New Fermion Superfluidity in a Gas of Fermi Atoms with a Feshbach Resonance

Yoji Ohashi

Aurel Bulgac, Yongle Yu

Cindy A. Regal, Jin, D. S.

This thesis presents experiments probing physics in the crossover between Bose-Einstein condensation (BEC) and BCS superconductivity using an ultracold gas of atomic fermions. Scattering resonances in these ultracold gases (known as Feshbach resonances) provide the unique ability to tune the fermion-fermion interactions. The work presented here pioneered the use of fermionic Feshbach resonances as a highly controllable and tunable system ideal for studying the cusp of the BCS-BEC crossover problem. Here pairs of fermionic atoms have some properties of diatomic molecules and some properties of Cooper pairs. I present studies of a normal Fermi gas at a Feshbach resonance and the work required to cool the gas to temperatures where superfluidity in the crossover is predicted. These studies culminated in our observation of a phase transition at the cusp of the BCS-BEC crossover through condensation of fermionic atom pairs. I also discuss subsequent work that confirmed the crossover nature of the pairs in these condensates.

Martin W. Zwierlein

C.-H. Pao, Shin-Tza Wu, Sungkit Yip

We consider a dilute atomic gas comprising two species of Fermions with unequal concentrations. We study at zero temperature the atom‐molecule mixture with inter‐species low‐energy scatterings induced by a Feshbach resonance. To examine the stability of this superfluid, we calculate the chemical potential and the superfluid density for fixed individual concentrations. We find that the system can have distinct properties compared to the ordinary Bose‐Fermi mixture due to the unbound fermions.

Yi Yu, Qijin Chen