Electron TMTSF

Saito et at.130 studied the salts of TMTSF and the sulfur analogue tetra-methyltetrathiafulvalene, TMTTF, with a polycyano dianion. Although the conductivity of both compounds was low (crrt = 10-5 Scm-1 for TMTSF vs. 10-7 Scm-1 for TMTTF) the conductivity of the Se-donor salt was improved by two orders of magnitude. Optical absorption spectroscopy was also used to assess the materials. The electronic transition between radical cations within the segregated donor columns occurred at considerably lower energy (8800 cm-1) in the TMTSF salt than in the TMTTF (11500 cm-1). A concurrent improvement... 

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. 

We saw in Section 1.1 for N2 that electron localization implies insulating ground states and that localization can be reduced by applying external pressure. When reducing temperature from RT down to about 20 K, (TMTTF)2X and (TMTSF)2X... 

A last example is (TMTSF)2C104. High-quality crystals can be obtained by oxidation of TMTSF in TCE containing TBA-C104. Typical dimensions are 4 x 0.2 X 0.2 mm (Bechgaard et al, 1981) but centimetre long crystals have been achieved. Again, the ID shape is related to its electronic structure. 

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. 

Fig. 3 Strategy for chemical modification of the TTF molecule to increase or decrease the electronic dimensionality (D) by the aid of enhance or suppress the self-aggregation ability of the donor molecules, respectively. Typical Fermi surfaces of TMTSF (a (TMTSF)2NbF6), BO (b (BO)2.4l3), TTP (c (BEDT-TTP)2l3, Scheme 8) [88], ET (d / -(ET)2l3), and EDO (e (EDO)2PEg) CT solids are depicted. PIPT photo-induced-phase-transition...

An important distinction must be made between the two-stack systems (like TTF TCNQ, where electrons travel on the TCNQ stacks while holes live on the TTF stacks) and the one-stack systems, or ion-radical salts (or radical ion salts), such as the (TMTSF)2X salts, the alkali TCNQ salts M (TCNQ), and the (ET)2X salts The holes are localized on the TMTSF or ET sublattice, whereas the electrons are on the TCNQ sublattice. 

Figure 26 Electron densities in the plane of a TMTSF molecule of (TMTSF)2 AsF6. Contours are 0.05 e/A3 per line starting at 0.05 e/A3 negative contours are shown by dashed lines. (From Ref. 193.)...

TTF type. The optical anisotropy of such two-dimensional conductors and their electron parameters may also be deduced from reflectance studies. As an example, from the (TMTSF)2X family we present the polarized reflectance of (TMTSF)2PF6 at three temperatures (Fig. 7). It is evident that optical anisotropy decreases at low temperature, and a reasonably well-defined plasma edge appears in the b direction at 25 K. The transverse reflectance edge appears at the frequency about 10 times lower than that of the stacking axis edge (tb< = 22 meV, about 10 times smaller than ta) [46]. Drude parameters for typical (TMTSF)2X salt are eq = 3.5, 1500 cm-1 < cop < 2000 cm-1, 250 cm-1 < y < 500 cm-1, and tb = 0.02 eV. 

The IR conductivity spectra of (TMTSF)2X and (TMTTF)2X compounds consist of a broad electronic band with superimposed vibrational fine structure. The spectra can be taken as evidence of considerable electronic coupling to some vibrational modes of TMTTF or TMTSF molecules, in particular to the methyl group modes. The model based on isolated dimers describes the experimental results quite well. Jacobsen et al. [61] have fitted the dimer model to the reflectance of some salts of this family. The chain-axis reflectance of (TMTTF)2PF6 at T = 300 K, measured and cal-... 

In the metallic regime the properties can usually be interpreted within the one-dimensional model. The low-temperature behavior is determined by the shape of the Fermi surface. If it is open as in the case of the TMTTF and TMTSF salts, the description of the properties is based on the physics of one-dimensional systems. This does not mean that the dynamics of the electrons is one-dimensional. The coupling between chains has an important role, allowing transitions at T > 0, derived from one-dimensional instabilities. 

Figure 22 Logarithm of the normalized resistance log(R/R0) versus the absorbed ionizing dose for samples of the organic charge-transfer salt TMTSF-DMTCNQ 1, electron irradiations 2, x-ray irradiations. (Adapted from Ref. 89.)...

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... 

Knowledge of the pressure-induced commensurability led to a series of beautiful experiments searching for evidence for a collective electron-phonon or CDW contribution to the low field conductivity in TTF-TCNQ above Tp. Clear evidence was indeed found for a substantial fall in ah between about 150 and 80 K in the narrow commensurability domain, as shown in Fig. 14 [85]. No such dip was found for the transverse conductivity [86], and the dips in ah were also shown to be suppressed by only a 2 x 10 3 molecular fraction of irradiation induced defects [87]. All of this leads to a consistent picture in favor of a collective electron-phonon CDW contribution to ah above Tp of TTF-TCNQ, as discussed in Ref. 2. However, the extra CDW conductivity is not more than 6000 (fl-cm)-1 at 80 K, that is, about one-half of the ambient pressure conductivity of (TMTSF)2PF6 at the same temperature (Fig. 1) and the latter is usually considered to be a single-particle contribution. So until the mechanism... 

Figure 6 (a) Resistivity [after Ref. 47] and (b) EPR spin susceptibility of (TMTTF)2X compounds [after Ref. 46] undergoing a one-dimensional Mott-Hubbard localization below 7p (c) electron spin susceptibility of (TMTSF)2PF6 [after Ref. 38] (d) plot of the l3C spin lattice relaxation rate versus 7xf(7) for three compounds in the TM2X series. The temperature below which the charges are localized is indicated by 7p. No localization is observed for Se compounds (dashed line) above the SDW or SC ordering. (From Ref. 41b.)... 

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