We report the observation of superconductivity in the spin-Peierls Fabre salt (TMTTF)2PF6 from pressure dependent electrical transport measurements above a threshold of 4.35 GPa. The data complete the sequence of ground states of this compound in the temperature and pressure plane adducing an empirical basis to the universal character of the phase diagram of the Fabre salts and their selenide analogues, the Bechgaard salts. The structure of the phase diagram at the approach of the crossover between spin-density wave and superconducting states is compared with the results of scaling theory of the interplay between both electronic instabilities under pressure. The comparison supports the view that magnetism and superconductivity in these compounds have a common electronic origin
The nature of the attractive electron-electron interaction, leading to the formation of Cooper-pairs in unconventional superconductors has still to be fully understood and is subject to intensive research. Here we show that the sequence spinPeierls, antiferromagnetism, superconductivity observed in (TMTTF)2PF6 under pressure makes the (TM)2X phase diagram universal. We argue that the suppression of the spin-Peierls transition under pressure, the close vicinity of antiferromagnetic and superconducting phases at high pressure as well as the existence of critical antiferromagnetic fluctuations above Tc strongly support the intriguing possibility that the interchain exchange of antiferromagnetic fluctuations provides the pairing mechanism required for bound charge carriers.The existence of a common border between the superconducting (SC) ground state and the insulating phase of spin density wave (SDW) nature 1 , was recognized as a remarkable property of the phase diagram of the Bechgaard salt (TMTSF) 2 PF 6 . It belongs to a broad family of isostructural compounds (TM) 2 X, where the flat organic molecule TM is either tetramethyltetraselenafulvalene (TMTSF) or tetramethyltetrathiafulvalene (TMTTF). Here X denotes a monovalent anion such as PF 6 , AsF 6 , ClO 4 or Br 2 . In the crystal, these molecules form stacks separated by chains of anions X. The overlap between the electron clouds of neighboring TM molecules along the stacking direction ( the a-axis) is about 10 (500) times larger than that between the stacks in the transverse b-(c−)direction. Provided that the longitudinal overlap is large compared to the on-site Coulomb repulsion, these organic materials become conducting with a pronounced one dimensional (1-D) character.The 1-D character of the Fermi surface of (TMTSF) 2 PF 6 , the presence of a spin-Peierls (SP) transition instead of the usual Peierls instability 3,4 as well as the existence of enhanced antiferromagnetic (AF) fluctuations at low temperature, evidenced by NMR relaxation experiments, raised several questions about the mechanism responsible for superconductivity in organic conductors 5 . Since 1-D physics is a relevant concept in these low dimensional systems, SDW and electron-electron pairing can develop simultaneously at low temperature in the interacting electron gas 1 . A cross-over from SDW to SC correlations could possibly be achieved through a small variation of the coupling constants either by applying pressure or changing X 6 . Furthermore, the nuclear spin-lattice relaxation rate data of (TMTSF) 2 PF 6 suggest that SDW correlations prevail at low temperature even under pressure when superconductivity is stabilized 5 . In the generic phase diagram proposed for the (TM) 2 X family 7 the sequence of ground states (SP, AF/SDW and SC) can be observed for different members of the series if they are placed according to their ambient pressure properties. Even parts of the sequence can be found for a given member of the series if pressure is applied. For instance, the SDW ground state of (TMT...
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