MPSD Exchange

Driving degenerate infrared-active phonons with circularly polarized laser pulses is predicted to generate magnetization [1]. Estimates based on the circular motion of the mode’s Born effective charge typically yield values on the order of one nuclear magneton per unit cell. Recent work has revealed large effective magnetic fields from circularly driven phonons in materials showing large phonon Zeeman effect [2], and – more surprisingly – phonon magnetizations up to four orders of magnitude larger than predicted by calculations even in systems without magnetic ions [3,4]. In most cases, this induced magnetization has been probed optically, via the Faraday rotation in the photo-excited volume, making it challenging to unambiguously separate the Faraday signal from other contributions arising by nonlinear optical interactions [5,6]. A promising, artefact-free approach is to quantify the phonon-induced magnetization by the fringing field it generates. Recently, we developed an ultrafast magnetometry technique that measures magnetic fields with sub-picosecond time resolution and sub-microtesla sensitivity [7,8]. Here, we study 6H-SiC, driven with circularly polarized pulses resonant with a doubly degenerate E1 phonon. By combining measurements of the fringing field from phonon-induced magnetization with polarization rotation measurements within the photoexcited volume, we find that the induced magnetization is at least an order of magnitude smaller than inferred from Faraday rotation alone [3]. This approach yields a more accurate estimate of the induced magnetization and helps clarify the origin of the large magnetization values reported in literature [3, 4]. [more]

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]

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]

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]

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]

UTe2 displays a tantalizing phase diagram under high magnetic fields, featuring multipleunconventional superconducting and magnetic phases with distinct order parameters. In thisMPSD exchange seminar, I will present our recent experimental results on probing nonlinearelectronic transport in these non-trivial states.First, I will discuss the field-reinforced superconducting state and its strongly anisotropic vortexresponse with respect to both current and magnetic-field orientation. These results reveal anunexpected quasi-2D superconducting state in a non-layered material. Next, I will provideevidence for a first-order transition modulated by high current densities in the high-field region,emphasizing its direct connection to a hidden order within the field-polarized phase. [more]

Chirality is a fundamental symmetry property of materials. Beyond the conventional classification of crystal systems as chiral or achiral, there exists a distinct subclass of achiral systems known as antiferrochiral. In this kick-off seminar of the MPSD internal seminar series, I will discuss the novel properties of such systems, including linear and nonlinear responses, as well as quasiparticle excitations.The concept of higher-order chirality will be introduced as a unified framework to describe these systems. Building on this perspective, new opportunities for chirality engineering will be highlighted, focusing on two recent experimental demonstrations from the Cavalleri department: light control of chirality and strain control of chirality. [more]