Summary of Research Accomplishments

Research Interests Qijin Chen Page 2 of 3
Current Research Interests
I have very broad research interests. For example, I have made significant scientific contributions
in experimental condensed matter physics, and also had one year’s training in superstring theory.
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 outlined below.
1. Ultracold atomic Fermi gases, optical lattices and quantum simulation
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
study of these systems may help us understand high Tc superconductivity. Another exciting tunable
parameter in atomic Fermi gases is the population imbalance between the two spin species. The
associated physics has turned out to be very rich. Added recently to this richness is the tunable
mass ratio between the pairing atoms. Finally, optical lattices of atomic traps may be used to simulate
typical as well as exotic condensed matter systems, e.g., the Hubbard model, so that study of
such simulated systems may provide a solution to unsolved problems in condensed matter physics.
Even further, one can engineer many new systems to study exotic quantum phenomena.
Over the past a few years, this field has seen very rapid progress. In 2003, the Jin group
at JILA and the Grimm group at Universität Innsbruck, Austria made a big breakthrough and
achieved molecular condensation in trapped atomic Fermi gases of 40 K and 6 Li, respectively. In
2004, condensation of Cooper pairs was observed in 40 K by the Jin group and in 6 Li by the Grimm
group and the Ketterle group at MIT. Evidence of superfluid phase transition were observed in
thermodynamic behavior of 6 Li by the Thomas group at Duke and the Levin group in Chicago. In
2005, the Ketterle group observed vortex lattices in 6 Li, which is the most definitive signature of
superfluidity. Population imbalance effects have become one of the hottest subjects since 2006, led
by the MIT group and the Hulet group at Rice University. Since then, there has been a bloom in
the study of optical lattices. Recently, synthetic gauge field and orbital effects in cold atoms have
become new hot topics.
A number of theorists have been working in the area of Fermi gases. However, their work has
mostly been based on either the mean-field BCS-Leggett theory at zero temperature or the finite
temperature Noziéres–Schmitt-Rink approach at Tc. The latter lacks self-consistency and cannot
possibly predict a pseudogap. Other theoretical work has been based on the Bose liquid theory,
and lacks proper treatment of the important fermionic pairing interaction.
Our pairing fluctuation theory, originally developed for the pseudogap physics in high Tc superconductivity,
has turned out very successful when applied to ultracold atomic Fermi gases. My
current research includes fixing a couple of minor defects of this theory and apply it to more experiments
or make more predictions. Of course, my research does not necessarily have to do with
this theory. For example, one may proceed with completely different theories with optical lattices,
synthetic gauge fields, etc.