Inelastic Collisions of a Fermi Gas in the BEC-BCS Crossover

Abstract

We measure inelastic three-body and two-body collisional decay rates for a two-component Fermi gas of 6Li, which are highly suppressed by the Pauli exclusion principle. Our measurements are made in the BEC-BCS crossover regime, near the two-body collisional (Feshbach) resonance. At high temperature (energy) the data show a dominant three-body decay process, which is studied as a function of bias magnetic field. At low energy, the data show a coexistence of two-body and three-body decay processes near and below the Feshbach resonance. Below resonance, the observed two-body inelastic decay can arise from molecule-atom and molecule-molecule collisions. We suggest that at and above resonance, an effective two-body decay rate arises from collisions between atoms and correlated (Cooper) pairs that can exist at sufficiently low temperature.

Influence of Cooper pairing on the inelastic processes in a gas of Fermi atoms

“…The recombination in this case is accompanied by the formation of a molecule at a deep energy level. Completely three-particle character of the relaxation in the case of the negative scattering length a < 0, including unitary region, is demonstrated in the work [4]. The formation of the weakly bounded dimers in the case of the positive scattering length a > 0 changes the kinetics of the strongly bound molecule formation essentially [5].…”

Fermion Pairing in Flatland

“…DOI: 10.1103/PhysRevLett.108.045302 PACS numbers: 67.85.Àd, 03.75.Lm, 05.30.Fk, 32.30.Bv Interacting fermions in coupled two-dimensional (2D) layers present unique physical phenomena and are central to the description of unconventional superconductivity in high-transition-temperature cuprates [1] and layered organic conductors [2]. Experiments on ultracold gases of fermionic atoms have allowed access to the crossover from Bose-Einstein condensation (BEC) of tightly bound fermion pairs to Bardeen-Cooper-Schrieffer (BCS) superfluidity of long-range Cooper pairs in three spatial dimensions [3,4] and, more recently, the confinement of interacting Fermi gases to two spatial dimensions [5][6][7][8][9]. A fermionic superfluid loaded into a periodic potential should form stacks of two-dimensional superfluids with tunable interlayer coupling [10][11][12][13], an ideal model for Josephsoncoupled quasi-2D superconductors [1,14].…”

“…We realize a system that is tunable from 3D to 2D with a gas of ultracold fermionic 6 Li atoms trapped in an optical trap and a standing-wave optical lattice. The lattice produces a periodic potential along the z direction,…”

“…The atomic gas consists of an equal mixture of 6 Li atoms in the first and third hyperfine states (denoted as j1i and j3i), chosen to minimize final-state interaction effects in the rf spectra [24]. Interactions between atoms in states j1i and j3i are greatly enhanced by a broad Feshbach resonance at 690.4(5) G [25].…”

“…An ultracold gas of 6 Li is produced by sympathetic cooling with 23 Na as described previously [21]. The 6 Li atoms are transferred from a magnetic trap to an optical dipole trap (wavelength 1064 nm, waist 120 m), with axial harmonic confinement (frequency 22.8 Hz) provided by magnetic field curvature.…”