Pressure-Induced Superconductivity of the Quasi-One-Dimensional Organic Conductor (TMTTF)(2)TaF(6)

We investigated the superconductivity of (TMTTF)(2)TaF(6) (TMTTF: tetramethyl-tetrathiafulvalene) by conducting resistivity measurements under high pressure up to 8 GPa. A cubic anvil cell (CAC) pressure generator, which can produce hydrostatic high-pressure, was used for this study. Since the generalized temperature-pressure (T-P) diagram of (TMTCF)(2)X (C = Se, S, X: monovalent anion) based on (TMTTF)(2)PF(6) (T(CO) = 70 K and spin-Peierls: SP, T(SP) = 15 K) was proposed by Jérome, exploring superconductivity states using high-pressure measurement beyond 4 GPa has been required to confirm the universality of the electron-correlation variation under pressure in (TMTTF)(2)X (TMTTF)(2)TaF(6), which has the largest octahedral-symmetry counter anion TaF(6) in the (TMTTF)(2)X series, possesses the highest charge-ordering (CO) transition temperature (T(CO) = 175 K) in (TMTTF)(2)X and demonstrates an anti-ferromagnetic transition (T(AF) = 9 K) at ambient pressure. A superconducting state in (TMTTF)(2)TaF(6) emerged after a metal-insulator transition was suppressed with increasing external pressure. We discovered a superconducting state in 5 ≤ P ≤ 6 GPa from T(c) = 2.1 K to 2.8 K, whose pressure range is one-third narrower than that of X = SbF(6) (5.4 ≤ P ≤ 9 GPa). In addition, when the pressures with maximum SC temperatures are compared between the PF(6) and the TaF(6) salts, we found that (TMTTF)(2)TaF(6) has a 0.75 GPa on the negative pressure side in the T-P phase diagram of (TMTTF)(2)PF(6).