Influence of Defects and Impurities on Charge Density Wave Systems

Abstract

Charge density-waves (CDW) have been the subject of intensive research for about twenty years. Most often they are found in synthetic compounds issuing from the assiduous efforts of chemists. These fascinating materials, inorganic and organic quasi-one-dimensional and quasi-two-dimensional conductors, have metallic properties that are unusual in many ways [1–4]. The key feature is the unstable metallic state, apt to spontaneously form a charge-density modulation ρ _CDW (r) and an associated lattice distortion. The modulation period is determined by the conduction electron density, i.e. related to the Fermi wave vector k _F,

$$\rho_{CDW}=\rho_osin(qr+\phi )=\rho_osin(2k_Fr+\phi )$$

in 1D presentation. As a consequence of this instability there is a rich variety of low temperature phase transitions driven by the temperature dependence of the CDW amplitude ρο and the interaction of its phase φ with the underlying lattice. One can find metal-insulator transitions, incommensurate and commensurate modulated structures for example. A central and most intriguing ingredient is the collective sliding mode conductivity [5–9] where the CDW condensate moves as a whole under an applied field. In practice this sliding is hindered by defects, since the phase φ of the modulation will have preferred values at defect sites, opposing the ideally free choice of position of the modulated charge density. This strong connection to defects influences the whole physics of charge density-waves with consequences that are manifest in a wide space and time scale from microscopic to macroscopic. Nevertheless the initial effects are found at the spatial scale of the defects and of the CDW wavelength. This is the reason why microstructural characterization of the CDW is of primary importance for understanding the CDW physics and of course not only with regard to the sliding CDW but also in the full scope of CDW related features.