▎ 摘　　要

The formula of weak-field magnetoconductivity Delta sigma(B) proportional to B-2, with B the strength of a magnetic field, is derived for systems corresponding to monolayer and bilayer graphenes. It is represented by Feynman diagrams given by three hexagons. In order to see qualitative features of the magnetoconductivity, it is calculated for monolayer graphene in a constant broadening approximation, in which a single imaginary self-energy is introduced. The results show that -Delta sigma(B) is exactly the same as the counter term due to the Hall current in the energy region away from zero energy, leading to the vanishing magnetoresistance in the Hall bar geometry. In the vicinity of zero energy, -Delta sigma(B) exhibits a prominent double-peak structure similar to that of the counter term. However, its absolute value becomes slightly smaller than the counter term, giving negative magnetoresistance having double-dip structure.