Overview of organic superconductors

Organic materials which are usually used for insulators, were shown to be an electrical conductor by H. Akamatsu, H. Inokuchi, and Y. Matsunaga in 1954. Moreover, J.P. Ferraris et al. showed that TTF · TCNQ was stably metallic down to around 60 K in 1973. Because of a low dimensionality of organic compound, however, a stabilization of an electronic state and a destabilization of a periodic lattice constructed a charge density wave which led a metal-insulator transition (a Peierls transition). After overcoming this low dimensionality, D. Jerome et al. discovered the first organic superconductor, (TMTSF)2 PF6 (Tc = 0.9 K (12kbar)) in 1980. Then with the resisting up of Tc constantly, the superconductor κ-(BEDT-TTF)2(NCS)2 (Tc = 10.4 K) was found in 1987 and the Tc of κ-(BEDT-TTF)2Cu[N(CN)2]Cl, 12.8 K at 0.3kbar, is the highest among the TTF family. Moreover, when the authors included the C60 system is included as molecular superconductor, the Tc = 33 K of Cs2RbC60 reaches the starting point of oxide superconductors. In this paper, the history of organic conductors is described in Sec. 2. In Sec. 3, the author chooses the typical example of organic superconductor, κ-(BEDT-TTF)2Cu(NCS)2, and the preparation, crystal structure, chemical and physical properties are reviewed. In Sec. 4, the mechanism of organic superconductors is discussed and the future of organic superconductors is finally presented. 118 refs