Charge density wave molecules

Figure 6 Idealized subunit of TTF7I5 showing conceptual mirror plane shared by molecules in an ideal stack charge density waves destroy this symmetry in the actual crystal...

To emphasize the importance of structural studies at LT or HP, let us take the example of TTF-TCNQ. This prototype charge density wave (CDW) system undergoes at ambient pressure a succession of three structural and electronic phase transitions, from a high-temperature metallic phase down to a low-temperature insulator phase. There has been a considerable debate about the mechanism of these transitions, and many distortional modes have been proposed to account for the physical properties of this material (e.g., rigid molecule displacement as translations [73,74] or librations [75,76] or even internal deformations of the molecules [77,78]. Indeed, an experi-... 

It is also remarkable to point out that superconductivity has also been discovered in a family of charge-transfer compounds based on the molecule M(dmit)2, with M = Ni, Pd with Tc -1.6 K under 7 kbar [12] (Fig. 23). In this interesting series, structural and NMR studies support the picture of a one-dimensional Fermi surface, and the ground state competing with superconductivity is a charge density wave modulation driven by the Ni(dmit)2 stacks [85]. 

Different superlattices with -v/S X /3 periodicity have been imaged. This periodicity has been related to rotation of graphite lattice [17]. These superlattices can be produced by either a multiple tip effect [17b] or electronic perturbations caused by adsorbed molecules [17c]. A hexagonal superlattice with a 4.4 nm periodicity, rotated 30° with respect to the HOPG lattice, and 0.38 nm corrugation has also been reported [17a]. This superlattice was also attributed to rotation of the surface layer of graphite. As this type of superstructures is most frequendy observed for thin layers of material, they have been associated with charge density waves [14, 18]. 

The synthesis of the first organic metal TTF-TCNQ was reported in 1973 by Coleman etal. and Ferraris et alP In this compound, TTF and TCNQ molecules are in 1 1 ratio and form separate stacked donor (TTF) columns and stacked acceptor (TCNQ) columns. A partial charge transfer between TTF and TCNQ transforms the molecular stacks into one-dimensional conductive paths via the formation of partially filled bands. Although TTF-TCNQ shows metallic conductivity down to around 60 K, it abruptly transfers to an insulator below 54 K, which was explained by a charge-density wave (CDW) phase-locking (Peierls transition ) due to its one dimensionality. 

The occurence of both polarizations (b" and c ) can bB understood in terms of charge density waves as both of thBSB components modify the inter-molecular spacing in chain direction because of the tilt angle of the molecules in the stacks. 

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