Materials Viewpoint of Organic Superconductors

Since the first discovery of an organic superconductor, TMTSF 2 PF 6 (TMTSF = tetramethyltetraselenafulvalene) with the superconducting transition temperature ( T c )=0.9 K under 12 kbar, by the suppression of spin density wave (SDW) state, approximately 120 organic superconductors have been found. Among them, the highest T c is 14.2 K under 82 kbar for β'-(BEDT-TTF) 2 ICl 2 [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 T c below 1 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 c below 2 K. (3) Yet another SC state is next to an antiferromagnetic (AF) order to afford a T c 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)(AuI 2 ) 0.441 [MDT-TS = 5 H -2-(1,3-diselenol-2-ylidene)-1,3,4,6-tetrathiapentalene] obtained by suppression with an incommensurate AF state and β-( meso -DMBEDT-TTF) 2 I 3 [DMBEDT-TTF = dimethylbis(ethylenedithio)tetrathiafulvalene] in competition with a checkerboard-type charge-ordered (CO) state, are introduced.