Kondo effect has been well studied both in heavy fermions and quantum dots
theoretically and experimentally; however, the investigation of multi-impurity
problems is far from complete. The double quantum dot provides a versatile
system for studying the strongly correlated behavior of multi-impurity Kondo
problems. The competition between the Kondo correlation and anti-ferromagnetic
impurity spin correlation in a carbon nanotube double quantum dot (CNT DQD) can
give rise to different quantum states. A SU(4)XSU(4) Kondo to SU(4) Kondo
transition in such system is thought to exist. Two isolated CNT QDs connected
by two leads on each sides shows SU(4)XSU(4) Kondo effect when T"<"Tk. Unlike
the double SU(2) quantum dot, CNT DQD in the region that each dot have only one
electron on the top level can be described by 6 dimensional representation of
SU(4) group when tuning on the tunneling. Thus, one can expect a transition
from SU(4)XSU(4) Kondo state to new SU(4) Kondo state. CNT DQD is modeled by
double SU(4) Anderson impurity model; a worldline Monte Carlo method along with
the directed loop updates is applied to solve this problem. Dynamic properties,
such as impurity spectral function and conductance, are studied by Maximum
Entropy Method and Analytic continuation of Quantum Monte Carlo data.
P.S.: Dong passed his Preliminary Exam 1 week after he gave this talk. Congratulations!