Attosecond spectroscopy in the x-ray regime in complex systems

Abstract

The ultrafast motion of electrons is a driving force for chemical reactions and the possibility to have control of it makes it a highly desirable avenue for study. This thesis uses the quantum mechanics of extended systems to simulate pump-probe experiments like ATAS in which the electrons can be promoted from inner shells and then are free to move through bands that are responsible for the physical properties of materials. The main tool that will be used is the resolution of the Von Neumann equation for the density matrix in crystal momentum representation, and so there will be the possibility to keep track of the real-time movement of electrons. The main subjects of this thesis are hexagonal lattices in 2D, graphene and hBN, as well as their extension in 3D, graphite. The simulations will allow to promote electrons from the core levels (K edge) to bands that are close to the Fermi energy, and all the spectra that will be observed are shown to be sensitive to the main properties of the materials; in case of injection of electrons from the valence band, also, the ATAS observed keep track of the interference felt by the electrons. Also, many computational methods related to high-performance computing will be investigated for the resolution of the Von Neumann equations, to speed up the simulations, as they are very demanding from the numerical point of view. The methodologies used and the results obtained pave the way for a deeper understanding of the physics of materials, from the electron coherence to the control of their quantum states