@PhdThesis{duepublico_mods_00082222,
  author = 	{Shomali M.Sc., Elaheh},
  title = 	{Investigation of carrier dynamics in laser-excited heterostructures from real-time time-dependent density functional theory},
  year = 	{2024},
  month = 	{Jul},
  day = 	{24},
  keywords = 	{Density functional theory; Real-time time-dependent density functional theory; Heterostructure; Metal/oxide heterostructure; Metal/metal heterostructure},
  abstract = 	{This thesis presents a systematic first-principles investigation of the layer-resolved dynamics of excited carriers in metal/oxide and metal/metal heterostructures based on time-dependent density functional theory (TDDFT) in the real-time domain. As a first step, we considered a minimal model Fe1/(MgO)3(001) heterostructure and investigated the impact of the laser frequency, peak power density and polarization direction. Different excitation energies were explored: below, in the vicinity and beyond the LDA band gap of bulk MgO. We noticed a marked anisotropy in the response to in- and out-of-plane polarized light, depending on laser frequency: The in-plane polarized light induces a much strong absorption in the Fe part and interface O for photon energies below and around the LDA band gap of bulk MgO, whereas the cross-plane polarized pulse leads to a significantly larger response in the MgO part for frequencies above the band gap. As a result, the laser frequency can be adjusted to selectively induce excitations in the metal, insulator, and interface parts. Furthermore, we analyzed the excitation process of Fe1/(MgO)3(001) heterostructure in terms of the changes in the layer-resolved time-dependent density of states (TDDOS), which helped us to identify a concerted excitation mechanism for photon energies above the charge transfer gap, which involves two simultaneous excitations via interface states close to the Fermi level: one from occupied states of the metal to the conduction band of the insulator and simultaneously, another from the valence band edge of MgO into Fe states above the Fermi level. This mechanism allows for an effective bidirectional relocation of excitations between the metallic and insulating subsystems. Going beyond the minimal model system, we also considered substantially larger heterostructures containing up to 5 monolayers of Fe and 7 monolayers of MgO. The electronic response of Fe3/(MgO)5(001) and Fe5/(MgO)7(001) heterostructures to an optical excitation was explored as a function of laser frequency, polarization direction and layer thickness. Although, the imaginary part of the dielectric tensor calculated within the random phase approximation (RPA) shows a reduction in the anisotropic response with increasing thickness of the metallic Fe part, the spatial redistribution of electronic charge after illumination from the RT-TDDFT calculation still indicates a strong dependence on the frequency and polarization direction of the laser pulse with a similar pattern for all thicknesses. We also compared the time-dependent DOS obtained for the heterostructures and bulk Fe, which helped us to reveal the origin of excited carriers. Furthermore, we demonstrated that for photon energies above the charge transfer gap, the abovementioned bidirectional transfer of excited carriers between the central (bulk-like) layers of the metal and insulator subsystems is active also in the thicker heterostructures. We further extended the modelling of the electron dynamics and propagation of optically exited carriers to the metal/metal Fe/Au(001) heterostructure, which was investigated in a back-side pump -- front-side probe experiment. The RT-TDDFT results on a Fe5/Au5(001) heterostructure excited by a 2 eV pulse allowed us to resolve the spectroscopic signatures observed in the 2PPE spectroscopy. Based on the analysis of the TDDOS, the presence of excited carriers at 1.7 eV in the Au layers of Fe5/Au5(001) heterostructure results from the transfer of carriers from the Fe layers across the interface, whereas the features at around 0.6 eV and below mainly arise from excitation in the Au subsystem. We further investigated the response of the Fe3/Au3(001) and Fe5/Au5(001) heterostructures to in-plane and out-of-plane laser pulses with different frequencies. The analysis of the imaginary part of the frequency-dependent dielectric tensor calculated within the RPA indicates a weak anisotropic behavior. The spatial redistribution of electronic charge after illumination exhibits a strong dependence on the frequency of the laser pulse with a similar pattern for both heterostructures. The analysis of the excitation pattern for the pulse with the lowest frequency illustrates larger changes in the occupation numbers in TDDOS compared to the highest frequency. Moreover, we observed an interface mediated excitation processes for the highest frequency, which leads to a transfer of hot carriers between the occupied and the unoccupied states of the Fe and Au subsystems. Carriers are excited via the interface from the Fe states below the Fermi level to the Au states above and simultaneously, carriers below the Fermi level in Au are excited to the Fe states above the Fermi level.},
  doi = 	{10.17185/duepublico/82222},
  url = 	{https://duepublico2.uni-due.de/receive/duepublico_mods_00082222},
  url = 	{https://doi.org/10.17185/duepublico/82222},
  file = 	{:https://duepublico2.uni-due.de/servlets/MCRFileNodeServlet/duepublico_derivate_00081727/Diss_Shomali.pdf:PDF},
  language = 	{en}
}