Theory of inplane magnetoresistance in two-dimensional massless Dirac fermion system

We present the theory of the inplane magnetoresistance in two-dimensional massless Dirac fermion systems including the Zeeman splitting and the electron-electron interaction effect on the Landau level broadening within a random phase approximation. With the decrease in temperature, we find a characteristic temperature dependence of the inplane magnetoresistance showing a minimum followed by an enhancement with a plateau. The theory is in good agreement with the experiment of the layered organic conductor \alpha-(BEDT-TTF)_2I_3 under pressure. In-plane magnetoresistsnce of graphene is also discussed based on this theory.