Energy dissipation on Dirac and semimetal surfaces: Understanding surface dynamics on the nano-scale.

The present thesis provides new insights into the mechanisms of energy dissipation on material surfaces. Dynamical disorder and energy dissipation are key sources of decoherence in quantum devices, yet we understand little about the underlying nature of these mechanisms. Fundamentally, interactions with phonons and electron-hole pairs at the surface determine the lifetime and decoherence rate of quantum states and the mechanisms that control surface dynamical processes.

The first part of this thesis deals with the experimental and theoretical study of various promising material surfaces, so-called Dirac materials. Among these, three-dimensional topological insulators such as Bi2Te3 exhibit an insulating gap in the bulk while the surface is electrically conducting. However, in real samples and at finite temperatures, their ideal zero-Kelvin behaviour is perturbed and scattering processes via electron-phonon (e-ph) coupling can give rise to energy losses. In this context it is demonstrated that atom-surface scattering provides a sensitive probe to determine the surface phonon dispersion and the e-ph interaction parameter and several examples of the phonon dispersion and the e-ph coupling of these materials are presented.

The second part illustrates how the lineshape broadening upon inelastic scattering from surfaces can be used to determine the characteristics of energy dissipation during the motion of atoms and molecules. The motion of an adsorbed molecule arises from the rate of energy transfer between the molecule and the surface. The presented experimental data provides information about the role and variety of energy dissipation channels during this process. Moreover, due to the low energy of the probing particle beam delicate adsorbates such as water can be studied without disruption of the motion or dissociation of the molecule.

In general, the study of these surface dynamical processes is a unique and challenging problem for experiments, as it requires both sub-nanometer spatial resolution and fast (pico- to nanosecond) temporal resolution. In an outlook it is shown that ultrahigh-resolution measurements provide experimental access to so-far unexplored concepts such as the lifetime of surface phonon modes. Furthermore, by combining reciprocal space with real space techniques, surface dynamical processes can be measured over 16 orders of magnitude, thus providing experimental data for the rate description of dynamics. The latter is crucial for an understanding of the underlying principles of chemical reactions where the rate prediction from accurate computational calculations suffers from the lack of experimental data.

The presented works thus give quantitative insights into the coupling between the system and its environment with results that provide benchmark data relevant to quantum simulation. It extends the molecular-level understanding of these processes from ``classical'' surface science systems to novel quantum mechanically designed surfaces.