arXiv subjects

Claus Ropers

Publications and source records attributed to Claus Ropers.

At least 19 recordsLinked to original sources

arXiv2026

Ultrafast order-selective electron imaging and spectroscopy

Ultrafast pump-probe spectroscopies in energy and momentum have become indispensable for probing non-equilibrium dynamics in quantum materials, revealing pathways in ultrafast energy conversion and light-induced phase transitions. Different modalities uniquely access the coupled lattice, charge, and spin degrees of freedom that underpin the functionality of these materials. However, investigating technologically relevant nanoscale devices also requires high spatial resolution. Although electron microscopy provides nanometer-scale imaging, it frequently lacks the simultaneous ultrafast temporal resolution and spectroscopic specificity needed for such studies. In this perspective, we review recent advances in dark-field electron and photoelectron microscopy, focusing on two complementary techniques: femtosecond photoelectron momentum microscopy and ultrafast transmission electron microscopy. Their application enables comprehensive insights into the ultrafast dynamics of the electron, lattice, and spin subsystems. These order-selective electron imaging and spectroscopy techniques open broad scientific opportunities for a microscopic understanding of non-equilibrium phenomena in quantum materials, nanostructures and devices.

cond-mat.mtrl-sci↗

arXiv2026

Surface-Sensitive Mapping of Anisotropic Phonon Cascades in T$_{d}$-WTe$_{2}$

Understanding how energy flows from photoexcited carriers into the lattice is essential for describing nonequilibrium phenomena in low-symmetry quantum materials. Here, we use ultrafast low-energy electron diffraction and diffuse scattering to probe momentum-resolved phonon dynamics at the surface of T$_d$-WTe$_2$, a strongly anisotropic semimetal. Following optical excitation, the Debye--Waller suppression of Bragg peaks exhibits a biexponential increase of the mean-squared atomic displacement, indicating sequential lattice relaxation. Analysis of the diffuse background reveals a preferential intensity build-up parallel to the tungsten-chain axis in the material, attributed to anisotropic electron--phonon coupling during electronic cooling which precedes anharmonic phonon--phonon scattering and subsequent thermalization across the surface Brillouin zone. The results identify a hierarchical relaxation pathway in which energy is first deposited into selected finite-momentum phonons before spreading through the broader lattice bath. Our work highlights the importance of momentum-resolved diffuse scattering for disentangling electron--phonon and phonon--phonon relaxation in anisotropic topological semimetals.

cond-mat.mtrl-sci↗

arXiv2025

Picosecond localization dynamics following ultrafast nanoscale magnetic switching

Ultrashort laser pulses provide the fastest known way to switch magnetic order. Such excitation commonly creates nanometer-scale domains, even after homogeneous illumination when the position of nucleated domains is not externally defined. However, the physics of domain localization during such ultrafast phase transitions remains unresolved. Here, we use shot-resolved pump-probe resonant x-ray scattering together with a material featuring a periodically modulated magnetic anisotropy landscape to track, in real time, the laser-driven nucleation and localization of nanometer-scale spin textures. We find that nucleation and localization are two distinct processes. Nucleation occurs homogeneously via fluctuations at early times, whereas spatially periodic structures emerge only later and, under suitable conditions, localize in less than one nanosecond. Real-space simulations show that this localization is governed by strong lateral variations in spin-texture lifetimes. Our results demonstrate that ultrafast phase-transition dynamics fundamentally differ from conventional transitions, yet still can be controlled through moderate nanometer-scale tailoring of the energy landscape.

cond-mat.mes-hall↗

arXiv2025

Tomographic reconstruction of free-electron quantum states

We give several algorithms for reconstructing quantum states of swift electrons, using maximum likelihood estimation, Bayesian inversion, and deep learning. We apply these algorithms to data previously recorded for an attosecond electron pulse-train to retrieve the density matrix and to analyse its physical properties. Based on the reconstructed quantum state, we obtain pulse-durations of about 245as and predict a degree of coherence of 36 per cent for radiations and excitations produced by these electrons.

quant-ph↗

arXiv2025

Revealing domain wall stability during ultrafast demagnetization

The ultrafast control of nanoscale spin textures such as magnetic domain walls or skyrmions is essential for advancing high-speed, high-density spintronics. However, imaging their dynamics will require a technique that combines nanometer spatial and femtosecond temporal resolution. Introducing ultrafast sub-wavelength imaging in the extreme ultraviolet, we track domain wall properties during ultrafast demagnetization in ferro- and ferrimagnetic thin films. We reveal that domain walls remain invariant in position, shape, and width, down to a demonstrated sub-nanometer precision, for up to 50% demagnetization. Stronger excitation causes stochastic nanoscale domain switching. This previously unobservable robustness of laser-excited domain walls highlights the localized nature of photoinduced demagnetization and presents both challenges and opportunities for all-optical magnetic control. The presented technique can be generalized to directly probe nanoscale dynamics in spintronic materials and devices.

cond-mat.mtrl-sci↗

arXiv2025

Observation of quantum entanglement between free electrons and photons

Quantum entanglement is central to both the foundations of quantum mechanics and the development of new technologies in information processing, communication, and sensing. Entanglement has been realised in a variety of physical systems, spanning atoms, ions, photons, collective excitations, and hybrid combinations of particles. Remarkably, however, photons and free electrons -- the quanta of light and their most elementary sources -- have never been observed in an entangled state. Here, we demonstrate quantum entanglement between free electrons and photons. We show that entanglement is produced when an electron, prepared in a superposition of two beams, passes a nanostructure and generates transition radiation in a polarisation state tied to the electron path. By implementing quantum state tomography, we reconstruct the full density matrix of the electron-photon pair, and show that the Peres-Horodecki separability criterion is violated by more than 7 standard deviations. Based on this foundational element of emerging free-electron quantum optics, we anticipate manifold developments in enhanced electron imaging and spectroscopy beyond the standard quantum limit. More broadly, the ability to generate and measure entanglement opens electron microscopy to previously inaccessible quantum observables and correlations in solids and nanostructures.

quant-ph↗

arXiv2025

Roadmap for Quantum Nanophotonics with Free Electrons

Over the past century, continuous advancements in electron microscopy have enabled the synthesis, control, and characterization of high-quality free-electron beams. These probes carry an evanescent electromagnetic field that can drive localized excitations and provide high-resolution information on material structures and their optical responses, currently reaching the sub-{\aa}ngstr\"om and few-meV regime. Moreover, combining free electrons with pulsed light sources in ultrafast electron microscopy adds temporal resolution in the sub-femtosecond range while offering enhanced control of the electron wave function. Beyond their exceptional capabilities for time-resolved spectromicroscopy, free electrons are emerging as powerful tools in quantum nanophotonics, on par with photons in their ability to carry and transfer quantum information, create entanglement within and with a specimen, and reveal previously inaccessible details on nanoscale quantum phenomena. This Roadmap outlines the current state of this rapidly evolving field, highlights key challenges and opportunities, and discusses future directions through a collection of topical sections prepared by leading experts.

cond-mat.mes-hall↗

arXiv2025

Tunable quantum light by modulated free electrons

Nonclassical states of light are fundamental in various applications, spanning quantum computation to enhanced sensing. Fast free electrons, which emit light into photonic structures through the mechanism of spontaneous emission, represent a promising platform for generating diverse types of states. Indeed, the intrinsic connection between the input electron wave function and the output light field suggests that electron-shaping schemes, based on light-induced scattering, facilitates their synthesis. In this article, we present a theoretical framework capable of predicting the final optical density matrix of a generic N-electron state that can also account for post-sample energy filtering. By using such framework, we study the modulation-dependent fluctuations of the N-electron emission and identify regions of Poissonian and super-Poissonian statistics. In the single-electron case, we show how coherent states with nearly 90% purity can be formed by pre-filtering a portion of the spectrum after modulation, and how non-Gaussian states are generated after a precise energy measurement. Furthermore, we present a strategy combining a single-stage electron modulation and post-filtering to harness tailored light states, such as squeezed vacuum, cat, and triangular cat states, with fidelity close to 100%.

quant-ph↗

arXiv2024

Femtosecond and attosecond phase-space correlations in few-particle photoelectron pulses

Temporal correlations in pulsed electron beams reflect the microscopic dynamics of emission and interparticle interaction. In femtosecond electron emission from nanoscale field emitters, Coulomb interactions result in structured few-electron states with strong correlations in energy, time, and transverse momentum. Interactions with external fields may be used to both probe and further manipulate these correlated states. Here, we combine femtosecond-gated, event-based detection with inelastic electron-light scattering to directly map the photoelectron phase-space distribution of two-electron states. Our experiments demonstrate a bimodal structure in longitudinal phase space, with distinct contributions from interparticle interaction and dispersion. Moreover, we theoretically reveal that global phase modulation coherently shapes the few-electron phase-space distribution to exhibit attosecond temporal correlations. This controlled phasing of few-electron states can be harnessed to produce tailored excitations and super-radiance via two-electron energy post-selection.

cond-mat.mes-hall↗

arXiv2024

Evidence for reduced periodic lattice distortion within the Sb-terminated surface layer of the kagome metal CsV$_3$Sb$_5$

The discovery of the kagome metal CsV$_3$Sb$_5$ sparked broad interest, due to the coexistence of a charge density wave (CDW) phase and possible unconventional superconductivity in the material. In this study, we use low-energy electron diffraction (LEED) with a $\mu$m-sized electron beam to explore the periodic lattice distortion at the antimony-terminated surface in the CDW phase. We recorded high-quality backscattering diffraction patterns in ultrahigh vacuum from multiple cleaved samples. Unexpectedly, we did not find superstructure reflexes at intensity levels predicted from dynamical LEED calculations for the reported $2 \times 2 \times 2$ bulk structure. Our results suggest that in CsV$_3$Sb$_5$ the periodic lattice distortion accompanying the CDW is less pronounced at Sb-terminated surfaces than in the bulk.

cond-mat.str-el↗

arXiv2024

Megahertz cycling of ultrafast structural dynamics enabled by nanosecond thermal dissipation

Light-matter interactions are of fundamental scientific and technological interest. Ultrafast electron microscopy and diffraction with combined femtosecond-nanometer resolution elucidate the laser-induced dynamics in structurally heterogeneous systems. These measurements, however, remain challenging due to the brightness limitation of pulsed electron sources, leading to an experimental trade-off between resolution and contrast. Larger signals can most directly be obtained by higher repetition rates, which, however, are typically limited to a few kHz by the thermal relaxation of thin material films. Here, we combine nanometric electron-beam probing with sample support structures tailored to facilitate rapid specimen cooling. Optical cycling of a charge-density wave transformation enables quantifying the mean temperature increase induced by pulsed laser illumination. Varying the excitation fluence and repetition rate, we gauge the impact of excitation confinement and efficient dissipation on the heat diffusion in different sample designs. In particular, a thermally optimized support can dissipate average laser intensities of up to 200 $\mu W/\mu m^2$ within a few nanoseconds, allowing for reversible driving and probing of the CDW transition at a repetition rate of 2 MHz. Sample designs tailored to ultrafast measurement schemes will thus extend the capabilities of electron diffraction and microscopy, enabling high-resolution investigations of structural dynamics.

cond-mat.mtrl-sci↗

arXiv2024

Electrons herald non-classical light

Free electrons are a widespread and universal source of electromagnetic fields. The past decades witnessed ever-growing control over many aspects of electron-generated radiation, from the incoherent emission produced by X-ray tubes to the exceptional brilliance of free-electron lasers. Reduced to the elementary process of quantized energy exchange between individual electrons and the electromagnetic field, electron beams may facilitate future sources of tunable quantum light. However, the quantum features of such radiation are tied to the correlation of the particles, calling for the joint electronic and photonic state to be explored for further applications. Here, we demonstrate the coherent parametric generation of non-classical states of light by free electrons. We show that the quantized electron energy loss heralds the number of photons generated in a dielectric waveguide. In Hanbury-Brown-Twiss measurements, an electron-heralded single-photon state is revealed via antibunching intensity correlations, while two-quantum energy losses of individual electrons yield pronounced two-photon coincidences. The approach facilitates the tailored preparation of higher-number Fock and other optical quantum states based on controlled interactions with free-electron beams.

quant-ph↗

arXiv2024

Optical switching of ferro-rotational charge-density wave states

Tailored optical excitations can steer a system along non-equilibrium pathways to metastable states with specific structural or electronic properties. The light-induced hidden state of 1T-TaS$_{2}$, with its strongly enhanced conductivity and exceptionally long lifetime, represents a unique model system for studying the ultrafast switching of correlated electronic states. We use surface-sensitive electron diffraction in combination with a femtosecond optical quench to reveal the coexistence of both charge-density-wave (CDW) 2D chiralities as a structural characteristic of the hidden state, corresponding to coexisting ferro-rotational CDW states. Density functional theory (DFT) simulations of interfaces between opposite CDW 2D chiralities predict a higher-level, fractal-type moir'{e} superstructure with a kagome band structure near the Fermi energy. More broadly, these findings suggest that heterochiral interfaces in CDW systems provide an additional structural degree of freedom, expanding the possibilities for electronic control via twist-angle engineering.

cond-mat.mes-hall↗

arXiv2024

Quantum eraser experiments for the demonstration of entanglement between swift electrons and light

We propose a tangible experimental scheme for demonstrating quantum entanglement between swift electrons and light, relying on coherent cathodoluminescence for photon generation in a transmission electron microscope, and a quantum eraser setup for formation and verification of entanglement. The entanglement of free electrons with light is key to developing free-electron quantum optics and its potential applications such as quantum sensing, novel photonic and electron state generation, and entanglement between free electrons.

quant-ph↗

arXiv2024

Valley-controlled photoswitching of metal-insulator nanotextures

Spatial heterogeneity and phase competition are hallmarks of strongly-correlated materials, promising tunable functionality on the nanoscale. Light-induced switching of a correlated insulator to a metallic state is well established. However, optical excitation generally lacks the specificity to select sub-wavelength domains and control final textures. Here, we employ valley-selective photodoping to drive the domain-specific quench of a textured Peierls insulator. Polarized excitation leverages the anisotropy of quasi-one-dimensional states at the correlated gap to initiate an insulator-to-metal transition with minimal electronic heating. We find that averting dissipation facilitates domain-specific carrier confinement, control over nanotextured phases, and a prolonged lifetime of the metastable metallic state. Complementing existing manipulation schemes, valley-selective photoexcitation will enable the activation of electronic phase separation beyond thermodynamic limitations, facilitating optically-controlled hidden states, engineered heterostructures, and polarization-sensitive percolation networks.

cond-mat.str-el↗

arXiv2024

Quantum effects in the interaction of low-energy electrons with light

The interaction between free electrons and nanoscale optical fields has emerged as a unique platform to investigate ultrafast processes in matter and explore fundamental quantum phenomena. In particular, optically modulated electrons are employed in ultrafast electron microscopy as noninvasive probes that push the limits of spatiotemporal and spectral resolution down to the picometer--attosecond--microelectronvolt range. Electron kinetic energies well above the involved photon energies are commonly employed, rendering the electron--light coupling efficiency low and, thus, only providing limited access to the wealth of quantum nonlinear phenomena underlying the dynamical response of nanostructures. Here, we theoretically investigate electron--light interactions when photons and electrons have comparable energies, revealing strong quantum and recoil effects that include a nonvanishing coupling of surface-scattered electrons to plane waves of light, inelastic electron backscattering from localized optical fields, and strong electron--light coupling under grazing electron diffraction by an illuminated crystal surface. Our results open new vistas in electron--light--matter interactions with promising applications in ultrafast electron microscopy.

cond-mat.mes-hall↗

arXiv2024

Femtosecond trimer quench cycled at megahertz rates in the unconventional charge-density wave material $1\textit{T'}-\text{TaTe}_2$

Ultrafast optical switching of materials properties is of great relevance both for future technological applications as well as gaining fundamental physical insights to microscopic couplings and nonequilibrium phenomena. Transition-metal dichalcogenides (TMDCs) combine photo-sensitivity with strong correlations, furthering rich phase diagrams and enhanced tunability. Owing to its chemical composition, $1\textit{T'}-\text{TaTe}_2$ exhibits an electronically and structurally unique set of charge-density waves (CDWs), featuring increased conductivity and a reduced prominence of amplitude modes. Compared to other charge-ordered TMDCs, only very few studies addressed the ultrafast response of this material to optical excitation. In particular, the question whether such unconventional properties translate to unusual quench dynamics remains largely unresolved. Here, we investigate the structural dynamics in $1\textit{T'}-\text{TaTe}_2$ by means of ultrafast nanobeam electron diffraction. The experiments are carried out with a tailored sample design that allows for excitation at 2$\,$MHz repetition rate, higher than any structural phase transformation probed thus far. Harnessing the enhanced resolution and sensitivity of this approach, we reveal a strongly directional cooperative atomic motion during the one-dimensional quench of the low-temperature trimer lattice. These dynamics are completed within less than 500$\,$fs, substantially faster than reported previously. In striking contrast, the periodic lattice distortion of the room-temperature phase is unusually robust against high-density electronic excitation. In conjunction with the known sensitivity of $1\textit{T'}-\text{TaTe}_2$ to chemical doping, we thus expect the material to serve as a versatile platform for tunable structural control by optical stimuli.

cond-mat.mtrl-sci↗

arXiv2023

Laser-induced real-space topology control of spin wave resonances

Femtosecond laser excitation of materials that exhibit magnetic spin textures promises advanced magnetic control via the generation of ultrafast and non-equilibrium spin dynamics. We explore such possibilities in ferrimagnetic [Fe(0.35 nm)/Gd(0.40 nm)]$_{160}$ multilayers, which host a rich diversity of magnetic textures from stripe domains at low magnetic fields, a dense bubble/skyrmion lattice at intermediate fields, and a single domain state for high magnetic fields. Using femtosecond magneto-optics, we observe distinct coherent spin wave dynamics in response to a weak laser excitation allowing us to unambiguously identify the different magnetic spin textures. Moreover, employing strong laser excitation we show that we achieve versatile control of the coherent spin dynamics via non-equilibrium and ultrafast transformation of magnetic spin textures by both creating and annihilating bubbles/skyrmions. We corroborate our findings by micromagnetic simulations and by Lorentz transmission electron microscopy before and after laser exposure.

cond-mat.mtrl-sci↗