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Register a new userCondensation of Pairs of Fermionic Atoms Near a Feshbach Resonance
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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 ^6Li_2 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 pre-existing molecules which are quasi-stable even above the two-body Feshbach resonance due to the presence of the degenerate Fermi gas.
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We studied the magnetic field dependence of the inelastic decay of an ultracold, optically trapped 6-Li gas of different spin compositions. The spin mixture of the two lowest hyperfine states showed two decay resonances at 550 G and 680 G due to two-body collisions, close to the predicted Feshbach resonance of the elastic s-wave collisions at 800 G. The rapid decay near Feshbach resonances found in bosonic gases was found to be suppressed by the Pauli exclusion principle. The observed lifetimes of several hundred milliseconds are much longer than the expected time for Cooper pair formation and the phase transition to superfluidity in the vicinity of the Feshbach resonance.
Within the framework of the variational approach the ground state is studied in a gas of Fermi atoms near the Feshbach resonance at negative scattering length. The structure of the originating superfluid state is formed by two coherently bound subsystems. One subsystem is that of quasi molecules in the closed channel and the other is a system of pairs of atoms in the open channel. The set of equations derived allows us to describe the properties of the ground state at an arbitrary magnitude of the parameters. In particular, it allows one to find a gap in the spectrum of single-particle Fermi excitations and sound velocity characterizing a branch of collective Bose excitations.
Feshbach resonances in lithium-6 were experimentally studied and theoretically analyzed. In addition to two previously known s-wave resonances, we found three p-wave resonances. Four of these resonances are narrow and yield a precise value of the singlet scattering length, but do not allow us to accurately predict the location of the broad resonance near 83 mT. Its position was previously measured in a molecule-dissociation experiment for which we, here, discuss systematic shifts.
We study the stability of the paired fermionic p-wave superfluid made out of identical atoms all in the same hyperfine state close to a p-wave Feshbach resonance. First we reproduce known results concerning the lifetime of a 3D superfluid, in particular, we show that it decays at the same rate as its interaction energy, thus precluding its equilibration before it decays. Then we proceed to study its stability in case when the superfluid is confined to 2D by means of an optical harmonic potential. We find that the relative stability is somewhat improved in 2D in the BCS regime, such that the decay rate is now slower than the appropriate interaction energy scale. The improvement in stability, however, is not dramatic and one probably needs to look for other mechanisms to suppress decay to create a long lived 2D p-wave fermionic superfluid.
Based on the analytic model of Feshbach resonances in harmonic traps described in Phys. Rev. A 83, 030701 (2011) a Bose-Hubbard model is introduced that provides an accurate description of two atoms in an optical lattice at a Feshbach resonance with only a small number of Bloch bands. The approach circumvents the problem that the eigenenergies in the presence of a delta-like coupling do not converge to the correct energies, if an uncorrelated basis is used. The predictions of the Bose-Hubbard model are compared to non-perturbative calculations for both the stationary states and the time-dependent wavefunction during an acceleration of the lattice potential. For this purpose, a square-well interaction potential is introduced, which allows for a realistic description of Feshbach resonances within non-perturbative single-channel calculations.
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