Summary of Research Accomplishments

Research Interests Qijin Chen Page 1 of 3
RESEARCH INTERESTS
Summary of Research Accomplishments
My research in the past several years has focused on superfluidity and superconductivity, beginning
with high temperature superconductors and moving more recently to ultracold Fermi gases.
In collaboration with my coworkers, I have previously developed a pairing fluctuation theory
for the pseudogap physics in high Tc superconductors [Phys. Rev. Lett. 81, 4708 (1998), cited over
130 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 finitemomentum
pairing fluctuations play a progressively more important role as the pairing strength
increases. It has been successful in addressing many high Tc experiments, including, among others,
the cuprate phase diagram and the highly unusual quasi-universal behavior of the superfluid
density.
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. In collaboration with Holland et al. at JILA,
we have performed the first theoretical study of pseudogap effects in atomic Fermi gases. We are
the first that have introduced the concept of pseudogap into the field of atomic Fermi gases. 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, etc. We have developed a theoretical
thermometry based on adiabatic magnetic field sweeps. Importantly, the analysis of the
specific heat measurements provides strong evidence for a superfluid phase transition. Recently,
we have developed a theory for momentum resolved rf spectroscopy, our theory results are in good
agreement with experiment. In particular, I propose that one can use the rf signal of minority
fermions in the presence of phase separation at low T and high population imbalance to probe the
homogeneous spectral function A(k, ω) of strongly interacting fermions.
What is unique to my own research are a number of collaborations (and parallel papers) with
leading experimentalists such as John Thomas at Duke University, Deborah Jin at JILA, Randall
Hulet at Rice University, and Cheng Chin at Innsbruck and now the University of Chicago, as well
as close interactions with the Ketterle group at MIT and Rudi Grimm group at Innsbruck, Austria.
I have published more than 60 papers over my career. Since 2005, I have co-authored 4 review
articles, written 1 Science paper, 7 Physical Review Letters, 11 Physical Review Rapid Communications,
and 8 regular Physical Review A and B papers. 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. It has also been highlighted in Nature and discussed in Physics
Today. Among other highlights is a collaboration with Debbie Jin which led to two publications.
One of these [Phys. Rev. A 73, 041601(R) (2006)] was the first to show that her ground breaking
experiments on the phase diagram are in good agreement with my theory.

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.

Research Interests Qijin Chen Page 3 of 3
2. Strongly correlated electrons
I am interested in strongly correlated electron systems in general. The cuprates, organic, and
heavy fermion (super)conductors are good examples of such systems. The new classes of Febased
pnictide superconductors are believed to be strongly correlated as well. These systems call
for new theories beyond the Landau Fermi liquid picture, which has been a foundation for modern
solid state physics.
High Tc, organic, heavy fermion and Fe-based pnictide superconductors
High Tc superconductivity is arguably the greatest challenge in condensed matter physics. There
has recently been a growing body of evidence which supports the relevance of precursor superconductivity,
either through pairing fluctuations or vortex fluctuations, in the underdoped (pseudogapped)
cuprates. Alternatively, new phases (e.g., antiferromagnet, insulating state) and quantum
critical physics may come into play. There are more questions than answers. I am interested in
any outstanding issues, such as the evolution of superfluid density with hole doping, transport
properties, the superconductor-insulator transition at the lower critical doping, vortices and Nernst
effects, Andreev reflection in the pseudogap regime, the one gap vs two gaps debate, etc.
The phase diagrams of organic superconductors are very similar to those of the cuprates. There
has been preliminary evidence for the existence of the pseudogap in these materials. The symmetry
of the order parameter is still far from clear. Our theoretical explanation for the highly unusual T 3/2
power law observed in the low T penetration depth measurements in the BEDT family has been an
significant step toward understanding the symmetry and possible pseudogap phenomena in these
superconductors. Heavy fermion superconductors also have similar phase diagrams, and some
may exhibit quantum critical behavior. Interestingly, the discovery of Fe-based superconductors
has invalidated the general belief that copper is the key to high Tc superconductivity whereas iron is
often thought to be detrimental to superconductivity. It has now seemed to be a consensus that the
order parameter of most Fe-based superconductors has an s± symmetry. Nevertheless, evidence
for other symmetries to exist are also found in some materials. There are still many important
questions to answer, including the most important pairing mechanism.
3. Topological insulators, nanoscience, spintronics, and quantum computing
Topological insulators have become a very hot field over the past couple of years. One important
question is to find interesting physics beyond the single particle picture and find potential
applications. Nanoscale physics, spintronics and quantum computing have great potentials for industrial
applications. They are often interrelated. I have always been interested in these exciting
areas. Optical lattices are also interrelated with nanoscale physics, quantum information science
and quantum computing. One may use optical lattices to study topological insulators. In particular,
the p + ip superfluid may be a topological superfluid, which can be used to study Majorana
fermions. These Majorana fermions are believed to be able to be used for topological quantum
computing.