On the low-temperature states of highly correlated BETS conductors

The λ-type BETS conductors (BETS=bis(ethylenedithio)tetrathiafulvalene), λ-BETS2MX4 (M=Ga, Fe; X=C1, Br) afford an extremely large variety of low-temperature properties by substitution of metal (M) and halogen (X) atoms. In this paper the superconducting properties of λ-BETS2GaBrxCl4−x and the coupling (or decoupling) of the antiferromagnetic and metal-insulator transitions of λ-BETS2FeBrxCl4−x are reported. λ-BETS2GaBrxCl4−x is an organic superconductor at x < 0.8. While λBETS2GaBr1.5Cl2.5 is a semiconductor at ambient pressure and becomes a superconductor at high pressure. The maximum Tc was almost the same as that of so-called “10 K-class organic superconductor” κ-ET2Cu(NCS)2. The anisotropic decrease of magnetic susceptibility of λ-BETS2GaBrxCl4−x (x > 1.0) suggested that non-antiferromagnetic insulating state is located near superconducting phase. The x-dependencies of resistivity and magnetic susceptibility of λ-BETS2FeBrxCl4−x were also examined. At x < 0.2, the metal-insulator and antiferromagnetic transitions take place cooperatively. Around x=0.3, these two transitions tend to be separated At x > 0.6, the coupling between π and d electron systems becomes very weak and the Fe3+ spins system undergoes an antiferromagnetic transition independently of π electron system. When the π-d coupling is strong (x < 0.2), the magnetic easy axis of the antiferromagnetic structure is parallel to c. But it becomes perpendicular to c when the coupling becomes weak (x > 0.6). The T-x phase diagrams of λ-BETS2GaBrxCl4−x and λ-BETS2FeBrxCl4−x resemble to each other. The superconducting phase of λBETS2 GaBrxCl4−x appears at the x-region where the π-d coupling can be observed in λ-BETS2FeBrxCl4−x.