phase of strongly correlated fermionic systems is an outstanding question in quantum many-body physics. We used spatially resolved radio-frequency spectroscopy to measure pairing energy of fermions across a wide range of temperatures and interaction strengths in a two-dimensional gas of ultracold fermionic atoms. We observed many-body pairing at temperatures far above the critical temperature for superfluidity. In the strongly interacting regime, the pairing energy in the normal phase considerably exceeds the intrinsic two-body binding energy of the system and shows a clear dependence on local density. This implies that pairing in this regime is driven by many-body correlations, rather than two-body physics. Our findings show that pairing correlations in strongly interacting two-dimensional fermionic systems are remarkably robust against thermal fluctuations.

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Section: Thementioning

confidence: 99%

Section: Thementioning

confidence: 99%

High-temperature pairing in a strongly interacting two-dimensional Fermi gas

Murthy

Neidig

Klemt

et al. 2018

Science

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“…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 .…”

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confidence: 99%

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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“…In our system, residual axial excitations grow with increasing lnðk F a 2D Þ [27]. Recently, it was predicted that they would lead to an increased critical temperature [55]. Additionally, the threedimensional internal structure of atom pairs might lead to corrections in the regime where E B ≈ ℏω z , which go beyond the two-body sector.…”

mentioning

confidence: 99%

“…Additionally, the equation of state can be extracted from the density distribution in the trap. Finally, the exploration of the dimensional crossover to 3D, in which an increased T c =T F is predicted [55], offers new opportunities to understand mechanisms which lead to high critical temperatures.…”

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confidence: 99%

Cooper Pairs Dance in a Disk

Pieri

2015

Physics

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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 [1-8], 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 BerezinskiiKosterlitz-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 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 the Fermi wave vector k F , the dimensionless crossover parameter is given by lnðk F a 2D Þ. The crossover regime is reached for j lnðk F a 2D Þj ≲ 1. For lnðk F a 2D Þ ≪ −1, the binding energy is large and the system consists of deeply bound b...

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Quasi-two-dimensional Fermi gases at finite temperatures

Cited by 18 publications

References 40 publications

High-temperature pairing in a strongly interacting two-dimensional Fermi gas

High-temperature pairing in a strongly interacting two-dimensional Fermi gas

Quasicondensation in Two-Dimensional Fermi Gases

Cooper Pairs Dance in a Disk

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