Pair fraction in a finite-temperature Fermi gas on the BEC side of the BCS-BEC crossover

Abstract

We investigate pairing in a strongly interacting two-component Fermi gas with positive scattering length. In this regime, pairing occurs at temperatures above the superfluid critical temperature; unbound fermions and pairs coexist in thermal equilibrium. Measuring the total number of these fermion pairs in the gas we systematically investigate the phases in the sectors of pseudogap and preformed-pair. Our measurements quantitatively test predictions from two theoretical models. Interestingly, we find that already a model based on classical atom-molecule equilibrium describes our data quite well.

Second sound in the crossover from the Bose-Einstein condensate to the Bardeen-Cooper-Schrieffer superfluid

“…The gas is confined by a combined magnetic and optical dipole trap with a trap depth of U 0 ≈ 1 μK × k B , for details see ref. 24 , 25 . The trap is nearly harmonic and cylindrically symmetric with trapping frequencies ω r = 2 π × 305 Hz and ω x = 2 π × 21 Hz.…”

Electronic Structure of Cuprates in a Boson–Fermion Mixture Theory

“…Integration over φ is performed by assuming the integrand in ( 13) to be φ-independent. Expressing Q as the new variable ε ¼ ℏ 2 Q 2 =2m allows writing Q 2 dQ ≡ ½ð2mÞ 3=2 =2ℏ 3 ε 1=2 dε in (13). This leads to…”

“…[ 12 ] In ref. [13], in addition to directly observing pairs of fermions in a Fermi gas of Li atoms, attempts were made to elucidate the relationship between pair formation and the PG, where unbound fermions coexist with ‐pairs.…”

Pair correlations in the normal phase of an attractive Fermi gas

“…Preformed pairs were recently studied in an experiment with a spin-balanced two-component Fermi gas of 6 Li in the normal phase [4], where the number of fermion pairs N p was determined by converting all atom pairs to tightly-bound diatomic molecules which afterwards were detected. The pairing fraction N p /N σ (where N σ is the number of all atoms per spin-state) was reported for various temperatures and couplings on the BEC side of the BCS-BEC crossover.…”

“…A preliminary theoretical account of the pairing fractions was already presented in Ref. [4], which was obtained by a statistical model of non-interacting atoms and molecules at equilibrium [5,6] as well as by an abinitio diagrammatic t-matrix approach [7]. However, the comparison between experiment and theory presented in Ref.…”