Ultrafast Electron Dynamics at Interfaces and in Heterostructures

This thesis utilizes ultrafast time-resolved photoelectron emission spectroscopy, in an angle-integrating and angle-resolved mode, to investigate phenomena interesting to the field of condensed matter physics. Supplemented by static linear and non-linear photoelectron emission spectroscopy. Electronic interactions are at the core of solid-state physics. The ability to probe these interactions in non-equilibrium states of matter is crucial for a comprehensive understanding of governing physics. By examining the non-equilibrium dynamics of electrons, this thesis provides additional insights into fundamental processes, like electron-electron scattering, charge carrier transport and transfer processes, exciton formation, electron-phonon coupling and phase transitions. As these processes happen on different time scales, experimenting in the time domain allows for the separation of the aforementioned processes. The investigated electronic states and interactions can be selected by tailoring the optical excitation and probe frequency to the investigated material system. Five sample systems exhibiting distinguishing physics are highlighted and explored within this thesis, starting with local changes of solvated electrons within the charge transfer resonance state of the Cu(111)/Na+/D2O heterosystem by systematic variation of the environment. Then, continuing with an investigation of the c(4x2) surface reconstruction of Si(100). This chapter aims to prepare the c(4x2) phase in sufficient quality for laser excitation experiments and investigate the electron dynamics and role of exciton-exciton interactions. In the third chapter, the focus is on two-dimensional collective excitations in the model charge density wave material class of rare-earth tritellurides. Here, the influence of the strength of the excitation on the response of the electronic structure of TbTe3 and its order along the major crystallographic axes is studied. Shedding light on the effects of the perturbation of the electronic system, inducing a transient momentum anisotropic response of the system. The second-to-last chapter will focus on the separation of the excitation volume from the probed surface in order to observe and understand electron propagation on a nanometer and femtosecond scale in a Fe/Au heterostructure. Ballistic electron transport can be distinguished from other transport channels by performing Au thickness-dependent measurements and supporting experimental results by Boltzmann transport calculations, as well as real-time time-dependent density functional theory. To extend the capabilities of time-resolved photoelectron emission spectroscopy using spatially separated excitation and detection volumes, the fifth and final chapter will discuss preparatory experiments of the addition of transition metal dichalcogenide crystallites and monolayers to a Ti/Au metallic heterostructure system. In that chapter, a thorough characterization of the samples using various methods, like profilometry, static photoemission and optical microscopy is performed. Moreover, it will conclude with the first results indicating the formation of a surface photovoltage at the Au/TMDC interface and electron transfer across it.