Development and physical properties of magnetic organic superconductors based on BETS molecules [BETS=Bis(ethylenedithio)tetraselenafulvalene] Original Research Article

Owing to the coupling between π and d electron systems, organic conductors based on BETS molecules and magnetic anions such as FeCl4− and FeBr4− exhibit very unique physical properties. The orthorhombic κ-(BETS)2FeBr4 system is the first antiferromagnetic organic superconductor. Due to the metamagnetic nature of Fe spin system, the magnetic field dependence of resistivity shows a strong anisotropy in the ac conduction plane below Tc. On the other hand, the triclinic modification, λ-(BETS)2FeCl4 undergoes coupled antiferromagnetic and metal–insulator (MI) transitions at 8.5 K. The easy axis of the antiferromagnetic spin structure was found to be deviated from the c axis (∥ needle axis of the crystal) by 30–35°. The field-induced insulator-to-metal transition and pressure-induced superconducting transition of λ-(BETS)2FeCl4 were briefly described from the viewpoint of the role of coupling between π and d electron systems.