Exotic Surfaces

The present application seeks to understand the properties of novel and unusual surfaces, for which few dynamical measurements exist. The applicant plans to spend a research period of 24 months at the University of Cambridge in the Surface Physics Group under the supervision of Dr. William Allison. The research group of Dr. William Allison is the world`s only group that operates a helium-3 spin-echo spectrometer which can resolve energy changes as small as 20 neV. Spin-echo measurements on the semimetal Bi(111) and the topological insulator PbBi 2 Te4 (111) will provide new benchmark data that will assist in the development of theoretical approaches to understand the fundamental atom-surface interaction on semimetal and topological insulator surfaces. In particular the spin-echo technique is the only method that can provide data with sufficient experimental resolution for direct measurements of the mode-selected electron-phonon interaction. Furthermore, the possible investigation of acoustic surface plasmons in a spectral region inaccessible to any other technique, is an extremely attractive aspect not yet explored. Based on the analysis of the spin-echo measurements a theoretical concept for the He-Bi(111) interaction will be developed which may also be adapted for other semimetals. In an additional return phase of 12 months at the Institute of Experimental Physics (Graz University of Technology) the knowledge obtained in the group in Cambridge and the first application of this concept for the He-Bi(111) system will be further developed. This includes the interpretation of the He-PbBi 2 Te4 (111) data measured in Cambridge. The data will be used to find an appropriate atom-surface interaction model for topological insulator surfaces. Finally the influence of adsorbed atomic hydrogen onto the Bi(111) surface with the heading phrase "the lightest on the heaviest" will be investigated experimentally. While an understanding of the model semimetal bismuth is of great interest for topological insulators and spintronics, topological insulators themselves are promising candidates for potential applications ranging from spintronics to quantum computation. Despite their great importance, especially in the case of topological insulators, semimetals have never been approached with helium atom scattering due to their intrinsic complexity of being surface conductors (even superconductors) and strongly corrugated at the same time. The applicant`s expertise in helium scattering on these surface systems combined with the world-leading experimental facilities available in Cambridge will generate new understanding at a fundamental level to these promising materials. Ongoing cooperation with the University of Cambridge will provide continuous exchange of knowledge and the realization of joint experiments.

The aim of this project was to understand the properties of novel and unusual surfaces, so-called semimetals and topological insulators, for which few dynamical measurements exist. Therefore the fellow spent a research period of 24 months at the University of Cambridge in order to run helium spin-echo measurements on the described surfaces followed by a return phase of 12 months at Graz University of Technology. Among the most important results are the development of a novel approach to measure the electron-phonon coupling of topological insulators and the first experimental observation of the atomic-scale motion of water molecules on the semimetal graphene. Both results are fundamentally new and are expected to gain interest by a broad community. A short description of these results is given below, a summary of all results obtained during the course of the project can be found in the detailed report.One of the major objectives for technical applications of topological insulators is surface dominated electron transport. However, the motion of electrons in a solid can be perturbed due to scattering by vibrations (phonons). Phonons are always present for a material at room temperature. The impact of these phonons on the electrons is described by a parameter, the so-called electron-phonon coupling ?. Depending on the material, the magnitude of the electron-phonon coupling can vary greatly. The fellow was able to develop a novel approach which allows to determine the electron-phonon coupling from experimental measurements. First results for the topological insulator Bi2 Te3 show that the electron-phonon coupling is rather weak for this material. With respect to potential applications it suggests that Bi2Te3 may be a particularly useful material for fast electronic devices.Furthermore, the fellow was able to study the atomic-scale motion (diffusion) of water molecules on the semimetal graphene. On a microscopic scale the molecule moves/hops along the surface whereupon the energy necessary for this motion is provided due to the thermal energy (temperature) of the sur- face. In contrast to the movement of water molecules on metal surfaces, its motion on the semimetal graphene is strongly influenced by interactions between the individual molecules. One reason for this might be the hydrophobic nature of the graphene surface. The measurements show that under certain conditions the water molecules tend to avoid each other, i.e. there is a repulsive interaction between the individual water molecules. This effect occurs due to the electric dipole of a water molecule. Loosely speaking, in analogy to a magnetic moment, one may imagine each water molecule being a little magnet: If these magnets are aligned the same way they will repel each other.