Observation of the Presuperfluid Regime in a Two-Dimensional Bose Gas

Abstract

In complementary images of coordinate-space and momentum-space density in a trapped 2D Bose gas, we observe the emergence of presuperfluid behavior. As phase-space density ρ increases toward degenerate values, we observe a gradual divergence of the compressibility κ from the value predicted by a bare-atom model, κ ba . κ/κ ba grows to 1.7 before ρ reaches the value for which we observe the sudden emergence of a spike at p = 0 in momentum space. Momentum-space images are acquired by means of a 2D focusing technique. Our data represent the first observation of non-mean-field physics in the presuperfluid but degenerate 2D Bose gas.PACS numbers: 05.30. Jp, 67.10.Ba, Because of the enhanced role of fluctuations in lowdimensional systems [1], a two-dimensional (2D) Bose gas at nonzero temperature does not have long-range phase coherence. In a homogeneous system there can be at best only a quasicondensate, no true Bose-Einstein condensation (BEC). Under the combined effect of interactions and quantum degeneracy, however, there is nonetheless a phase transition known as Berezinskii-KosterlitzThouless (BKT) associated with the unbinding of vortex pairs [2]. Below the critical temperature T BKT , the system is superfluid.Experiments in 2D atomic gases [3][4][5][6] are usually conducted in the presence of an inhomogeneous trapping potential. In the complete absence of interactions, the confining potential can resurrect a traditional BEC [7], but for realistic experimental parameters, interatomic interactions tend to suppress BEC by smoothing out the spatial profile [3-6, 8, 9] of the mean density to the point where the sample can be understood as a collection of locally uniform spatial regions, each of which is characterized by a particular local density and thus a particular local value of T BKT . Although these local regions may be too small to test in detail the coherence-related predictions of BKT theory, qualitative effects have been observed in experiment [3,6].Our particular interest is in the region just to the warm side of T BKT . In an earlier experiment on bosons trapped in a 2D optical lattice, we observed a proliferation of vortices as we warmed through the discrete-case equivalent of T BKT [10]. But in that experiment a great many mesoscopic condensates were present, one at each lattice site, on both sides of the BKT transition, because they had condensed at a T BEC distinct from and well above T BKT . For the continuous case, in contrast, there is no corresponding second transition temperature above T BKT . But if the cooling gas has by T BKT already become a medium that can support vortices, whether bound or not, then heuristically we see that it must have continuously evolved from a fully fluctuating nondegenerate gas into a sort of presuperfluid with suppressed density fluctuations [11]. Theory [8,[11][12][13][14][15] validates this intuition, and experiments [5] have in turn been consistent with predictions of that theory. Up until now, however, experiments have not been directly sensitive to the proper...

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

“…This change of the density distribution illustrates the crossover from a dense condensate of bosonic molecules to a degenerate Fermi gas whose density is reduced by the Fermi pressure. However the phase transition into a condensed phase, which manifests itself in the enhanced density of pairs with vanishing momentum, is not directly visible in the measured density distributions.We thus conceived a method to probe the in situ pair momentum distribution of our strongly interacting system by combining a quench of interactions with a matter wave focusing technique, in which the sample expands ballistically in a weakly confining radial harmonic potential [29][30][31][32].Due to its large aspect ratio, our sample expands rapidly and almost exclusively in the z-direction after the release from the optical trap. Hence, its density suddenly drops and interactions between the expanding particles are quenched.…”

“…We thus conceived a method to probe the in situ pair momentum distribution of our strongly interacting system by combining a quench of interactions with a matter wave focusing technique, in which the sample expands ballistically in a weakly confining radial harmonic potential [29][30][31][32].…”

Matter-wave Fourier optics with a strongly interacting two-dimensional Fermi gas

“…In order to access the momentum distribution we realize the technique theoretically discussed above in a way similar to the one presented in [10] for a 2D Bose gas: we switch off the optical trap and let the gas evolve in the weak magnetic potential, which has a harmonic trapping frequency ω exp = 2π × 10 Hz in radial direction [26]. As the strong confinement along the z-direction is switched off the sample rapidly expands in z-direction and quickly enters the ballistic regime.…”

“…Therefore, to access the true momentum distribution of a strongly interacting quantum gas, it is crucial to develop methods that overcome the limitations of this technique. So far the best candidate for weakly interacting systems has been the Bose-gas focusing technique [6][7][8][9][10] which brings the far field limit to finite TOF. In [10] this is achieved by letting the cloud expand in a weak harmonic potential.…”

“…So far the best candidate for weakly interacting systems has been the Bose-gas focusing technique [6][7][8][9][10] which brings the far field limit to finite TOF. In [10] this is achieved by letting the cloud expand in a weak harmonic potential. However all these methods crucially rely on a ballistic expansion of the sample, which is challenging to achieve for strongly interacting systems.…”