Ultrafast photoelectron spectroscopy of quantum materials with a novel 10.8 eV photon source

This thesis presents the study of three quantum materials conducted by means of a novel photon source for Time- and Angle-Resolved Photoelectron Spectroscopy (TR-ARPES) developed at the T-ReX laboratory at FERMI (Elettra, Trieste). This source provides pulses at a photon energy of 10.8 eV up to the MHz repetition rate, while offering picosecond time resolution and few tens of meV energy resolution. The results from the three science cases studied highlight how the unique capabilities of this setup are beneficial to tackle open scientific questions in different areas of modern condensed matter physics. The three systems pertain to different material classes, namely a topological insulator, a kagome metal and a high-temperature superconductor. The topological insulator Bi2Se3 is studied to address the effect of spin-orbit coupling in shaping the 2D dispersion of the topological surface state. Ab-initio calculations of the photoemission with variable spin-orbit coupling strength at the surface are compared with equilibrium and nonequilibrium measurements performed at 10.8 eV. The dispersion is reproduced by spin orbit coupling strength reduced to 70-85 % of its full atomic value. The study of the kagome metal CoSn focusses on the dynamics of its two flat bands, that arise due to the lattice-induced localization of electrons and the resulting enhancement of the effect of correlations. The flat bands are investigated at large momentum values, where they can be distinguished from each other. The effect of photoexcitation is an ultrafast shift and broadening, both of the order of the meV, suggesting a small electron delocalization induced by the pump pulse. The study on the copper-based superconductor Bi2212 comprises the use of three sources. The results of TR-ARPES performed with the 10.8 eV source show that this setup allows accessing the antinodal region, located at large k values. However, in the present configuration the setup is not optimized for providing detailed insight on the nonequilibrium response of these states. Improving the time resolution of the setup will allow access also to the antinodal dynamics. The investigation of the O 2p states, performed by means of an HHG source, shows that an infrared pump pulse redistributes charges in the valence band involving also the oxygen states. Time-resolved X-ray photoelectron spectroscopy measurements performed at a free-electron laser facility clarify that the main response to the charge redistribution induced by infrared photoexcitation occurs in the copper-oxygen planes, where the fundamental interactions of cuprates take place.