  A decrease in current with increasing voltage, often referred to as negative
differential resistance (NDR), has been observed in many electronic devices and
can usually be understood within a one-electron picture. However, NDR has
recently been reported in nanoscale devices with large single-electron charging
energies which require a many-electron picture in Fock space. This paper
presents a generic model in this transport regime leading to a simple criterion
for the conditions required to observe NDR and shows that this model describes
the recent observation of multiple NDR's in Spin Blockaded transport through
weakly coupled-double quantum dots quite well. This model shows clearly how a
delicate interplay of orbital energy offset, delocalization and Coulomb
interaction lead to the observed NDR under the right conditions, and also aids
in obtaining a good match with experimentally observed features. We believe the
basic model could be useful in understanding other experiments in this
transport regime as well.
