Charge Transport in Charge-Ordered States of Two-Dimensional Organic Conductors, α-(BEDT-TTF)2I3 and α'-(BEDT-TTF)2IBr2

Electric conductance and dielectric constant have been measured in the charge-ordered states of the quasi two-dimensional organic conductors α-(BEDT-TTF) 2 I 3 and α'-(BEDT-TTF) 2 IBr 2 to understand the charge transport mechanism. The current–voltage ( I – V ) characteristics show a strong nonlinear behavior, which is qualitatively explained by the transport of individual carriers thermally excited from the charge-ordered states in BEDT-TTF layers. The discrepancy with the simulation based on the thermal activation model at low temperatures is likely due to quantum tunneling through the potential barrier. The dielectric constants show a large difference between the in-plane and out-of-plane directions of the samples, showing a highly two-dimensional nature of Coulomb potential. The temperature dependence of dielectric constant is ascribed to the polarization effect of the thermally excited electron–hole pairs, consistent with the model of nonlinear I – V characteristics. All these features are qualitativ...