Christopher Gilbreth

Christopher Gilbreth's picture
Lead Physicist
Quantum
Research Areas: 
Nuclear Physics
Research Type: 
Theorist
Education: 
Ph.D. 2013, Yale University
Advisor: 
Yoram Alhassid
Dissertation Title: 
Ultracold Fermi Gases: Effective Interactions and Superfluidity
Dissertation Abstract: 

Cold atomic Fermi gases are clean, highly experimentally tunable systems with connections to many different fields of physics. However, in the strongly-interacting regime they are nonperturbative and difficult to study theoretically. One challenge is to calculate the energy spectra of few-body cold atom systems along the crossover from a gas described by a Bose-Einstein condensate (BEC) to a gas described by Bardeen-Cooper-Schrieffer (BCS) theory. The configuration-interaction (CI) method is widely used for such problems, but the finite model spaces employed require carefully chosen interactions with good convergence characteristics. We study a recently introduced effective interaction in the CI approach for the unitary Fermi gas, extending it to the BEC-BCS crossover and examining its properties analytically and numerically. We find it exhibits fast convergence away from the unitary limit, which allows us to calculate the low-lying energy spectrum of three- and four- particle systems along the crossover.

For larger systems of cold atoms, the superfluid phase transition is a subject of principle interest, but is still incompletely understood. Realistic ab initio calculations of the heat capacity across the superfluid phase transition have not to date been achieved, and the nature of the pseudogap effect in the unitary regime is still a subject of debate. We apply the auxiliary field quantum Monte Carlo (AFMC) method to shed light on the superfluid phase transition by studying a finite-size trapped gas in the canonical ensemble. The AFMC method permits fully nonperturbative calculations of strongly-interacting systems without introducing uncontrolled approximations, but can be computationally intensive. Our calculations are made feasible by introducing a new stabilization technique to improve the scaling of the method. Applying this method, we present new results concerning the signatures of the superfluid phase transition and pseudogap phase in this system.