Summary of Research Accomplishments

My research in the past three years has focused on superfluidity and superconductivity, beginning with high temperature superconductors but with a current primary focus on ultracold Fermi gases and quantum simulations.

In collaboration with my coworkers, I have previously developed a pairing fluctuation theory for the pseudogap physics in high T_c superconductors [Phys. Rev. Lett. 81, 4708 (1998), cited over 260 times]. This theory naturally interpolates between the BCS and BEC limits, and extends the mean-field BCS theory to short coherence length superconductors in a natural way so that finite-momentum pairing fluctuations play a progressively more important role as the pairing strength increases. It has been successful in addressing many high T_c experiments, including, among others, the cuprate phase diagram and the highly unusual quasi-universal behavior of the superfluid density as a function of temperature and hole doping concentration.

Since 2005, I have extended our pairing fluctuation theory and applied it to trapped atomic Fermi gases. I have made great progress in this area. We are the first that have introduced the concept of pseudogap into the field of atomic Fermi gases. The existence of a pseudogap in Fermi gases has now been established with conclusive evidence. We have successfully addressed the pairing gap in cold Fermi gas superfluids, the density profiles in a trap, the thermodynamic behavior in the strongly interacting regime, the phase diagrams with and without population imbalance, the collective mode behavior, theory on rf spectroscopy, Fermi-Fermi mixtures, Fulde-Ferrel-Larkin-Ovchinnikov physics, etc. Our recent works on Fermi gases in optical lattices have uncovered a series of new quantum phenomena and made predictions to be verified in future experiments.

I have published about 100 papers over my career in high profile journals, including Science, Nature, Reviews of Modern Physics, Physics Reports, Reports on Progress in Physics, Physical Review Letters, National Science Review, etc., with a total SCI citation over 3300, and an H-index of 31, (or alternatively, by google scholar, a citation over 4700, and H-index of 36). Among them, the Science paper in collaboration with the Thomas group has been cited as one of several ground-breaking works in the field of ultracold Fermi gases. The Physics Reports article has been the first review in the field of ultracold Fermi gases.

Current Research Interests

My primary focus is on the physics of superfluidity and related phenomena in ultracold atomic Fermi gases, while I am also interested in other areas.

1. Ultracold atomic Fermi gases, optical lattices and quantum simulations

Superfluidity in ultracold atomic Fermi gases is one of the most exciting research areas in condensed matter and atomic physics in recent years. Via Feshbach resonances, one can tune the attractive interaction between fermionic atoms from very weak to very strong. This makes it possible to observe Bose-Einstein condensation (BEC) in quantum degenerate Fermi gases directly over the entire range of the BCS-BEC crossover. Furthermore, this has created a strong hope that these systems may help us understand high T_c superconductivity. Besides interaction strength, there have now been a collection of many other tunable parameters, including population imbalance, geometric ratio or dimensionality, mass imbalance, pairing symmetry, optical lattices and lattice depth, lattice geometry, synthetic gauge field, spin-orbit coupling, etc. This brings us an opportunity to simulate existing and engineer new and exotic quantum systems, with very rich physics to explore. In particular, with optical lattices, atomic Fermi gases provide an ideal platform to simulate the condensed matter system, and thus provide the hope to offer a solution to unresolved difficult issues. One of our key focuses is on the understanding of Fermi Hubbard physics, as a quantum simulator for the high Tc cuprate superconductors. To this end, we need to solve with high level of quantitative accuracy for the phase diagram for both attractive and repulsive Hubbard model, in both 3D and 2D, with and without population imbalance, at a varing filling factor and interaction strength.

This is the main focus of my group.

2. Strongly correlated electrons

This involves mainly superconductivity in the cuprates, Fe-based superconductors, heavy fermion systems, as well as graphene based low-dimensional superonductors. It also includes other 2D lattices, such as the Lieb lattice and Kagome lattice. Issues of interest include the pairing mechanism, pseudogap phenomena, superfluid behavior, pair density wave, etc., These systems call for new theories beyond the Landau Fermi liquid picture, which has been a foundation for modern solid state physics.