Charge density waves conduction

Many phenomena such as dislocations, electronic structures of polyacetylenes and other solids, Josephson junctions, spin dynamics and charge density waves in low-dimensional solids, fast ion conduction and phase transitions are being explained by invoking the concept of solitons. Solitons are exact analytical solutions of non-linear wave equations corresponding to bell-shaped or step-like changes in the variable (Ogurtani, 1983). They can move through a material with constant amplitude and velocity or remain stationary when two of them collide they are unmodified. The soliton concept has been employed in solid state chemistry to explain diverse phenomena. 

In conducting solids, the conduction electron density is spatially modulated, forming charge density waves (CDW) the periodic distortion accompanying the CDW (due to interaction between the conduction electron and the lattice) is responsible for the incommensurate phase (Overhauser, 1962 Di Salvo Rice, 1979 Riste, 1977). The occurrence of CDW and the periodic distortion can be understood in terms of the model proposed by Peierls and Frdhlich for one-dimensional metals. Let us consider a row of uniformly spaced chain of ions (spacing = a) associated with conduction electrons of energy E k) and a wave vector k. At 0 K, all the states are filled up to the Fermi energy, = E(kp). If the electron density is sinusoidally modulated as in Fig. 4.15 such that... 

Optical study indicates that at low temperatures the low-energy electronic properties of some organic metal-like conductors (e.g., TTF-TCNQ) are dominated by charge density wave (CDW) effects. Frequency-dependent conductivity of TTF-TCNQ, obtained from the IR reflectance, at 25 K displays a double-peak structure with a low-frequency band near 35 cm-1 and a very intense band near 300 cm-1 [45]. The intense band may be ascribed to single-particle transitions across the gap in a 2kF (Peierls) semiconducting state, while the 35-cm-1 band is assigned to the Frohlich (i.e., CDW) pinned mode. Low-temperature results based on the bolometric technique [72,73] (Fig. 15) confirm the IR reflectance data. Such a con-... 

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 dynamics of impurity pinning of the charge density wave and the frequency dependence of conductivity are investigated in the one-dimensional Peierls-Frohlich state. 

We discuss the interaction of a partially filled electronic conduction band in a segregated donor-acceptor stack system with libra-tional modes of the solid. The orientational Peierls instability predicted by us earlier leads to the formation of chiral charge density waves, which interact and phase-lock below the metal-insulator transition via the Coulomb interaction. The effect of the resulting order on the physical properties of the system and the implications for the understanding of the recent neutron scattering data for the occurrence of several transitions in TTF-TCNQ will be discussed. 

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