THE ELECTRICAL AND MAGNETIC-PROPERTIES OF A NOVEL 2-DIMENSIONAL ANTIFERROMAGNET BASED ON BEDT-TTF - THETA-(BEDT-TTF)(2)CU-2(CN)[N(CN)(2)](2)

A crystal-structure analysis and a band calculation, as well as conductivity, thermoelectric power, ESR, and static susceptibility measurements were performed on the new BEDT-TTF salt, theta-(BEDT-TTF)(2)Cu-2(CN)[N(CN)(2)](2). This salt has a unique polymerized anion comprising infinite Cu-N(CN)(2)-Cu double helices bridged by disordered CN groups. Although the two-dimensional arrangement of crystallographically equivalent BEDT-TTF+0.5 cations provides a two-dimensional closed Fermi surface within the tight-binding approximation, this salt is semiconductive even at room temperature, probably due to a strong electron correlation in this system. A semiconductor-semiconductor transition has been observed around 220 K. Below 220 K the electronic system is represented as a Mott insulator and a two-dimensional Heisenberg antiferromagnet. Rapid decreases in the ESR line width and spin susceptibility below 50 K indicate the existence of an additional magnetic transition.