Archive 2026

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Ultrafast electron diffraction (UED) enables direct observation of structural dynamics in photoexcited solids, liquids, and gases on femtosecond timescales [1-6]. In this talk, I will tell you about our recent efforts to push UED across complementary keV and MeV regimes. I will focus primarily on our new 90 keV instrument, where radiofrequency (RF) compression allows us to generate high-brightness electron pulses with unprecedented high throughput. By synchronizing the RF compression to the drive laser with better than 6 fs (FWHM) precision [9], we achieve 50-114 fs (FWHM) electron pulses at 40-100 kHz [10,11]. I will show you how shot-noise limited UED is powerful in tracking ultrafast structural dynamics in solids, using aluminium thin films as a model system, where we resolve non-thermal lattice dynamics governed by short- or long-range interactions [12,13]. In the final part of the talk, I will briefly turn to complementary 30 fs MeV UED experiments [7,8] in molecules and liquids. I will highlight how the combination of elastic and inelastic electron scattering gives access to the coupled electronic-nuclear dynamics [14] in the photochemistry of gas-phase aldehydes. I will also reveal the charge transfer and nuclear dynamics in donor-acceptor molecules in highly polar liquid environments, illustrating the powerful combination of keV and MeV UED to study photochemical reactions across all three states of matter with femtosecond temporal and picometre spatial resolutions. [1] G. Mourou and S. Williamson, Appl. Phys. Lett. 41, 44-45 (1982). [2] J. C. Williamson et al., Nature 386, 159-162 (1997). [3] X. Shen et al., Struct. Dyn. 6, 054305 (2019). [4] T. Xiong et al., Phys. Rev. Res. 2, 043064 (2020). [5] D. Filippetto et al., Rev. Mod. Phys. 94, 045004 (2022). [6] K. Amini, A. Rouzée, M. J. J. Vrakking (Eds.), “Structural Dynamics with X-ray and electron scattering”, RSC books (2023). [7] F. Qi et al., Phys. Rev. Lett. 124, 134803 (2020). [8] Y. Yang et al., arXiv:2508.03946 (2025). [9] M. R. Otto et al., Struct. Dyn. 4, 051101 (2017). [10] K. Amini et al., arXiv:2601.02597 (2026). [11] A. Ryabov and K. Amini, arXiv:2601.02333 (2026). [12] F. Rodriguez Diaz et al., Struct. Dyn. 11, 054302 (2024). [13] F. Rodriguez Diaz and K. Amini, arXiv:2601.02055 (2026).[14] J. Yang et al., Science 368, 885 (2020). [more]

Magnet-Superconductor Hybrid (MSH) systems have proven to be versatile platforms for the engineering of topological superconductivity and the ensuing Majorana zero modes, an important step towards the realization of topological quantum computing. In particular, the experimental ability to create MSH system with widely varying magnetic structures -- from ferromagnetic and skyrmion-like to antiferromagnetic – has provided an unprecedented opportunity to manipulate and explore topological phases. In this talk, I will review some recent progress in the theoretical prediction and experimental realization of novel topological superconducting phases – ranging from strong and higher order topological superconductors to topological nodal-point superconductivity -- in MSH systems. Moreover, I will demonstrate how the manipulation of the magnetic structure in MSH systems provides a new path to braiding MZMs and to the real time simulation of topologically protected quantum algorithms. [more]

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X-ray free-electron lasers (XFELs) provide unique opportunities to investigate nonequilibrium structural dynamics in nanoscale materials with unprecedented spatial and temporal resolution. By recording diffraction patterns from individual nanoparticles and nanocrystals, XFEL-based methods can reveal structural heterogeneity and transient lattice distortions that are inaccessible to conventional ensemble-averaged techniques. This talk will discuss how XFEL single-particle imaging and serial femtosecond crystallography can be used to probe photoinduced structural dynamics in semiconductor quantum dots and metallic nanoparticles. Using time resolved serial femtosecond crystallography (TR-SFX), we investigate ultrafast lattice responses in colloidal semiconductor quantum dots. In recent experiments on CsPbBr₃ quantum dots, resonant optical excitation produced measurable changes in diffraction intensities and Bragg peak profiles, demonstrating sensitivity to subtle photoinduced lattice distortions at the single-particle level [1].In parallel, we apply XFEL single-particle imaging to large ensembles of gold nanoparticles undergoing nonequilibrium processes such as growth and plasmon-induced melting. In these experiments, each diffraction pattern originates from an individual particle, and a single dataset can contain millions of snapshots. Unlike conventional methods that primarily provide ensemble-averaged information, single-particle imaging offers access to the full distribution of structures within the ensemble. To extract structural information from these highly heterogeneous datasets, we developed a high-throughput reconstruction framework based on Monte Carlo sampling of particle shape and orientation parameters. By combining diffraction patterns from millions of particles and exploiting geometric correlations across the dataset, the method reconstructs statistically significant distributions of transient particle morphologies and reveals structural evolution pathways during nonequilibrium transformations [2]. Together, these examples illustrate how XFEL diffraction imaging, combined with advanced analysis methodologies, enables direct observation of structural heterogeneity and ultrafast dynamics in nanomaterials. The talk will highlight both the experimental capabilities and computational challenges of extracting transient structural information from large-scale XFEL datasets. [1] Shen, Zhou, et al. "Direct Observation of the Exciton-Polaron in Single CsPbBr3 Quantum Dots." ACS nano 19.31 (2025): 28372-28382. [2] Shen, Zhou, et al. "Resolving nonequilibrium shape variations among millions of gold nanoparticles." ACS nano 18.24 (2024): 15576-15589. [more]

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Interfaces between different material phases or materials of different chemical composition often define or allow tuning the properties of a system of interest. An atomistic level of understanding such interfaces provides an invaluable insight into the electronic properties and microscopic mechanisms that lead to interfacial reactions, atomic restructuring, and emergent electronic properties. In the first part of this seminar, I will introduce several fundamental concepts of the methodology our group uses in its research, such as the Born—Oppenheimer approximation for the dynamics of nuclei and the notion of the first-principles potential energy surface, the efficient and rigorous treatment of nuclear quantum effects via the imaginary-time path-integral formulation, and the representation of first-principles electronic structure by machine learning models. I will then discuss in more detail how one can connect to experiments by using such simulations, in particular regarding the simulation of Raman scattering signals. I will discuss our developments of first-principles simulations of tip-enhanced Raman spectroscopy (TERS) images of molecular adsorbates on metal substrates. The results demonstrate that accurate simulations are capable of reproducing experimental measurements and serve as a powerful interpretative tool that allows us to shed light on the role of the metal substrate in shaping TERS images and discuss the underlying physics. To reach beyond the harmonic approximation, we demonstrate that the methods of molecular dynamics and machine learning can be seamlessly integrated into the TERS simulations and enable efficient, large-scale, quantitative predictions of nuclear quantum and finite-temperature effects on such spectra. [more]

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Surface Acoustic Waves (SAWs) are directed phonon modes whose amplitude, wavelength, andfrequency can be precisely controlled via nanofabrication techniques. From transportmeasurements in 2D quantum materials to optical spectroscopy, SAWs represent a tool forexploring and manipulating light-matter-phonon coupling in low-dimensional condensed mattersystems.This talk will introduce the fundamental concept of SAW generation and propagation,highlighting how piezoelectric transduction enables coherent phonon control. We will showcaseopportunities for interdisciplinary collaboration across the Max Planck Institute for the Structureand Dynamics of Matter, where SAW technology bridges multiple experimental domains:modeling phonon-driven dynamics and strain-engineered band structures, optical spectroscopyof SAW-induced modulation in 2D materials, and tailored microfabrication for ultra-shortwavelength devices. [more]

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When an intense laser is emitted on a solid, the material can emit light at many multiples of the laser’s color, so‑called high harmonics. These harmonics act like fingerprints of how electrons move and interact on an extremely short timescale. We review recent theoretical advances for describing the all-optical spectroscopic technique of high-order harmonic spectroscopy in condensed matter systems. Theoretical methods for characterizing the underlying ultrafast electron processes, their interactions beyond the electric dipole, and the presence of excitons will be discussed. With it, predictions are made concerning the spectroscopic signatures of beyond-electric-dipole interactions, exciton processes, phase transitions, or other aspects of the ultrafast generation process. [more]

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