Experimental Results: Surface Phonons

In this chapter some exemplary high resolution HAS experiments on surface phonon dispersion curves of single crystal surfaces based largely on the TOF technique are reviewed. A selection is made of significant results for insulator, semiconductor and conducting surfaces, with special attention to the HAS results for metals, semimetals, and layered materials, where evidence is found for important effects of, for example, electron-phonon coupling and the related observation of Kohn anomalies, anomalous longitudinal branches and deep subsurface modes. Studies on complex surfaces with large-period unit cells due to reconstruction or high index orientation are also reviewed. Here also the comparatively high resolution of HAS permits to observe low energy dynamical excitations and their dispersion. Each section is meant as an introduction to the specific phenomena and issues characteristic to each of these different surface types, with special consideration to the questions solved and the new theoretical issues raised. This chapter is complemented by Appendix A, which contains a comprehensive list of the available HAS experimental data on phonon dispersion curves and of the theoretical studies for the different classes of materials, as of early 2018 which update previous reviews: [1, 2] (all systems), [3, 4] (layered compounds), [5, 6] (semiconductors), [7] (metals) and Landot-Bornstein Chapters [8, 9, 10, 484].