HMTSF conductivity

Figure 6.47 Electrical conductivity of HMTSF-TCNQ and TTF-TCNQ as a function of temperature.

This is the most puzzling requirement. It is not known why certain crystals—for example, HMTSF TCNQ or Cu(DMDCNQI)2—conduct very well at low temperatures but do not form Cooper pairs. One may wonder whether certain intramolecular or intermolecular vibrations or rigid-body librational modes must be "right" for superconductivity. 

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]. 

Figure 19 Hall coefficient versus temperature for three organic metals, with current flowing along the highly conducting axis HMTSF-TCNQ and TTF-TCNQ (from Ref. 39) and (3L-ET2I3 [from Refs. 20 (circles and diamonds) and 98 (squares)]. The horizontal dashed line corresponds to a carrier concentration of 0.5 hole per ET molecule, the dotted line to Eq. (7) for the TCNQ chains of TTF-TCNQ.

Fig. 1. Molecules that form conducting crystals described in this article. The left column contains the donors perylene (Per), tetrathiafulvalene (TTF), hexamethyl-enetetraselenafulvalene (HMTSF), tetramethyltetraselenafulvane (TMTSF), and bis(ethylenedithio)-TTF (BEDT-TTF or ET ). To the right are the acceptor molecules tetracyanoquinodimethane (TCNQ) and tetracyanonaphthalene (TNAP).

Polarized single crystal reflectance studies of HMTSF-TCNQ are presented. One reflectance component perpendicular to the highly conducting direction is flat and rather low, the... 

Fig. 2 Optical conductivity Of HMTSF-TCNQ for fell c /solid...

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. 

Tne charge transfer salt HMTSF-TCNQ is exceptional in that it remains a good electrical conductor down to very low temperatures, with a conductivity... 

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... 

Parameters estimated from low T Hall coefficient and conductivity of HMTSF-TCNQ. 

Some of the early Hopkins conductivity data for the series TMTTF-TCNQ, TTF-TCNQ (for whom the conductivity anisotropy is 500 5 <1), TMTSF-TCNQ, and HMTSF-TCNQ are shown in Fig. 10 [25, 26, 112-115]. From other (mainly magnetic) data it is surmised that... 

---