Metal-insulator transitions and superconductivity in ditetramethyltetraselenafulvalenium fluorosulfonate [(TMTSF)2FSO3]

We have investigated the phase diagram of the orgainic charge-transfer salt ditetramethyltetraselenafulvalenium fluorosulfenate [(TMTSF)2FSO3] which contains an asymmetric dipolar anion. At low pressures there is a metal-insulator transition related to anion ordering. Between 5 and 6 kbar, there is a dipolar glass phase with increased resistivity. However, thermopower and superconducting measurements indicate that the Fermi surface remains ungapped. Above 5 kbar there is a superconducting transition with Tc>2 K. This is the highest Tc for this family of compounds and suggests that the anions can enhance Tc.