Anisotropy of Upper Critical Field in a One-Dimensional Organic System, (TMTTF)2PF6 under High Pressure

Abstract

We have measured the temperature-dependent resistivity of (TMTTF) 2 PF 6 up to 7 GPa using a turnbuckle-type diamond anvil cell (DAC) and at magnetic fields of up to 5 T. Unlike previous reports on organic compounds, a zero resistance was observed in the superconducting state even at high pressures. Superconductivity was observed in the range of P = 4.18 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 H c2 ( T ) displays a positive curvature without saturation. This could be related to the wide pressure range of the coexistence between the spin density wave (SDW) and superconductivity (SC) ground state, and may be caused by an FFLO state, for a magnetic field along the a - and b '-axes with T ≥0.5 K for P = 4.48 GPa. This feature is suppressed with increasing pressure when the orbital pair breaking mechanism becomes dominant. The Ginzburg–Landau coherence lengths for three different axes obtained from this work show that (TMTTF) 2 PF 6 is an anisotropic three-dimensional superconductor.