▎ 摘　　要

Recently, graphene has received a great deal of attention both experimentally and theoretically because of its unusually device-friendly material properties, including high mobility, high electron density, and long-carrier mean-free-path, etc. It has also been shown that graphene-based chemical sensors are capable of detecting minute concentrations (1 part per billion) of various active gases due to its high sensitivity. We consider the role of interband coherence in a kinetic equation approach to the analysis of transport in graphene with long-range electron-impurity scattering for an ac electric field in terahertz regime. We find that there is a minimum conductivity in the electron-density dependence of conductivity when the frequency of the ac electric field omega is less than a critical value omega(0). For omega > omega(0) the conductivity monotonically decreases with decreasing electron density. The dependence of omega(0) on the form of the electron-impurity scattering potential is examined.