| 1. |
- Engel, Robin Y., et al.
(author)
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Electron population dynamics in resonant non-linear x-ray absorption in nickel at a free-electron laser
- 2023
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In: Structural Dynamics. - : American Institute of Physics (AIP). - 2329-7778. ; 10:5
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Journal article (peer-reviewed)abstract
- Free-electron lasers provide bright, ultrashort, and monochromatic x-ray pulses, enabling novel spectroscopic measurements not only with femtosecond temporal resolution: The high fluence of their x-ray pulses can also easily enter the regime of the non-linear x-ray-matter interaction. Entering this regime necessitates a rigorous analysis and reliable prediction of the relevant non-linear processes for future experiment designs. Here, we show non-linear changes in the L-3-edge absorption of metallic nickel thin films, measured with fluences up to 60 J/cm(2). We present a simple but predictive rate model that quantitatively describes spectral changes based on the evolution of electronic populations within the pulse duration. Despite its simplicity, the model reaches good agreement with experimental results over more than three orders of magnitude in fluence, while providing a straightforward understanding of the interplay of physical processes driving the non-linear changes. Our findings provide important insights for the design and evaluation of future high-fluence free-electron laser experiments and contribute to the understanding of non-linear electron dynamics in x-ray absorption processes in solids at the femtosecond timescale.
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| 2. |
- Ligges, Manuel, et al.
(author)
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Ultrafast doublon dynamics in photo-excited 1T-TaS2
- 2018
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In: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 120:16
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Journal article (peer-reviewed)abstract
- Strongly correlated materials exhibit intriguing properties caused by intertwined microscopic interactions that are hard to disentangle in equilibrium. Employing nonequilibrium time-resolved photoemission spectroscopy on the quasi-two-dimensional transition-metal dichalcogenide 1T-TaS2, we identify a spectroscopic signature of doubly occupied sites (doublons) that reflects fundamental Mott physics. Doublon-hole recombination is estimated to occur on timescales of electronic hopping ℏ/J≈14 fs. Despite strong electron-phonon coupling, the dynamics can be explained by purely electronic effects captured by the single-band Hubbard model under the assumption of weak hole doping, in agreement with our static sample characterization. This sensitive interplay of static doping and vicinity to the metal-insulator transition suggests a way to modify doublon relaxation on the few-femtosecond timescale.
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| 3. |
- Lojewski, Tobias, et al.
(author)
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The interplay of local electron correlations and ultrafast spin dynamics in fcc Ni
- 2023
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In: Materials Research Letters. - : Taylor & Francis. - 2166-3831. ; 11:8, s. 655-661
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Journal article (peer-reviewed)abstract
- The complex electronic structure of metallic ferromagnets is determined by a balance between exchange interaction, electron hopping leading to band formation, and local Coulomb repulsion. By combining high energy and temporal resolution in femtosecond time-resolved X-ray absorption spectroscopy with ab initio time-dependent density functional theory we analyze the electronic structure in fcc Ni on the time scale of these interactions in a pump-probe experiment. We distinguish transient broadening and energy shifts in the absorption spectra, which we demonstrate to be captured by electron repopulation respectively correlation-induced modifications of the electronic structure, requiring to take the local Coulomb interaction into account.
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| 4. |
- Malik, Rameez Saeed, 1987-
(author)
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Ultrafast Magnetization Dynamics : Element-selective studies of magnetic alloys using ultra short XUV pulses
- 2020
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Doctoral thesis (other academic/artistic)abstract
- In this thesis, I investigate the ultrafast magnetization dynamics in 3d ferromagnets and their alloys with ultrashort laser pulses. The high harmonics generation (HHG) setup provides extreme-ultraviolet photons with energies 35-72 eV, which is the energy range where 3d metals have their M2,3 absorption edges. By employing HHG with the transverse magneto-optical Kerr effect, the magnetization of multiple elements in a magnetic system is probed and their dynamics are resolved separately on femtoseconds time scales.The magneto-optical response of elemental Fe and Ni during demagnetization is investigated. This magneto-optical response is measured as an asymmetry in the intensity of reflected light for two opposite sample magnetization directions. Experiment and density functional theory calculations show that for Fe, the asymmetry is strongly dependent on the particular type of magnetic excitation. However, for Ni, it is relatively insensitive to the magnetic excitation. Next, the element-specific magnetization dynamics of FeNi alloys are investigated. A time delay in the Ni demagnetization relative to Fe is observed for all alloy compositions. This Ni-delay depends on the alloy composition and is related to changes in the exchange interactions.Co2FeAl (CFA) Heusler alloys are unique due to their peculiar electronic structure and because they can exhibit very low damping. Experimentally, CFA films show a decreased damping with an increase in structural ordering. The demagnetization times of Fe and Co in CFA samples with different amount of ordering are similar for all samples. However, the remagnetization times exhibit a dependence on the structural ordering. Both the theoretical and experimental damping parameters correlate well with the remagnetization times. In FeCo alloys, the damping can be changed by doping with heavy metals. Here, the magnetization dynamics of Fe65Co35 films as a function of Re doping are investigated. We find no observable change in the demagnetization times for samples with increased damping. However, when increasing the Re doping and the damping, the remagnetization time becomes faster. Also, a fast increase of the asymmetry signal is observed at the Ru-edge during the demagnetization of FeCo. This effect is attributed to a super-diffusive spin current going from the FeCo layer to the Ru capping layer.Last, the magnetization dynamics of a ferrimagnetic insulator is studied. The NiFe2O4 asymmetry shows oscillatory dynamics after an ultrashort laser pulse excitation. With 1.55 eV pump, these oscillations are strong. For 3.1 eV pump, demagnetization becomes dominant and the oscillations diminish.
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