Thema der Dissertation:
Ultrafast 4f multiplet excitation in rare-earth metals studied with Xrays
Ultrafast 4f multiplet excitation in rare-earth metals studied with Xrays
Abstract: Optically driven fundamental processes initiating ultrafast magnetization dynamics in 4f rare-earth metals have been studied for many years. Attempts were made to explain the various demagnetization timescales observed for different 4f metals, considering e.g. magnon generation or electron-phonon driven spin-flip scattering. Excitations within the 4f electronic system, though, were not involved in the discussion, since they are not directly accessible by optical stimuli.
This thesis reports on time-resolved experiments performed at the freeelectron laser facilities EuXFEL and FLASH which identified an up to now disregarded mechanism provoking 4f electronic excitations on ultrafast timescales indirectly via interaction with the laser-excited 5d6s valence electrons in 4f metals.
By analyzing ultrafast spectral changes in X-ray absorption (XAS) and resonant inelastic X-ray scattering (RIXS) signals from terbium metal with the help of atomistic simulations, inelastic 5d-4f electron-electron scattering is found to transfer energy and angular momentum between the 4f and 5d systems. As a consequence, 4f electrons are excited from the ground state 7F6 into energetically higher 7FJ multiplets.
Based on these findings, a three-temperature model has been developed, separating temperatures for the 4f electrons, the 5d valence electrons and the lattice. By fitting the simulation based on the three-temperature model to the experimental data, the coupling between 4f and 5d electrons is found to increase with the amount of valence electrons, able to distribute enough energy to lift the 4f electronic system to the energetically lowest excited 4f multiplet 7F5.
According to the 3rd rule of Thole and van der Laan, the absorption branching ratio of M5 to M4 decreases with the reduction of the total angular momentum J, as expected for transitions into energetically higher multiplets 7FJ=5,4,..0 of the 4f electronic system. With time-resolved XAS experiments at the FemtoSlicing facility BESSY II, it was demonstrated, that this correlation can be used to probe and evaluate 4f multiplet excitations, even with sources exhibiting limited energy resolution.
The reported 5d-4f scattering mechanism transiently alters the magnetocrystalline anisotropy and hence the coupling of the 4f system to the lattice. With up to 22% of all probed atoms being excited to energetically higher multiplets, inelastic 5d-4f scattering is deemed to have a significant impact on the ultrafast demagnetization in terbium metal and must be taken into account for a full description of the magnetization dynamics in rare-earth metals.
This thesis reports on time-resolved experiments performed at the freeelectron laser facilities EuXFEL and FLASH which identified an up to now disregarded mechanism provoking 4f electronic excitations on ultrafast timescales indirectly via interaction with the laser-excited 5d6s valence electrons in 4f metals.
By analyzing ultrafast spectral changes in X-ray absorption (XAS) and resonant inelastic X-ray scattering (RIXS) signals from terbium metal with the help of atomistic simulations, inelastic 5d-4f electron-electron scattering is found to transfer energy and angular momentum between the 4f and 5d systems. As a consequence, 4f electrons are excited from the ground state 7F6 into energetically higher 7FJ multiplets.
Based on these findings, a three-temperature model has been developed, separating temperatures for the 4f electrons, the 5d valence electrons and the lattice. By fitting the simulation based on the three-temperature model to the experimental data, the coupling between 4f and 5d electrons is found to increase with the amount of valence electrons, able to distribute enough energy to lift the 4f electronic system to the energetically lowest excited 4f multiplet 7F5.
According to the 3rd rule of Thole and van der Laan, the absorption branching ratio of M5 to M4 decreases with the reduction of the total angular momentum J, as expected for transitions into energetically higher multiplets 7FJ=5,4,..0 of the 4f electronic system. With time-resolved XAS experiments at the FemtoSlicing facility BESSY II, it was demonstrated, that this correlation can be used to probe and evaluate 4f multiplet excitations, even with sources exhibiting limited energy resolution.
The reported 5d-4f scattering mechanism transiently alters the magnetocrystalline anisotropy and hence the coupling of the 4f system to the lattice. With up to 22% of all probed atoms being excited to energetically higher multiplets, inelastic 5d-4f scattering is deemed to have a significant impact on the ultrafast demagnetization in terbium metal and must be taken into account for a full description of the magnetization dynamics in rare-earth metals.
Time & Location
Aug 06, 2025 | 04:00 PM
Hörsaal A (1.3.14)
(Fachbereich Physik, Arnimallee 14, 14195 Berlin)