TY  - THES
AU  - Heber, Michael
TI  - Studies on ultrafast dynamics in correlated electron systems with time- and angle-resolved photoemission spectroscopy
PB  - University of Hamburg
VL  - Dissertation
CY  - Hamburg
M1  - PUBDB-2025-01079
SP  - 102
PY  - 2024
N1  - Dissertation, University of Hamburg, 2024
AB  - Angle-resolved photoemission spectroscopy (ARPES) is a potent technique for studying the electronic band structure of condensed matter. The use of the pump-probe technique extends ARPES into the time domain, allowing the electron dynamics to be observed on an ultrafast timescale. High harmonic generation (HHG) sources and free electron lasers (FELs) generate extreme ultraviolet (EUV) or X-ray pulses that have a temporal width of 100 fs or less. HHG converts optical or infrared laser radiation into the EUV, whereas in the FEL, relativistic electrons emit coherent synchrotron radiation in an alternating magnetic field. The FEL FLASH (Free electron LASer Hamburg) is a high-repetition-rate FEL that extends the photon energy into the soft X-ray range. Electron spectrometers with high detection efficiency are essential because the repetition rate of the pump and probe source frequently constrains time-resolved (tr-)ARPES measurements. Therefore, time-of-flight photoelectron spectrometers with a 3D detection scheme are best suited for this purpose. The tr-ARPES setups, the wide-angle photoelectron spectrometer WESPE and the momentum microscope HEXTOF, are based on the time-of-flight technique and designed for the plane grating beamline PG2 at FLASH. In this thesis I present the time resolved studies on three different quantum materials.Frequently, the momentum microscope is also paired with a HHG source. In the first scientific application of this thesis, a sample of pristine graphene on iridium(111) is investigated to evaluate the potential of this laboratory-based experimental setup. The results demonstrate a sub-100 fs time resolution and a field of view larger than the first Brillouin zone. The multispectral nature of the HHG spectrum enables measurement of kz-dependent ARPES signal which were used to reconstruct a tomogram of the Fermi surface of iridium.The Dirac cone is not a unique feature of Graphene. It also appears as a characteristic feature of the surface state of topological insulators, which were investigated in the second experiment chapter. Despite being insulating in the bulk, topological insulators possess an intriguing conducting topological state at the surface. The topological insulator bismuth selenide (Bi2Se3) was investigated using the HEXTOF setup. Tr-ARPES data was obtained for the topological surface state, and tr-XPS data was obtained for the Bi 4d and Se 3d core levels. Our study aimed to explore the electron dynamics at high pump fluences within the topological surface state, as well as to examine the robustness of the state itself. Even at fluences that result in surface ablation, our experiments have demonstrated that the topological surface state remains intact, and its dynamics maintain their characteristics.The last scientific case in this thesis deals with a heavy fermionic system, where 4f and 5f electrons interact with conduction band electrons, forming quasi-particles in the band structure. Thulium selenide (TmSe) is a heavy fermionic system with mixed valence character on the Tm site. The Tm 4f electrons hybridize with the electrons in the conduction band, resulting in the formation of a heavy fermion multiplet in the PES spectra near the Fermi level. When TmSe is doped with tellurium, the lattice expands, causing a transition from the mixed valence state to a predominantly divalent state. The dynamics of the 3H6 multiplet peak shows a dependence on the Te concentration. In the mixed valence state, a delayed long-lasting dynamic is observed.This thesis not only demonstrates how versatile and powerful tr-ARPES is for the study of condensed/quantum matter in general, but also explicitly shows that recent developments in momentum microscope instrumentation can satisfy a wide range of needs, paving the way to a “one fits all”-design which includes besides time and angle also spin resolution.
LB  - PUB:(DE-HGF)11
DO  - DOI:10.3204/PUBDB-2025-01079
UR  - https://bib-pubdb1.desy.de/record/625263
ER  -