Charge transport in charge-ordered states of two-dimensional organic conductors, α-(BEDT-TTF) ₂I ₃ and α'-(BEDT-TTF) ₂IBr ₂

Electric conductance and dielectric constant have been measured in the charge-ordered states of the quasi two-dimensional organic conductors α-(BEDT-TTF) ₂I ₃ and α'-(BEDT-TTF) ₂IBr 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 electronhole pairs, consistent with the model of nonlinear I-V characteristics. All these features are qualitatively the same between α-(BEDT-TTF) ₂I ₃ and α'-(BEDT-TTF) ₂IBr ₂, despite the different structural and magnetic properties.