Insulating, conducting and superconducting states of (TMTSF)2AsF6 under pressure and magnetic field

Abstract

We report resistivity measurements along the high conductivity axis of (TMTSF)2AsF6 for various hydrostatic pressures up to 12 kbar, and for transverse magnetic fields of up to 70 kOe, in the temperature range 0.1 K-300 K. We derive a phase diagram which shows a « triple » point near Tc = 1.22 K and Pc = 10.5 kbar, and a reentrant behaviour of the superconducting state inside the semiconducting state. Moreover we observe a striking anisotropy in the transverse critical fields H c2 and large changes in these fields near the « triple » point. These features suggest we are dealing, in these compounds, with non usual superconducting properties. The transverse magnetoresistance also shows very interesting features including a large anisotropy and a very similar behaviour in the conducting and semiconducting phases. These peculiarities might be rationalized if one assumes (on account of the low dimensionality of the electron system) that first the superconducting instability can grow from below about 30 K and secondly that it can coexist to some extent with the competing SDW instability responsible for the semiconducting phase due to the existence of significant electron-electron couplings