Charge ordering and nonlinear electrical transport in quasi-one-dimensional organic chains with strong electrostatic interchain interactions

We here examine the electrical and magnetic properties of the isostructural NT3 center dot MCl4 (NT=naphtho[2,1-d:6,5-d']bis([1,2,3] dithiazole and M=Ga and Fe). The crystal structure of NT3 center dot MCl4 consists of one-dimensional pi-stacking chains of NT with strong interchain interactions caused by electrostatic S delta+center dot center dot center dot N delta- contacts. This structure includes four NT molecules with significant differences in molecular structure and charge, exhibiting a characteristic charge ordering, namely, three-dimensional alternation of charge-rich (or -intermediate) and -poor molecules. NT3 center dot GaCl4 and NT3 center dot FeCl4 are found to be semiconductors with sigma(RT)similar to 0.5 S cm(-1) and to exhibit a nonlinear electrical transport at room temperature with a very low threshold field of 80 V cm(-1) for the negative differential resistance. This threshold field significantly increases with a decrease in temperature. The X-band electron paramagnetic resonance (EPR) spectra of NT3 center dot GaCl4 consist of a single-line absorption ascribable to that of the NT+ cation. When the sample is exposed to a current at room temperature, this signal exhibits a drastic decrease in intensity with little change in linewidth. This is attributed to the inhomogeneous formation of EPR-silent conducting pathways for the nonlinear transport. The temperature dependence of the EPR spin susceptibility chi(s) of NT3 center dot GaCl4 suggests a transition toward a spin-gap state below 20 K; chi(s) exhibits a Bonner-Fisher-type temperature dependence above 20 K, but gradually collapses to zero below this temperature.