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

Abstract

We have measured the current-voltage $(I\text{\ensuremath{-}}V)$ characteristics, dielectric properties, and magnetoresistances of insulating layered organic crystals $\ensuremath{\theta}\text{\ensuremath{-}}{(\text{BEDT-TTF})}_{2}M\text{Zn}{(\text{SCN})}_{4}$ $(M=\text{Cs},\text{Rb})$, in which electron-electron Coulomb interactions are considered to induce charge ordering. The in-plane $I\text{\ensuremath{-}}V$ characteristics follow the power law with a large exponent that exceeds 10 in the low-temperature limit. The nonlinear $I\text{\ensuremath{-}}V$ 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 $M=\text{Cs}$ are explained by the polarization of the electron-hole bound pairs, consistently with the $I\text{\ensuremath{-}}V$ characteristics. The large dielectric anisotropy ($\ensuremath{\approx}100$ at 0.6 K) observed for $M=\text{Cs}$ suggests two-dimensional long-range Coulomb interaction, which is also consistent with the explanation of the nonlinear $I\text{\ensuremath{-}}V$ curves. The organic crystals have a large positive magnetoresistance ratio, e.g., $\ensuremath{\approx}10\text{ }000\mathrm{%}$ for $M=\text{Cs}$ 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.