Physikalische Chemie - Direktor: Prof. Dr. Martin Wolf
Department Online Seminar
Chair: Ralph Ernstorfer

Friday, April 22, 2022, 11:00 am
Daniela Zahn
FHI Department PC
Ultrafast Lattice Dynamics and Microscopic Energy Flow in Ferromagnetic Metals and in an Anisotropic Layered Semiconductor
In this talk, I will give an overview of my PhD thesis, focusing on two topics: lattice dynamics in black phosphorus and ultrafast energy flow in 3d ferromagnets.
The layered semiconductor black phosphorus exhibits a peculiar structure with in-plane anisotropy. Here, we use femtosecond electron diffraction to access the lattice response to laser excitation. The optical excitation and subsequent electron-phonon coupling lead to a pronounced non-thermal state of the lattice, which is characterized by a transiently reduced anisotropy of the atomic vibrations. On timescales of tens of picoseconds, thermal equilibrium is restored via phonon-phonon coupling [1,2]. Our results yield insights into both electron-phonon and phonon-phonon coupling and provide pathways to control the timescale of lattice thermalization in black phosphorus.
The 3d ferromagnets iron, cobalt, and nickel exhibit ultrafast demagnetization on timescales of hundreds of femtoseconds following laser excitation. Here, three subsystems contribute to the ultrafast response: electrons, spins, and the lattice. We employ experimentally measured lattice responses to study the ultrafast energy flow between these subsystems quantitatively. Energy-conserving atomistic spin dynamics (ASD) simulations combined with DFT calculations are used to model the microscopic energy flow. A comparison to the experimental results shows the pronounced influence of the magnetization dynamics on the lattice dynamics and demonstrates that energy-conserving ASD simulations provide a quantitative description of the microscopic energy flow. In addition, a non-thermal behavior of the spin system is observed in the simulations, showing that thermal descriptions cannot capture the full non-equilibrium dynamics in magnetic materials [3,4].
[1] Zahn et al., Nano Lett. 2020, 3728-3733 (2020)
[2] Seiler et al., Nano Lett. 2021, 6171-6178 (2021)
[3] Zahn et al., Phys. Rev. Research 3, 023032 (2021)
[4] Zahn et al., Phys. Rev. Research 4, 013104 (2022)

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