Speaker
Description
Femtosecond electron diffraction (FED) enables the direct observation of the crystal lattice’s response to laser excitation. It is therefore ideally suited to study the ultrafast energy flow from electrons to phonons as well as other photo-induced changes of the lattice, such as structural phase transitions, coherent phonons, and lattice distortions.
We have employed FED to probe the nuclear dynamics accompanying the singlet exciton fission process in pentacene single crystals [1]. Our combined experimental and theoretical study shows that long-range motions play a decisive part in the electronic decoupling of the electronically correlated triplet pairs.
In magnetic materials, ultrafast demagnetization is observed following laser excitation, which is determined by the interplay between spins, electrons, and the lattice. By combining FED with density functional theory (DFT and atomistic spin dynamics simulations, we obtained a quantitative picture of the ultrafast microscopic energy flow in the elemental ferromagnets (Fe, Co, Ni) [2]. Furthermore, in FED studies of the strongly correlated antiferromagnet NiO, we observed a prolonged transient non-thermal state of the lattice and a transient crystallographic asymmetry indicative of an exchange striction-induced lattice distortion [3].
In layered materials, material properties can be tuned by layer stacking, down to monolayer limit. In FED studies of bulk and monolayer WSe2 samples, we extracted an element-specific vibrational response (i.e. W and Se). This demonstrated that monolayer geometry primarily affects Se, and characterize the nonthermal lattice state.
Femtosecond electron diffuse scattering (FEDS) provides a momentum-resolved view of phonon dynamics in photo-excited materials. We report our recent developments in FEDS, using black phosphorus as a prototypical semiconductor of contemporary relevance [4-7]. In particular, we demonstrate an efficient first-principles methodology for the calculation of the all-phonon inelastic scattering in solids [5-6]. We then employ ultrafast dynamics simulations based on the time-dependent Boltzmann formalism supplemented by structure factor calculations to reproduce the experimental signatures of nonequilibrium structural dynamics [7].
References
[1] H. Seiler et al., Science Advances, 7, eabg0869 (2021).
[2] D. Zahn et al., Phys. Rev. Research 3, 023032 (2021).
[3] Y. W. Windsor et al., Phys. Rev. Lett. 126, 147202 (2021).
[4] D. Zahn et al., Nano Letters 20, 3728 (2020).
[5] M. Zacharias et al., Phys. Rev. Lett., in print (2021).
[6] M. Zacharias et al., Phys. Rev. B, in print (2021).
