TMTSF parameters

We saw previously the example of TMTSF thin films where the lattice parameters could be obtained, and proof that the triclinic phase coincides with that found for... 

Takahashi T, Kobayashi Y, Nakamura T, Kanoda K, Hilti B, Zamhounis IS (1994) Symmetry of the order parameter in organic superconductors (MDT-TTF)2Aul2 vs. (TMTSF)2C104. Physica C 235-240 2461-2462... 

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. 

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.

The low value of U in (TMTSF)2X salts in view of their filled electron band (or the hole band) is in striking contrast to the high Us in -filled M(TCNQ)2 conductors, which often have a susceptibility enhancement over the Pauli susceptibility of factors of 10-30, suggesting that C//4i 1. As pointed out by Mazumdar and Bloch (100), U is an effective parameter which is magnified at the band filling of . This makes it much easier to understand why M(TCNQ)2 and (TMTTF)2X salts show strong correlation effects and why in (TMTSF)2X salts U is so low. 

The anisotropy of superconducting properties has further been seen in Meissner and shielding measurements from which Bd was determined [111]. The values at 50 mK for (TMTSF)2C104 are 0.02 mT, B i 0.1 mT, and Bd 1 mT. Together with the values for the upper critical fields the anisotropic GL parameter Ki can be calculated by... 

Fig. 2.12. Proton nuclear relaxation rate 1/Ti vs temperature of (TMTSF)2C104. The solid line is a calculation with an anisotropic order parameter. Prom [117]...

The crystal structures of the two systems are also very alike. Both are triclinic with comparable unit-cell parameters and with the "herring-bone" structure known from TTF-TCNQ /8/. The most pronounced crystallographic difference between the two systems is a rather elongated /6.1 %/ b-axis in TMTSF-DMTCNQ. This is to be expected because the b-direc-tion is across the width of the molecules where the additional methyl-groups tend to push the chains apart thereb. y probably reducing the interchain coupling. 

Organic superconductivity now exists in two families of organic compounds. In the first series, (TMTSF)2X, the quasi-one-dimensionality manifests itself, first, in the competition between various ound states, which is governed by parameters such as the efficiency of Coulomb interactions, the amplitude of the interchain coupling, etc. and, secondly, in the remarkable field-induced spin density wave states. 

Input data for the calculation were the known room-temperature structure [17] and vibrational studies of TMTSF [18]. As suggested by the structural data, the dimerization amplitude u was taken to be 0.03 A. The parameters, whose value is adjustable and determined by the fitting of the experimental data, are the transfer integral t, the gap amplitude Ak, the e-mv coupling constants g and the core dielectric constant Coo- Use of the selFconsistent relations mentioned before allows us to determine the e-p parameter (di/d u) and the percentage contribution of the static potential to the gap A. ... 

Table 1. Values of the parameters used in the calculation of the T = 300 K spectra of (TMTSF)2PF5 for the U — o case.

Table 2. Stmctural and electronic interaction parameters for (TMTSF)2X and (TMTTF)2X salts.

As for a more detailed comparison between theory and experiment, we cannot expect, of course, that all the features of the measured spectra can be accurately reproduced with a model as simple and rough as the one we have used for our calculations. For instance, in the case of (TMTSF)2C104, in order to reproduce the amplitudes of the observed vibronic structures, we have to use a gap parameter that causes the c culated conductivity maximum to be higher in frequency than the observed one. This kind of discrepancy is much less pronounced in the case of (TMTSF)2PF5, although both the gap parameter and the transfer integral are veiy similar to those of the 104" salt. 

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