Coexistence of superconductivity and spin density wave orderings in Bechgaard and Fabre salts

Abstract
In a small range of pressure, superconductivity (SC) and Spin Density Wave (SDW) states are shown to coexist in the Bechgaard salt (TMTSF) ₂PF ₆ and the Fabre salt (TMTTF) ₂BF ₄. In (TMTSF) ₂PF ₆, a precise investigation of the (P,T) phase diagram has led us to demonstrate the coexistence of the two phases with a superconducting critical temperature which is pressure independent while the critical current at zero field is strongly depressed as the pressure is decreased. In (TMTTF) ₂BF ₄, using non-linear transport measurements, we present the signature of the presence of 1D superconducting filaments in a small range of pressure. We also investigate the compound under a magnetic field applied along the c ^*-axis: the upper critical field is more or less pressure independent and is about 2 Tesla (at zero temperature). We suggest that such a high critical field is compatible with the penetration of the magnetic field in the insulating regions of the compound in a similar way of Josephson vortices in layered superconductors.