Negative Differential Resistance due to Nonlinearities in Single and   Stacked Josephson Junctions
  Josephson junction systems with a negative differential resistance play an essential role for applications. As a well known example, long Josephson junctions of the BSCCO type have been considered as a source of THz radiation in recent experiments. Numerical results for the dynamics of the fluxon system have demonstrated that a cavity induced negative differential resistance plays a crucial role for the emission of electromagnetic radiation. We consider the case of a negative differential resistance region in the McCumber curve itself of a single junction and found that it has an effect on the emission of electromagnetic radiation. Two different shapes of negative differential resistance region are considered and we found it is essential to distinguish between current bias and voltage bias. 
G. Filatrella, V. Pierro, N.F. Pedersen, and M.P. Soerensen
cond-mat.supr-con