Materials viewpoint of organic superconductors

Since the first discovery of an organic superconductor, TMTSF2PF6 (TMTSF = tetramethyltetraselenafulvalene) with the superconducting transition temperature (T-c) = 0.9K under 12kbar, by the suppression of spin density wave (SDW) state, approximately 120 organic superconductors have been found. Among them, the highest T-c is 14.2K under 82kbar for beta'-(BEDT-TTF)(2)ICI2 [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene]. Organic superconductors are categorized into three groups. (1) A superconducting (SC) state neighbors the SDW state, and appears by the suppression of the SDW state to afford a Tc below I K. (2) Another SC state is competitive with a nonmagnetic insulating state induced by anion ordering, charge density wave (CDW); or charge ordering, afford a T, below 2 K. (3) Yet another SC state is next to an antiferromagnetic (AF) order to afford a T, up to 14 K under an ambient condition or a pressurized condition. In this article, organic superconductors are reviewed on the basis of a variety of molecules, and finally, recent novel organic superconductors (MDT-TS)(AuI2)(0.441) [MDT-TS = 5H-2-(1,3-diselenol-2-ylidene)-1,3,4,6-tetrathiapentalene] obtained by suppression with an incommensurate AF state and beta-(meso-DMBEDT-TTF)(2)I-3 [DMBEDT-TTF = dimethylbis(ethylenedithio)tetrathiafulvalene] in competition with a checkerboard-type charge-ordered (CO) state, are introduced.