Anisotropy of upper critical field in one-dimensional organic system, (TMTTF)$_2$PF$_6$ under extremely high pressure

TL;DR

This study investigates the anisotropic upper critical magnetic field in the organic superconductor (TMTTF)$_2$PF$_6$ under high pressure, revealing superconductivity over a specific pressure range and evidence of FFLO state behavior.

Contribution

First measurement of the temperature-dependent upper critical field anisotropy in (TMTTF)$_2$PF$_6$ under high pressure, showing possible FFLO state and 3D anisotropic superconductivity.

Findings

Superconductivity observed between 4.18 GPa and 6.03 GPa with a peak T_c of 2.25 K.

H_{c2}(T) exhibits positive curvature suggestive of FFLO state at certain conditions.

(TMTTF)$_2$PF$_6$ is confirmed as an anisotropic three-dimensional superconductor.

Abstract

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.