Appearance of Negative Differential Conductivity in Graphene Nanoribbons   at High-Harmonics
  We theoretically study current dynamics of graphene nanoribbons subject to bias dc and ac driven fields. We showed that graphene nanoribbons exhibit negative high-harmonic differential conductivity. Negative differential conductivity appears when bias electric filed is in the neighborhood of applied ac filed amplitude. We also observe both even and odd high-harmonic negative differential conductivity at wave mixing of two commensurate frequencies. The even harmonics are more pronounced than the odd harmonics. A possible use of the present method for generating terahertz frequencies at even harmonics in graphene is suggested. 
M. Rabiu, S. Y. Mensah and S. S. Abukari
cond-mat.mtrl-sci cond-mat.mes-hall