Recently, scientists found a new class of materials with properties that were unknown in any traditional material. Crystals made from these materials do not conduct electricity in their interior, yet their surfaces possess qualities usually assigned to metals, such as a good surface conductivity. These materials are called 'topological insulators' (TI) and have been the focus of experimental and theoretical studies worldwide throughout the last years.
One of the many peculiar features of the conducting TI surface is the 'forbidden backscattering'. Even the 'flattest' surfaces contain steps or small defects, which means that electrons travelling along this surface are likely to encounter an obstacle. On usual materials, these obstacles lead to a high probability for an electron to be reflected several times by such disturbances before reaching its destination. Due to the 'forbidden backscattering' on TI surfaces, electrons will ignore most obstacles on a TI surface and just pass them as if they were not there, an important property for future applications in quantum computing. Recently, the investigation of the TI materials Bi2Te3 and Bi2Se3 resulted in the discovery of a so-called 'Kohn anomaly', which could mean that the excitation of surface vibrations would interrupt the smooth charge transport and give rise to the undesirable backscattering. The method of surface scattering of helium atoms can imply creation or annihilation of phonons, i.e. surface vibrations, and provides an excellent opportunity to verify this hypothesis.
Another class of 'topological' materials are the so-called 'Weyl semimetals' (WS), which possess different properties on opposite material surfaces. This special feature is caused by the inherent asymmetry of the underlying crystal structure.
In this project, the helium scattering group at the Institute of Experimental Physics at TU Graz will investigate the surface vibrational properties of topological materials, particularly the TI Bi2Te2Se and the WS TaAs and NbAs. Special attention will be paid to the possible existence of a surface Kohn anomaly on the TI surface as well as the differences of material properties on opposite WS surfaces. Results from helium atom scattering experiments will also provide insight into the coupling of surface electrons to the vibrational motion of the nuclei of the crystal lattice as well as the charge transport mechanism of these topological materials.