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

In a small range of pressure, superconductivity (SC) and Spin Density Wave (SDW) states are shown to coexist in the Bechgaard salt (TMTSF)2PF6 and the Fabre salt (TMTTF)2BF4. In (TMTSF)2PF6, 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)*BF4, 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.

organic superconductivity; spin density wave

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