Tuning Charge Order in (TMTTF)2X by Partial Anion Substitution

Pressure-Induced Superconductivity of the Quasi-One-Dimensional Organic Conductor (TMTTF)2TaF6

We investigated the superconductivity of (TMTTF)2TaF6 (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)2X (C = Se, S, X: monovalent anion) based on (TMTTF)2PF6 (TCO = 70 K and spin-Peierls: SP, TSP = 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)2X (TMTTF)2TaF6, which has the largest octahedral-symmetry counter anion TaF6 in the (TMTTF)2X series, possesses the highest charge-ordering (CO) transition temperature (TCO = 175 K) in (TMTTF)2X and demonstrates an anti-ferromagnetic transition (TAF = 9 K) at ambient pressure. A superconducting state in (TMTTF)2TaF6 emerged after a metal-insulator transition was suppressed with increasing external pressure. We discovered a superconducting state in 5 ≤ P ≤ 6 GPa from Tc = 2.1 K to 2.8 K, whose pressure range is one-third narrower than that of X = SbF6 (5.4 ≤ P ≤ 9 GPa). In addition, when the pressures with maximum SC temperatures are compared between the PF6 and the TaF6 salts, we found that (TMTTF)2TaF6 has a 0.75 GPa on the negative pressure side in the T-P phase diagram of (TMTTF)2PF6.