Electron 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. 

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...

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(a genuine two-dimensional compound) [25,26], and the more three-dimensional compound (TSeT)2Cl [18] (also, unpublished results of M. Mil-jak and B. Hilti).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]. 

HMTSF-TCNQ exhibit unusual properties as a result of a more 3-dimensional character of the electronic states at low temperatureCref. 1 and 2). The 3-dimensio-nal character is broken if small amounts of MTCNQ or DMTCNQ are introduced in the lattice probably due to the increase in size of the dopants,when compared to TCNQ. Also the low temperature properties of HMTSF-TCNQ depend somewhat on the conditions of crystallization. ... 

Hall effect measurements are reported for three single crystals of the charge transfer salt HMTSF-TCNQ in the temperature range 1.4-200 K at ambient pressure and under hydrostatic pressures of approximately 6 Kbars. There is evidence that the high conductivity of this material at low temperatures arises from a small number of electrons with a high mobility and a low degeneracy temperature as suggested by other experiments and a recent band-structure calculation. 

A one electron band structure proposed recently for HMTSF-TCNQ accounts very well for the observed diamagnetism and many other electronic properties of this material. In this picture the Fermi surface consists of small el ect ron-1 i ke ellipsoids and hole-like cylinders or vice versa at k = - kp. 

Therefore from 60-200 K the materia 1 s show quite different behaviour which may be related to the larger transverse electron overlap in one direction for HMTSF-TCNQjas deduced from the crystal structure, which shows... 

To summarise, it seems clear that at low T the conductivity of HMTSF TCNQ arises from small pockets of electrons with a degeneracy temperature of about 2CK and there is mounting evidence that these are an intrinsic property of the material. The proposed band structure accounts for many of the experimental observations but a number of problems remain. One interesting experiment is X-ray diffuse scattering at low T. It is known that there is a longitudinally polarised "2kp" soft phonon down to 100, if this condenses to... 

In fact it is even possible that a 3D superlattice could be responsible for the electron pockets at low Tjand the change in sign of R i In HMTSF-TCXQ. the transverse transfer integral, t say, may well be large enough i satisfy the con-02) a... 

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