Bechgaard salts

The neutral insulator TMTSF, which shows field-effect conduction with /Th — 0.2 cm s (Nam et al, 2003), when transformed into a Bechgaard salt also becomes superconducting, but at lower temperatures. In this case the perfect segregation of organic and inorganic molecular planes leads to confined electronic systems, which in the normal state are quasi ID. Organic superconductors based on the BEDT-TTF molecule represent the case of pure 2D electronic systems. 

It is worth noting that when trying to obtain other polymorphs of Bechgaard salts incorporating octahedral, fluorine-containing anions as in (TMTSF)2PF6 or (TMTSF)2ShF6 only the triclinic phase is obtained. 

TMTSF)2C104 [TMTSF is tetramethyltetraselenafulvalene], an ambient-pressure one-dimensional superconductor of the Bechgaard salt family... 

Figure 5 Crystal packing of the Bechgaard salt (TMTSF)2C104. The independent intermolecular spacings within a TMTSF stack are labeled dl and d2. The dashed lines show the short Se---Se distances. (From Ref. 45.)...

Fig. 7) was operated routinely for many years on the neutron diffractometer DIO at the Institut Laue Langevin and was used for studies on, for example, Bechgaard salts at temperatures close to the superconducting transition [87,88]. Since 1990, 1.6 K can be reached routinely with an improved design and a dilution cryostat has been tested successfully in the laboratory which fits the Euierian cradle and is capable of 70 mK (C. M. E. Zeyen and S. Pujol, private communication, 1993). The helium flow cryostat of Argoud and Muller [80] is also well adapted to four-circle geometry and can be installed even on a small Euierian cradle. 

X-ray diamond anvil cells made feasible the challenge of measuring full high-pressure structures of, for example, some Bechgaard salts (Refs. 70... 

The temperature dependence of the NMR relaxation rate Tf1 for the Au compound (Fig. 9) exhibits a typical behavior of one-dimensional conductors with deviations to the Korringa law (Tf1 T) shown by the upward curvature at high temperatures similarly to (TMTTF)2PF6 [41] and TTF[Ni(dmit)2] [42]. Since there are no localized spins on the dithiolate chain, the relaxation comes from the hyperfine contact and dipolar interactions, 7 1 + r j, produced by the spins of the itinerant electrons along the perylene stacks. The enhancement of the relaxation is, however, less important than that shown by the Bechgaard salts [45]. 

Earlier chapters in this volume have dealt with the molecular properties and crystal structures of organic metals. The large planar molecules under consideration here are usually stacked face to face in chains sometimes the molecular planes are perpendicular to the stacking axis, as in the case of the Bechgaard salts, or they may be tilted by as much as 30°, as in TTF-TCNQ. Because the overlap of the partially occupied tt orbitals is much better along the stacking axis, their electronic band structures are often quasi-one-dimensional. 

Single-chain compounds such as the Bechgaard salts (TMTSF)2X, where the chains are (nearly) uniform and all chains are equivalent. 

For TTF-TCNQ the anomalous pressure derivative d In u,Jd In p falls approximately linearly with T [68], while for the single-chain Bechgaard salt (TMTSF)2C104 it remains in the range 22 to 25% per kbar between... 

An important precursor to the Bechgaard salts, TMTSF-DMTCNQ [2], also showed evidence for a low-temperature semimetallic state at pressures below ca. 12 kbar [83], but in view of the similarities with the Bechgaard salts, it is probable that a fully metallic state is formed at higher pressures [2]. 

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