TMTSF under pressure

Exactly this approach led to the discovery of the first organic superconductors, the Bechgaard CT salts of tetramethyltetraselenafulvalene (TMTSF, 3) with various counteranions (TMTSF)2PF6 (under pressure) in 1980120 and... 

Important further illuminations of the relationships between the structural and transport properties is provided by crystallographic studies under pressure. As yet, very limited information is available. However, from studies of (TMTSF)2PF6, the volume compressibility is estimated to be 0.5% kbar-1 (49). It, therefore, requires 6 kbar to compress (TMTSF)2PF6 into the same volume as (TMTSF)2C104, which is in rough agreement with the critical pressure for superconductivity in the PF6 compound. 

Because of the softness of organic metals one expects them to show interesting behavior under applied pressures. This had been demonstrated earlier by Jerome and co-workers on several compounds and in the case of TMTSF-DMTCNQ (DMTCNQ = dimethyltetracyanoqui-nodimethane) a pressure of 10 kbar transforms it abruptly from a Peierls semiconductor with Tm = 50 K to a metal at all temperatures (91). When the temperature-dependent resistance of the (TMTSF)2X family became known, the very low transition temperatures in some of the compounds suggested that these salts would easily become metallic, and maybe even superconducting, under pressure. 

The resistivity measurements along the transverse c direction are also interesting. Actually one can infer that is directly related to the physics of the a — b planes and therefore could probe whether transport proceeds via collective modes or independent quasi-particles. Jacobsen et al. [80], were the first to report a non-monotonic temperature dependence of p,. in (TMTSF)2PFj passing through a well characterized maximum at r , = 80 K (under ambient pressure) at variance with the T-dependence in the a direction [80]. A recent pressure study of this effect has shown that T evolves under pressure and reaches about 300 K at 10 kbar [81]. The constant volume data for p,(7) in the metallic regime below... 

The critical divergence of Eq. (20) is followed accurately for the onset of SDW ground states arising either from a quasi-one-dimensional metal as for (TMTSF)2PF6 or from a one-dimensional Mott-Hubbard paramagnetic insulator [i.e., (TMTTF)2Br at ambient pressure or (TMTTF)2PF6 under 13 kbar] [57] (Fig. 11). 

Figure 20 Superconducting transition of (TMTSF)2PF6 under 8 kbar and k - (ET)2Cu(SCN)2 at ambient pressure.

Two TMTSF molecules transfer one electron to PFg. This monovalent electron acceptor can be replaced by AsFg, SbFg, and TaFg, all with octahedral S3unmetry and superconducting transition temperatures in the compound of 1K under applied pressures of lOkbar. The anions with tetra-... 

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