TMTSF conductivity

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... 

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. 

Since the early discovery of the large conductivity peak in TTF-TCNQ around 60 K (Coleman et al., 1973) (Fig. 6.47) many studies have been carried out on TTF-related systems (Subramanyam, 1981 Soos Klein, 1976). Some aspects related to conduction in these systems were mentioned in the previous section. (TMTSF)2C104 and related compounds (TMTSF = tetramethyltetraselenafulvalene) are found to show superconductivity at low temperatures (Jerome, 1985). 

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-... 

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 21 Resistivity of (TMTSF)2PF6 for current flow in the high-conductivity direction (a) with various magnetic fields applied in the low-conductivity direction (c ). (From unpublished results of W. Kang, J. R. Cooper, D. J6rome, and K. Bechgaard.)...

Figure 1 Prototype molecules TTF (tetrathiafulvalene and TMTSF (tetramethyl-tetraselenafulvalene) which are used for the elaboration of organic conductors TTF-TCNQ or superconductors (TMTSF)2X. View of the crystal structure of some molecular superconductors (a) (TMTSF)2PF6 (b) (alkali)3 C (c) (BEDT-TTF)2Cu(SCN)2 side view (d) view along the axis perpendicular to the conducting plane.

As far as all selenium molecular compounds are concerned, the shallow minimum of the resistivity is no longer observed, but different behaviors are observed for the representative compounds (TMTSF)2PF6 and (TMTSF)2C104. The conductivity of the former compound reaches 106 (ft cm)-1 at 12 K and then vanishes abruptly as the system undergoes a metal-insulator transition [15] toward an antiferromagnetic (SDW) ground state, as indicated by NMR [53] and susceptibility measurements [54]. Furthermore, 13C NMR has proved the incommensurate nature of this SDW ground state [52]. It is the presence of this additional periodicity in... 

Figure 12 (a) Temperature dependence of the longitudinal resistivity of (TMTSF)2PF6 obtained with the clamped contact technique. (From W. Kang, private communication, and O. Traetteberg, Thesis, Univ. Orsay, 1993.) (b) The conductivity anisotropy is temperature independent as long as the transverse motion remains incoherent. Data for (TMTSF)2PF6 under 12 kbar. (After Ref. 6.)... 

The very large pressure coefficient of the susceptibility (Fig. 14a) and conductivity in the metallic regime (d In room temperature [6]) raises a serious problem for the comparison with theory, which usually computes constant-volume temperature dependences. Hence the temperature dependence at constant pressure that is observed in actual experiments must be transformed into constant-volume data since the change of volume (due to the thermal expansion) cannot be ignored between 300 and 50 K. No detailed determinations of the constant-volume resistivity have been performed so far. However, a crude estimate of the intrinsic temperature dependence can be performed using the thermal expansion and the pressure dependence of the a axis at various temperatures [59] (Fig. 14b). 

Figure 14 (a) Pressure dependence of the spin susceptibility x (T,T)-l/2 from NMR data. (From Ref. 41b.) (b) Constant-pressure and constant-volume temperature dependences of the resistivity of (TMTSF)2AsF6 derived point by point from the constant-pressure data of Fig. 12. The lattice parameters are from Ref. 33 and the pressure coefficient of the conductivity from Ref. 57.

Figure 26 (a) Single-crystal optical reflectance data for organic conductions with decreasing conductivity from top to bottom (from Ref. 95) (b) optical reflectance of (TMTSF)2PF6 at different temperatures in the conducting regime (from Refs. 95 and 98) (c) anisotropy of the optical reflectance (from Ref. 98) (d) optical reflectance of p(ET)2I3 (from Ref. 97). 

Figure 29 Far-infrared conductivity of (TMTSF)2C104, relaxed phase. The dashed line is the Drude law with 1/r 3.5 cm-1 and

---