Direct observation of a surface charge density wave

A CHARGE density wave (CDW) is a periodic symmetry-lowering redistribution of charge within a material, accompanied by a rearrangement of electronic bands (such that the total electronic energy is decreased) and usually a small periodic lattice distortion^1,2. This phenomenon is most commonly observed in crystals of reduced symmetry, such as quasi-two-dimensional³ or quasi-one-dimensional⁴ materials. In principle, the reduction of symmetry associated with surfaces and interfaces might also facilitate the formation of CDWs; although there is some indirect evidence for surface charge density waves^5-12,14, none has been observed directly. Here we report the observation and characterization of a reversible, temperature-induced CDW localized at the lead-coated (111) surface of a germanium crystal. The formation of this new phase is accompanied by significant periodic valence-charge redistribution, a pronounced lattice distortion and a metal-nonmetal transition. Theoretical calculations confirm that electron-phonon coupling drives the transition to the CDW, but it appears that some other factor-probably electron-electron correlations-is responsible for the ground-state stability of this phase.