Anisotropy of upper critical field in one-dimensional organic system, (TMTTF)₂PF₆ under extremely high pressure

We have measured the temperature dependent resistivity of (TMTTF)$_2$PF$_6$ to 7 GPa using a turnbuckle DAC (diamond anvil cell) and in magnetic field up to 5 T. Unlike many other organic conductors, a zero resistance was observed in the superconducting state even under high pressures. Superconductivity was observed over a range of $P$ = 4.18 GPa to 6.03 GPa and showed a peak $T_c$ of 2.25 K at 4.58 GPa. The temperature dependence of the upper critical magnetic field $H_{c2}(T)$ was determined via resistivity at $P$ = 4.58 GPa, for the intrachain ($a$), interchain ($b'$), and interlayer ($c^*$) configurations and the $H_{c2}(T)$ displays positive curvature without saturation, which may be originated by a FFLO state, for magnetic field along a-axis and $b'$-axis in T $\ge$ 0.5 K for $P$ = 4.48 GPa. This feature is suppressed with increasing pressure and the orbital pair breaking mechanism becomes dominant. The values of the Ginzberg-Landau coherence length for three different axes obtained from this work shows that (TMTTF)$_2$PF$_6$ is an anisotropic three - dimensional superconductor.