Surface dynamics of topological materials

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.

Topological materials hold great promises for future use in quantum techno logy. These promises are based on fundamental aspects that were originally predicted by theorists who found that for certain crystal structures, electrical transport may be restricted to the materials surface while inside, the material would be an electrical insulator. Moreover, the charge transport at the surface should experience no resistance. After the first experimental discovery in 2007, these topological insulators (TIs) have been the focus of experimental and theoretical studies worldwide. One of the important 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 mater ials, 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, important for future applications in quantum computing. As another important application, so-called quantum sensing devices could be developed that are based on electronic changes caused by single atoms or molecules adsorbed at the surface of a TI. However, all these ideal properties require near zero Kelvin temperatures. In real material samples at finite temperature, scattering processes via electron-phonon (e-ph) coupling may give rise to energy losses and the role of impurities may also become more serious. Here, the FWF funded project set in with new sensitive measurements of (a) the electronic structure, (b) the lattice vibrations (phonons), (c) the e-ph coupling and (d) the influence of impurity atoms in the surface layer. The scattering of helium atoms from the surface of the materials of interest provides accurate information about the distribution of electronic charges at the surface, the different modes of nuclear oscillations of surface atoms, and their mutual influence upon each other, i.e. the coupling of their motions that cause energy losses from the surface into the interior. Several international collaborations were essential for the successful outcome of the project: Topological materials of high purity were grown at a centre in Aarhus, Denmark, and could be influenced in their final properties by controlled implantation of different atoms in the crystal lattice. Colleagues from theoretical physics in Milan and Madrid helped to interpret the experimental results. Complementary measurements were conducted at the Cavendish Laboratory at the University of Cambridge so that a whole class of TIs with lattice structures built from chemical elements of the groups 15 and 16 of the periodic table could be well characterized in terms of the properties named under (a) to (d) above.