We present the pressure–temperature phase diagram of the bilayer-type organic conductor (ETTM-STF)${}_2{\mathrm{BF}}_4$, established by ${}^1\mathrm{H}, {}^{19}\mathrm{F}$, and ${}^{77}\mathrm{Se}$- NMR and ac susceptibility measurements under ambient and applied pressures. At ambient pressure, the system exhibits critical slowing down of the conformational dynamics of the ETTM-STF terminal group and undergoes a nonmagnetic transition at 20 K, accompanied by tetramerization associated with molecular shape distortion. Upon applying pressure, the nonmagnetic phase is progressively suppressed and vanishes near 10 kbar. Around this pressure region, clear diamagnetic signals are observed, demonstrating bulk superconductivity. The superconducting transition temperature reaches 8.0 K at 7 kbar, which is notably high among non-BEDT-TTF organic superconductors. Our combined nuclear magnetic resonance (NMR) and ac susceptibility results reveal that the superconducting phase is directly adjacent to the nonmagnetic phase in the pressure-temperature phase diagram. This finding stands in stark contrast to the majority of correlated-electron superconductors, where superconductivity typically emerges in proximity to magnetically ordered phases, suggesting a potentially unconventional pairing mechanism in this system.

Close

PrevNext

View figure in article

• Superconductivity
• Organic superconductors
• Strongly correlated systems
• Unconventional superconductors
• AC susceptibility measurements
• Nuclear magnetic resonance