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.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.

arXiv:cond-mat/0601054Bulletin of the American Physical SocietyJan 3, 200635 citations

The Fermi Superfluid

Hui Zhai

1. Compare the difference between the two-body problem in a vacuum and on top of the Fermi sea. 2. Introduce the BCS mean-field theory and the BCS ground state wave function and its excitation. 3. Introduce the physical picture of BCS pairing. 4. Introduce the concept of contact using the BCS state as an example. 5. Discuss how the BCS state responds to the Zeeman field. 6. Discuss different pairing symmetries. 7. Discuss how the BCS-BEC crossover mean-field equation recovers features in both the BCS and the BEC limit. 8. Discuss different behaviors of the excitation spectrum in the BCS and the BEC regimes and determine superfluid critical velocity from the excitation spectrum. 9. Discuss how the superfluid transition temperature changes from the BCS regime to the BEC regime. 10. Introduce a list of experimental observations on the BCS-BEC crossover. 11. Discuss the challenging issues in studying the unitary Fermi gas. 12. Introduce the concept of holographic duality and the prediction of the $\eta/s$ bound.1. Compare the difference between the two-body problem in a vacuum and on top of the Fermi sea. 2. Introduce the BCS mean-field theory and the BCS ground state wave function and its excitation. 3. Introduce the physical picture of BCS pairing. 4. Introduce the concept of contact using the BCS state as an example. 5. Discuss how the BCS state responds to the Zeeman field. 6. Discuss different pairing symmetries. 7. Discuss how the BCS-BEC crossover mean-field equation recovers features in both the BCS and the BEC limit. 8. Discuss different behaviors of the excitation spectrum in the BCS and the BEC regimes and determine superfluid critical velocity from the excitation spectrum. 9. Discuss how the superfluid transition temperature changes from the BCS regime to the BEC regime. 10. Introduce a list of experimental observations on the BCS-BEC crossover. 11. Discuss the challenging issues in studying the unitary Fermi gas. 12. Introduce the concept of holographic duality and the prediction of the $\eta/s$ bound.

Cambridge University Press eBooksJan 30, 20210 citations

BCS-BEC crossover in strongly interacting Fermi gases

Giancarlo Calvanese Strinati

Bulletin of the American Physical SocietyMar 22, 20050 citations

Studying the BCS-BEC crossover regime with a Fermi gas of

^{40}

K atoms

Studying the BEC-BCS crossover with an ultracold gas of

^{6}

Li atoms

\eta/s

bound.