Sliding charge density waves

The idea of a charge density wave transition goes back to R. E. Peierls and, independently, to H. Frohlich in 1954. Prominent examples of charge density wave systems are the organic charge transfer salts, such as tetrathiafulvalene-tetracyanoquino-dimethane and layered compounds formed from transition metals and chalcogens, such as TaS₂ and NbSe₂. However, until 1976, none of the charge density wave systems had exhibited the sliding conductivity envisioned by Frohlich. Impurities are sensitive to the charge density oscillations and can lock the charge density wave in place, a phenomenon very analogous to static friction. A model for the noise is that it arises when the charge density wave moves through the impurities or the lattice, much like a particle rolling down a washboard. An image of an area of the sample is formed using the superlattice reflection alone; thus any feature in the imaging is due to the charge density wave formation.