Pair Correlations in the Two-Dimensional Fermi Gas

Ngampruetikorn, Vudtiwat; Levinsen, Jesper; Parish, Meera M.

doi:10.1103/physrevlett.111.265301

Cited by 58 publications

(94 citation statements)

References 34 publications

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“…The crossover to a fermionic description should thus occur at positive values of ln(k F a 2D ). This is in line with recent theoretical predictions [6,41].Far on the BCS side, fermionic theories predict an exponential decrease of T c /T F [7,42]. Although we can only give an upper limit for the critical temperature T c /T F ≤ 0.16 for ln(k F a 2D ) ≥ 2, the observed nonGaussian fraction is consistent with a decrease towards the BCS limit.…”

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confidence: 77%

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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confidence: 77%

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

et al. 2015

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“…Here we adopt the diagrammatic approach in the framework of the Virial expansion [62][63][64][65][66][67]. The advantage of this method is that it is accurate at high temperature, and can systematically incorporate all the two-body and three-body contributions which allow us to extract the Efimov effect in a controllable way.…”

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confidence: 99%

Visualizing the Efimov Correlation in Bose Polarons

2017

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The Bose polaron is a quasi-particle of an impurity dressed by surrounding bosons. In fewbody physics, it is known that two identical bosons and a third distinguishable particle can form a sequence of Efimov bound states in the vicinity of inter-species scattering resonance. On the other hand, in the Bose polaron system with an impurity atom embedded in many bosons, no signature of Efimov physics has been reported in the existing spectroscopy measurements up to date. In this work, we propose that a large mass imbalance between a light impurity and heavy bosons can help produce visible signatures of Efimov physics in such a spectroscopy measurement. Using the diagrammatic approach in the Virial expansion to include three-body effects from pairwise interactions, we determine the impurity self-energy and its spectral function. Taking 6 Li-133 Cs system as a concrete example, we find two visible Efimov branches in the polaron spectrum, as well as their hybridizations with the attractive polaron branch. We also discuss the general scenarios for observing the signature of Efimov physics in polaron systems. This work paves the way for experimentally exploring intriguing few-body correlations in a many-body system in the near future.Top-down and bottom-up are two major approaches to studying correlations in a quantum many-body system. The cold atom system has intrinsic advantage for the bottom-up approach since it is a dilute system and the few-body problems therein are well understood. In this approach, one would like to understand how manybody physics is built up from few-body correlations. In cold atom system, one of the most intriguing three-body correlations lies in Efimov physics, which is characterized by an infinite number of trimer states nearby a two-body resonance and following a universal scaling law [1,2]. Efimov physics has been observed in a number of cold atoms experiments, while all of them are at the few-body level [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. The manifestation of Efimov physics in the many-body system has yet to be observed.In this context, a convenient and non-trivial testbed is the highly-polarized ultracold gases, which consist of minority impurity atoms interacting with the majority of fermionic or bosonic atoms, respectively called the Fermi or the Bose polarons. Lots of theoretical efforts have been paid to study the Fermi polaron [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] and the Bose polaron [36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51]. Nearby a Feshbach resonance, a Fermi polaron displays an attractive branch [20][21][22][23][24][25]29] and a repulsive branch [26][27][28], which directly manifests two-body correlations in this system. In the past few years, the Fermi polaron has been studied by a number of experiments [52][53][54][55][56][57], while the Bose polaron has only recently been explored [58][59][60]. Most of these experiments are the injection radio-frequency spectroscopy measurements, with which both the r...

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“…One can note that the present formalism can be adapted to evaluate the spectral densities of atomic gases in waveguides including the closed channel by expanding the self-energy in terms of the fugacity [37,46,47]. Appendix: Gas without interaction…”

Section: Discussionmentioning

confidence: 99%

“…On the theoretical side, highly anisotropic traps can be modeled by considering harmonic atomic waveguides where interatomic collisions are well known in the low energy limit [33][34][35]. The first terms of the virial expansions for strictly 1D or 2D gases using the single-channel approach have also been studied and the Beth-Uhlenbeck formula in pure 2D or 1D systems is well established [36,37].…”

Section: Introductionmentioning

confidence: 99%

Second-order virial expansion for an atomic gas in a harmonic waveguide

2016

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The virial expansion for cold two-component Fermi and Bose atomic gases is considered in the presence of a waveguide and in the vicinity of a Feshbach resonance. The interaction between atoms and the coupling with the Feshbach molecules is modeled using a quantitative separable two-channel model. The scattering phase-shift in an atomic waveguide is defined. This permits us to extend the Beth-Uhlenbeck formula for the second-order virial coefficient to this inhomogeneous case.

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Pair Correlations in the Two-Dimensional Fermi Gas

Cited by 58 publications

References 34 publications

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

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

Visualizing the Efimov Correlation in Bose Polarons

Second-order virial expansion for an atomic gas in a harmonic waveguide

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