The discovery and realization of graphene as an ideal two-dimensional (2D) material has triggered extensive efforts to create similar 2D materials with exciting spin-dependent properties. Here, we report on a novel Sn 2D superstructure on Au(111) that shows similarities and differences to the expected electronic features of ideal stanene. Using spin-and angle-resolved photoemission spectroscopy, we find that a particular Sn/Au superstructure reveals a linearly dispersing band centered at the Γ-point and below the Fermi level with antiparallel spin polarization and a Fermi velocity of v F ≈ 1×10 6 m/s, the same value as for graphene. We attribute the origin of the band structure to the hybridization between the Sn and the Au orbitals at the 2D Sn-Au interface. Considering that free-standing stanene simply cannot exist, our investigated structure is an important step towards the search of useful stanene-like overstructures for future technological applications.

Comprehending far-from-equilibrium many-body interactions is one of the major goals of current ultrafast condensed matter physics research. Here, a particularly interesting but barely understood situation occurs during a strong optical excitation, where the electron and phonon systems can be significantly perturbed and the quasiparticle distributions cannot be described with equilibrium functions. In this work, we use time-and angle-resolved photoelectron spectroscopy to study such far-from-equilibrium many-body interactions for the prototypical material graphene. In accordance with theoretical simulations, we find remarkable transient renormalizations of the quasiparticle self-energy caused by the photoinduced nonequilibrium conditions. These observations can be understood by ultrafast scatterings between nonequilibrium electrons and strongly coupled optical phonons, which signify the crucial role of ultrafast nonequilibrium dynamics on manybody interactions. Our results advance the understanding of many-body physics in extreme conditions, which is important for any endeavor to optically manipulate or create non-equilibrium states of matter.

Comprehending far-from-equilibrium many-body interactions is one of the major goals of current ultrafast condensed matter physics research. Here, a particularly interesting but barely understood situation occurs during a strong optical excitation, where the electron and phonon systems can be significantly perturbed and the quasiparticle distributions cannot be described with equilibrium functions. In this thesis, we use time-and angle-resolved photoelectron spectroscopy (trARPES) to study such far-from-equilibrium many-body interactions for the prototypical material graphene by evaluating the low-energy non-equilibrium quasiparticle self-energy at the femtosecond timescale. The low-energy quasiparticle response, however, is at first inaccessible due to resolution artifacts caused by the spectrally our broad XUV probe pulses, which are unavoidable due to the time-bandwidth-product. Therefore, we apply a Lucy-Richardson deconvolution algorithm (LRD) to the ARPES spectra. Furthermore, we introduce rigorous benchmarking of the LRD to assure physical-meaningful results. To deliver angle-resolved photoemission spectroscopy (ARPES) data in the necessary quality for this LRD approach, we built a trARPES setup with an extreme ultraviolet (XUV) probe source in this thesis. This XUV probe source can be switched between an energy-resolution optimized beamline and time-resolution optimized beamline. With this experiment, we find remarkable transient renormalizations of the quasiparticle self-energy caused by the photo-induced nonequilibrium conditions, which are in accordance with first-principles theoretical modeling.These observations can be understood by ultrafast scatterings between far-from-equilibrium electrons and strongly-coupled optical phonons, which signifies the crucial role of ultrafast non-equilibrium dynamics on many-body interactions. Our results advance the understanding of many-body physics in extreme conditions, which is important for any endeavor to optically manipulate or create emergent states of matter. iii Acknowledgements Hier würde ich gerne einigen Personen danken. Ich danke meiner ganzen Familie und allen Freunden für die Unterstützung während der Arbeit. Ich danke Prof. Dr. Stefan Mathias für die Ermöglichung und Betreuung bei dieser Arbeit. Ich danke Dr. Marcel Reutzel, Dr. Sabine Steil, Dr. Matthijs Jansen und Dr. Daniel Steil für die Unterstützung und Betreuung während der Arbeit. Ich danke meiner Prüfungskommission, Gutachtern und dem