Pressure-induced Mott transition and signatures of superconductivity in β′-(BEDT-TTF)2RCF2SO3 (R = CF3, SF5)

ABSTRACT We investigated pressure-induced phases in the organic dimer–Mott insulators $\beta ^\prime\hbox{-} {\rm \lpar BEDT{\hbox{-}}TTF\rpar }_2{\rm CF}_3{\rm CF}_2{\rm SO}_3$β′-(BEDT-TTF)2CF3CF2SO3 and $\beta ^\prime {\hbox{-}}{\rm\lpar BEDT{\hbox{-}}TTF\rpar }_2$β′-(BEDT-TTF)2 ${\rm SF}_5{\rm CF}_2{\rm SO}_3$SF5CF2SO3 [BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene] by measuring their electrical resistivity under pressures up to 12 GPa utilizing a cubic-anvil cell. Although previous high-pressure studies on BEDT-TTF salts using cubic anvil techniques achieved a maximum pressure of approximately 10 GPa, the use of an MgO gasket and Daphne 7373 as the pressure-transmitting medium in this study enabled stable measurements up to 12 GPa. Increasing pressure produced a substantial reduction in room-temperature resistivity, and both salts exhibited insulator–metal transitions. Near the Mott boundary, a pronounced decrease in resistivity was observed at low temperatures. Although zero resistance was not achieved, this behavior is consistent with that of a superconductivity precursor. These findings indicate that the vicinity of the Mott critical pressure is a promising regime for superconductivity in $\beta ^\prime$β′-type dimer–Mott systems and demonstrate the effectiveness of cubic-anvil high-pressure transport measurements for systematic exploration of this regime.