HMTSF

As will be discussed later (Section 1.5), molecules containing no metallic elements are able to combine and form materials exhibiting metallic character, e.g., HMTSF-TCNQ, TTF-TCNQ, etc., or even lose any electrical resistance below a given temperature and thus become superconductors, e.g., (TMTSF)2C104. Metal-free molecules can also, in the solid state, show magnetic order, such as / -NPNN and /7-NC-C6F4-CNSSN, where in the absence of -electrons the magnetic properties are related to unpaired -electrons. 

Figure 6.47 Electrical conductivity of HMTSF-TCNQ and TTF-TCNQ as a function of temperature.

Figure 4 Relationship of donor and acceptor stacks in crystal structure of (HMTSF)(TCNQ) showing shared planes of symmetry between donor and acceptor stacks...

Schematic diagrams appropriate to NMP/TCNQ and TTF/TCNQ are shown in Fig. 30 and are based on experimental studies. Application of the one-dimensional Hubbard model to analyse low and high temperature data for NMP/TCNQ yielded consistent values of U and t. For TTF/TCNQ and HMTSF/TCNQ, the increased cation polarizability is believed to have successfully reduced the strength of the effective electron-electron interaction with the result that a true metal-semiconductor transition is observed at 58 K for TTF/TCNQ which disappears completely for HMTSF/TCNQ. At present the advantages of using complex salts as against simple salts of charge-transfer systems to produce organic metals are not clear, particularly since the...

This is the most puzzling requirement. It is not known why certain crystals—for example, HMTSF TCNQ or Cu(DMDCNQI)2—conduct very well at low temperatures but do not form Cooper pairs. One may wonder whether certain intramolecular or intermolecular vibrations or rigid-body librational modes must be "right" for superconductivity. 

TMTSF)2PF6 shows a typical SDW transition as a knee in the susceptibility (powder sample). As expected for a SDW, Xs does not go to zero as T— 0. HMTSF-TCNQ is a special case. Xpauiiwas calculated from NMR... 

Figure 2 Temperature dependence of the susceptibility (xD extracted) for the organic metals TTF-TCNQ, (TMTSF)2PF6 (powder), and HMTSF-TCNQ. (From Ref. 3.)...

Some materials are very sensitive to disorder [3]. In general, the low-temperature susceptibility follows x a T a (a5=3 0.6 to 0.9). (NMP)(TCNQ) and Qn(TCNQ)2 are examples of this effect, the disorder being intrinsic, attributed to the asymmetry of the cation. (HMTSF)(TNAP) has a similar behavior at low temperature, the disorder being attributed to the TNAP molecule. In (TTT)2I3+6 the disorder results from nonstoichiometry. Similar effects have been obtained when disorder is induced by irradiation... 

It should be noted that the behavior of the susceptibility due to disorder is distinct from that of paramagnetic impurities. The susceptibility is usually smaller for selenium compounds. HMTSF-TCNQ suggests prac-tially no enhancement due to correlations. 

HMTSF-TCNQ, which is believed to have strong coupling between chains, does not show ESR signal at room temperature [37]. The linewidth is presumably on the order of 4000 Oe. On the contrary, TMTSF-DMTCNQ, a Se compound with a narrow line ( 70 Oe at 300 K), has been taken as evidence of weak interchain coupling [38]. 

Figure 1 Log-log plots of longitudinal resistivity p versus temperature T for representative organic conductors TTF-TCNQ (from Ref. 14), HMTSF-TCNQ (from Ref. 15), (TMTSF)2PF6 (from Refs. 17 and 19), TMTSF2C104 (from Ref. 16), (TSeT)2Cl (from Ref. 18), and pL-ET2I3 (from Ref. 20).

One characteristic feature of the behavior of Xs(T) for organic metals is illustrated in Fig. 4. In contrast to ordinary metals, Xs(X) increases quite substantially with temperature from (say) 60 to 300 K. This increase is strongest for the most one-dimensional compound, TTF-TCNQ [53], and becomes progressively weaker for (TMTSF)2C104 [54], (3-(BEDT-TTF)2I3 (a genuine two-dimensional compound) [25,26], and the more three-dimensional compound (TSeT)2Cl [18] (also, unpublished results of M. Mil-jak and B. Hilti). For HMTSF-TCNQ [33] such a discussion is complicated by the presence of Landau-Peierls diamagnetism from small pockets of electrons and holes, although estimates of Xs(T) have been made by Soda... 

One of the initial motivations for pressure studies was to suppress the CDW transitions in TTF-TCNQ and its derivatives and thereby stabilize a metallic, and possibly superconducting, state at low temperatures [2]. Experiments on TTF-TCNQ and TSeF-TCNQ [27] showed an increase in the CDW or Peierls transition temperatures (Tp) with pressure, as shown in Fig. 12 [80], Later work on materials such as HMTTF-TCNQ showed that the transitions could be suppressed by pressure, but a true metallic state was not obtained up to about 30 kbar [81]. Instead, the ground state was very reminiscent of the semimetallic behavior observed for HMTSF-TCNQ, as shown by the resistivity data in Fig. 13. One possible mechanism for the formation of a semimetallic state is that, as proposed by Weger [82], it arises simply from hybridization of donor and acceptor wave functions. However, diffuse x-ray scattering lines [34] and reasonably sharp conductivity anomalies are often observed, so in many cases incommensurate lattice distortions must play a role. In other words, a semimetallic state can also arise when the Q vector of the CDW does not destroy the whole Fermi surface (FS) but leaves small pockets of holes and electrons. Such a situation is particularly likely in two-chain materials, where the direction of Q is determined not just by the FS nesting properties but by the Coulomb interaction between CDWs on the two chains [10]. 

Figure 19 Hall coefficient versus temperature for three organic metals, with current flowing along the highly conducting axis HMTSF-TCNQ and TTF-TCNQ (from Ref. 39) and (3L-ET2I3 [from Refs. 20 (circles and diamonds) and 98 (squares)]. The horizontal dashed line corresponds to a carrier concentration of 0.5 hole per ET molecule, the dotted line to Eq. (7) for the TCNQ chains of TTF-TCNQ.

Fig. 1. Molecules that form conducting crystals described in this article. The left column contains the donors perylene (Per), tetrathiafulvalene (TTF), hexamethyl-enetetraselenafulvalene (HMTSF), tetramethyltetraselenafulvane (TMTSF), and bis(ethylenedithio)-TTF (BEDT-TTF or ET ). To the right are the acceptor molecules tetracyanoquinodimethane (TCNQ) and tetracyanonaphthalene (TNAP).

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