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

Abstract

We have investigated the phase diagram of the orgainic charge-transfer salt ditetramethyltetraselenafulvalenium fluorosulfenate [${\mathrm{}\left(\mathrm{TMTSF}\right)\mathrm{}}_2$FS${\mathrm{O}}_3$] 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 $T_c>2\mathrm{}$ K. This is the highest $T_c$ for this family of compounds and suggests that the anions can enhance $T_c$.