Coexistence of superconductivity and spin density wave orderings in the organic superconductor (TMTSF)2PF6

The phase diagram of the organic superconductor (TMTSF)(2)PF6 has been revisited using transport measurements with an improved control of the applied pressure. We have found a 0.8 kbar wide pressure domain below the critical point (9.43 kbar, 1.2 K) for the stabilisation of the superconducting ground state featuring a coexistence regime between spin density wave (SDW) and superconductivity (SC). The inhomogeneous character of the said pressure domain is supported by the analysis of the resistivity between T-SDW and T-SC and the superconducting critical current. The onset temperature T-SC is practically constant (1.20+/-0.01 K) in this region where only the SC/SDW domain proportion below T-SC is increasing under pressure. An homogeneous superconducting state is recovered above the critical pressure with T-SC falling at increasing pressure. We propose a model comparing the free energy of a phase exhibiting a segregation between SDW and SC domains and the free energy of homogeneous phases which explains fairly well our experimental findings.