Charge transport in charge-ordered layered crystals θ- (BEDT-TTF )₂ MZn (SCN )₄ (M=Cs,Rb ): Effects of long-range Coulomb interaction and the Pauli exclusion principle

We have measured the current-voltage (I-V) characteristics, dielectric properties, and magnetoresistances of insulating layered organic crystals θ- (BEDT-TTF)₂ MZn (SCN)₄ (M=Cs,Rb), in which electron-electron Coulomb interactions are considered to induce charge ordering. The in-plane I-V characteristics follow the power law with a large exponent that exceeds 10 in the low-temperature limit. The nonlinear I-V characteristics are attributed to electric field induced unbinding of pairs of an electron and a hole that are thermally excited and attracted to each other due to two-dimensional long-range Coulomb interaction. The temperature and frequency dependences of the in-plane dielectric constant for M=Cs are explained by the polarization of the electron-hole bound pairs, consistently with the I-V characteristics. The large dielectric anisotropy (≈100 at 0.6 K) observed for M=Cs suggests two-dimensional long-range Coulomb interaction, which is also consistent with the explanation of the nonlinear I-V curves. The organic crystals have a large positive magnetoresistance ratio, e.g., ≈ 10000% for M=Cs in a magnetic field of 10 T at 0.1 K. The magnetoresistance is nearly independent of the magnetic field orientation despite the highly two-dimensional charge transport, indicating that it is electron-spin related. The magnetoresistance may be caused by magnetic field induced parallel alignment of spins of mobile and localized electrons, both in the highest occupied molecular orbital of a BEDT-TTF molecule, and by the resulting suppression of conduction due to the Pauli exclusion principle.