Direct observation of a surface charge density wave

Carpinelli, Joseph M.
Weitering, Hanno H.
Ward Plummer, E.
Stumpf, Roland

(Carpinelli, Weitering, Ward Plummer) Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee 37996, USA, and Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
(Stumpf) Sandia National Laboratory, Albuquerque, New Mexico 87185, USA.

Nature 381(6581):p 398-400, May 30, 1996.

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 . This phenomenon is most commonly observed in crystals of reduced symmetry, such as quasi-two-dimensional or quasi-one-dimensional 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 , 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.