Electron TMTTF

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... 

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... 

One way of experimentally exploring the electronic structure of solids is by means of photoemission spectroscopies such as UPS and X-ray photoelectron spectroscopy (XPS), where photoexcited electrons are analyzed dispersively as a function of their kinetic energy. The electronic structure of the reference material TTF-TCNQ will be extensively discussed in Section 6.1. Figure 1.31 shows the XPS spectra of the S2p core line for (TMTTF)2PF6 (black dots) and BEDT-TTF (grey dots). 

LB films prepared from hexadecyl-TCNQ TMTTF and (heptadecyl-dimethyltetrathia-fulvalene)2 TCNQ and from their mixtures Electron microscopy and electron diffraction Molecular packing determined conductivities best lateral d.c. conductivity was 0.5 S cm -1 765... 

Another demonstration of the validity of these calculations is provided by BEDT-TTF-based salts. The calculated Fermi surface of these materials exhibit closed orbits characteristic of two-dimensional electronic interactions and this has been confirmed experimentally. For example, in the case of (BEDT-TTF)2I3, the calculated surface of these orbits (Fig. 21) [61] agrees well with the one measured by magnetic experiments [161]. However, the overall good agreement between calculation and experiment must not hide the fact that some qualitative discrepancies may arise in some cases. For example, (TMTTF)2X salts exhibit a resistivity minimum at a temperature at which no structural transition has yet been observed. The resistivity minimum is not explained by the one-electron band structure, and to account for this progressive electron localization, it is necessary to include in the calculations the effect of the electronic correlations [162]. Another difficulty has been met in the case of the semiconducting materials a -(BEDT-TTF)2X, for which the calculated band structure exhibits the characteristic features of a metal [93,97,100] and it is not yet understood... 

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. 

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]. 

Finally, it may be remarked that the two salts MEM(TCNQ)2 and TEA(TCNQ)2 bear several points of resemblance in their physical properties with the most interesting salts of the (TMTTF)2X series (TMTTF = tetramethyltetrathiafulvalene) [14]. They are all organic conductors with a quasi-one-dimensional character, with p = 5 (or ), and with a dominant electron-electron interaction. They exhibit comparable modest values of the electrical conductivity at high temperature, which indicate that the electrons are not very delocalized in the materials, and in all of them an underlying 4kF dimerization is also present, due to the cations. These common features, which are thought to be at the origin of sizable Umklapp scattering effects in the salts of the (TMTTF)2 series [14], could also be able to produce the same kinds of effects in MEM(TCNQ)2 and TEA(TCNQ)2 (see also Chapter 2). 

We may incidentally remark that the same kinds of arguments could also be reasonably invoked for the electrical properties of some materials of more recent concern. For instance, a resistivity minimum that is not related to a phrase transition is observed at Tp in (DMDCNQI)2Ag (DMDCNQI = dimethylcicyanoquinonediimine) [59] as in (TMTTF)2PF6 [60]. This minimum is attributed to the opening of a gap in the electronic spectrum of the materials below Tp, under the effect of Umklapp scattering, with a corresponding Mott-Hubbard localisation [14]. It is, however, also... 

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.)... 

ESR spectra of the LB film of TMTTF-C18TCNQ show a single line without structures, which together with the observed g value indicates that there is strong coupling between TMTTF radical spins and TCNQ radical spins [59] as in the TTF-TCNQ crystal [60]. The temperature dependence of the spin susceptibility of the film suggests the presence of spin species due to conduction electrons. 

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. 

When magnitude of the electron-electron Coulomb interaction increases, the system is expected to show a ehange from the SDW dominated state to the spin-Peierls one. This was verified in another system, (TMTTF(tetramethyltetra-thiafulvalene))2X and (TMTSF)2X [74]. The former materials have the narrower band width than the latter. This means that the role of Coulomb interaction is more important in the former system than in the latter. The former system is expected to have the spin-Peierls state and the latter one the SDW state. Systematic studies have proposed the phase diagram shown in Fig. 23 [74]. 

Let us assume for this estimation that the susceptibilities of both chains of the compounds are equal, though we know that the experiments give 0.3 s (T Q), < o, 1+ /14, 57, 58/. We feel justified in doing so since we are interested, for the moment, in factors 5 to 10 and not in 20% effects or so. The values of the experimental susceptibility are summarized in Table 2, either per molecule of TMTTF /TCNQ/ or per electron at P=0 kbar and P= 8kbar, assuming a 0.55 elec/mole-cule charge transfer and i -0.23 eV. Assuming that X (S 0Kj ( T °), we derive for / ]a =0.55 from... 

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