Charge Density Waves Surface Deformation Studied by Helium Atom Diffraction

Abstract

Helium atom diffraction has been extensively described in this book; the technique appears to be a very effective tool to obtain detailed information on the structural features of complex surfaces such as stepped metal [11, adsorbate-covered [2], and reconstructed [3] surfaces. The angular position of elastic diffraction peaks determines the symmetry and the dimensions of the unit mesh, while the peak intensity is very sensitive to the location of the scattering centers in the mesh. The aim of present paper is to show a typical example in which helium atom diffraction was used to probe surface deformation. I will present data concerning the study of two transition-metal dicalcogenides, the 1T-TaS and the 2H-TaSe₂. These layered materials exhibit charge-density wave (CDW) instabilities and superlattice reconstruction at low temperatures, which appear as weak extra spots in X-ray, neutron or electron diffraction. Similar extra spots also appear in the helium diffraction study of the outer layer of these compounds, and the observed super-lattice peaks have in some cases the same intensity as that of the main peaks, thus indicating a high sensitivity of atom diffraction to surface deformation. Some of the data presented here have already been published [4]; some other data are still unpublished and their presentation must be considered as the result of a preliminary study [5].