Charge transport in antiferromagnetic insulating phase of two-dimensional organic conductor λ-(BETS)₂FeCl₄

Resistance and dielectric constants have been measured in the antiferromagnetic insulating phase of the quasi-two-dimensional organic conductor λ-(BETS)₂FeCl₄ to understand charge transport. Nonlinear current-voltage characteristics are observed at low temperatures, which are explained by a charge transport model based on the electric-field dependent Coulomb potential between the thermally excited electron and hole. A small dip in the magnetic field dependence of the resistance is found at 1.2 T, which is ascribed to a spin-flop transition. The large difference between the in-plane and out-of-plane dielectric constants shows the two-dimensionality of the Coulomb potential, which is consistent with the charge transport model. The angular dependence of the metal-insulator transition field is determined, which suggests that the Zeeman effect of the 3d spins of the Fe ions plays an essential role.