  A new mechanism is proposed to explain the origin of negative differential
resistance (NDR) in a strongly coupled single molecule-metal junction. A
first-principles quantum transport calculation in a Fe-terpyridine linker
molecule sandwiched between a pair of gold electrodes is presented. Upon
increasing applied bias, it is found that a new phase in the broken symmetry
wavefunction of the molecule emerges from the mixing of occupied and unoccupied
molecular orbital. As a consequence, a non-linear change in the coupling
between molecule and lead is evolved resulting to NDR. This model can be used
to explain NDR in other class of metal-molecule junction device.
