Negative Differential Conductivity in Carbon Nanotubes in the Presence   of an External Electric Field
  We study theoretically the electron transport properties in carbon nanotubes under the influence of an external electric field E(t) using Boltzmann's equation. The current-density equation is derived. Negative differential conductivity is predicted when {\omega}{\tau}<<1 (quasi-static case). We observed this in the neighbourhood where the constant electric field E_o is equal to the amplitude of the AC electric field E_1 and the peak decreases with increasing E_1. This phenomenon can also be used for the generation of terahertz radiation without electric current instability. 
S. S. Abukari, S.Y.Mensah N. G. Mensah, K.W. Adu, K. A. Dompreh, and   A. K. Twum
cond-mat.mes-hall