Charge density wave layered materials

At the time of Overhauser s work in 1964, experimental examples were not available and the concept was generally considered academic. Several years later, however, investigators working with layered materials (electrons essentially move 111 only two directions) and with linear conductors (motion is essentially in one direction) attributed a charge density wave phenomenon to what has been termed the Pierls instability. The latter effect, which involves lowering electron energy and lattice distortion, currently is not believed to apply to the simple, three-dimensional metals (K etc). In summary, the Pierls instability and (he Overhauser charge density waves concept appear to be similar, but different. 

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]. 

It is now generally agreed, however, that the observed increase in results from suppression of charge density wave formation rather than some exotic quasi-two-dimensional mechanism. However, the effect of the two-dimensional anisotropy of the material is of considerable interest. It has been found that the critical field behavior is in broad agreement with theoretical predictions based on a model of a layered compound containing two-dimensional superconducting layers weakly coupled via Josephson tunneling. ... 

Electronic conduction in insulating materials has been a subject of considerable interest in the quest to understand charge transport in the thin film layers of organic electronic devices. In typical dielectric materials, the electronic states near the Fermi level are usually localized states, and the electron wave functions decay exponentially over a distance known as the localization length. In constrast, metals have a high, generally uniform density of states, whereas semiconductors have well-separated conduction and valence bands (separated... 

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