  Motivated by recent experiments, we have studied transport behavior of
coupled quantum dot systems in the Coulomb blockade regime using the master
(rate) equation approach. We explore how electron-electron interactions in a
donor-acceptor system, resembling weakly coupled quantum dots with varying
charging energy, can modify the systems response to an external bias, taking it
from normal Coulomb blockade behavior to negative differential resistance (NDR)
in the curent-voltage characteristics.
