Superconductivity in the family of organic salts based on the tetramethyltetraselenafulvalene (TMTSF) molecule: (TMTSF)₂X (X=ClO₄, PF₆, AsF₆, SbF₆, TaF₆)

The authors report resistivity measurements along the high-conductivity axis of (TMTSF)₂ClO₄, (TMTSF)₂TaF₆ and (TMTSF)₂SbF₆ for various hydrostatic pressures up to 12.5 kbar, and for transverse magnetic fields of up to 50 kG, in the temperature range 100 mK-4K. These measurements show that all three compounds have a superconducting phase transition near 1K when sufficient pressure is applied (TMTSF)₂SbF₆ and (TMTSF)₂TaF₆ become superconducting for pressures above 10 kbar and 11 kbar respectively. At lower pressures a metal-insulator phase transition is observed which is probably associated with a spin density wave (SDW) instability. They observe a superconducting phase transition in (TMTSF)₂ClO₄ at much lower pressures (<3 kbar) and at 1.5 kbar there is a minimum in the resistivity above the superconducting phase transition temperature indicating the possible coexistence of the SDW and superconducting phases. They show that the critical pressure above which the superconducting phase is stabilised can be correlated with the separation between the sheets of TMTSF molecules and anions and discuss possible implications with reference to all members of the family of (TMTSF)₂X salts.