Condensation of Pairs of Fermionic Atoms near a Feshbach Resonance

Abstract

We have observed Bose-Einstein condensation of pairs of fermionic atoms in an ultracold 6Li gas at magnetic fields above a Feshbach resonance, where no stable 6Li2 molecules would exist in vacuum. We accurately determined the position of the resonance to be 822+/-3 G. Molecular Bose-Einstein condensates were detected after a fast magnetic field ramp, which transferred pairs of atoms at close distances into bound molecules. Condensate fractions as high as 80% were obtained. The large condensate fractions are interpreted in terms of preexisting molecules which are quasistable even above the two-body Feshbach resonance due to the presence of the degenerate Fermi gas.

Lattice Monte Carlo calculations for unitary fermions in a harmonic trap

“…II has been implemented on a number of clusters and massively parallel architectures. As a result of the low memory footprint, which scales 12 Although we have not proved the convergence of m (Nκ) ef f (τ ) as a function of τ , all of of our numerical evidence suggests that this quantity tends to a constant at late times.…”

“…Such an N * κ then defines a best estimate value for the effective mass at a given time τ . Alternatively, we may define an energy E Nκ = lim τ →∞ m (Nκ) ef f (τ ) 12 and study its convergence as a function of N κ . In all of our studies, we use the latter approach.…”

“…Thus the s-wave phase shift satisfies δ(k) = π/2 for all k and the field theory describing the many-body system is at a conformal fixed point 1 ; in 1998 it was suggested that unitary fermions could serve as the starting point for an effective field theory expansion for nuclear physics [2,3]. Since then the unitary fermion gas has been created and studied experimentally by trapping atoms tuned to a Feshbach resonance by means of an applied magnetic field, exhibiting collective effects interpolating between the well understood phenomena of BCS pairing and Bose-Einstein condensation [4][5][6][7][8][9][10][11][12][13]. The nonperturbative nature of the strongly coupled interaction between unitary fermions poses a nontrivial challenge for theory, and numerical simulation has played an essential role in making progress.…”

Functional renormalization for the Bardeen–Cooper–Schrieffer to Bose–Einstein condensation crossover

“…We briefly discuss the example of 6 Li [3], which besides 40 K has been realized in current experiments [4][5][6][7][8][9] (figure 1a).…”

“…However, the quantitatively precise understanding of BCS-BEC crossover physics requires non-perturbative methods. The experimental realization of molecule condensates and the subsequent crossover to a BCS-like state of weakly attractive fermions [4][5][6][7][8][9] pave the way to future experimental precision measurements and provide a testing ground for non-perturbative methods. An understanding of the crossover on a quantitative level at and near the resonance has been developed through numerical quantum Monte Carlo (QMC) methods [12][13][14][15][16].…”