Charge-density waves in transition-metal compounds

The vast majority of compounds crystallize into a regular form in which a unit cell is repeated indefinitely, except for generally localized defects, impurities and boundaries. In a few compounds, however, at sufficiently low temperatures interactions between electrons and ions across unit cells make this regular array unstable with respect to small distortions. The stable state is one in which the charge density, the spin density, or the ion positions display long-period modulations. The period of these modulations may be incommensurate with the spacing of the underlying lattice, so that the material is no longer truly periodic, having two unrelated periods. In this article we shall focus on charge-density waves, in which the electron density and also the ion positions exhibit a periodic variation.