arXiv subjects

Baerbel Rethfeld

Publications and source records attributed to Baerbel Rethfeld.

At least 19 recordsLinked to original sources

arXiv2026

Interplay of non-local transport and local scattering during electron thermalization and spatial equilibration in laser-excited metals

Ultrafast laser excitation of metals induces electronic nonequilibrium both in space and locally in the energy distribution. The subsequent dynamics are governed by the interplay between non-local transport and local scattering of hot electrons, yet combined microscopic descriptions of these processes remain sparse. Here, we disentangle the influence of these processes on thermalization using a reformulation of the Boltzmann transport equation in energy space that consistently describes both spatial equilibration and scattering through full collision integrals. Our results reveal that transport accelerates the apparent thermalization observed at the irradiated surface by removing athermal carriers, while the same spatial redistribution delays complete equilibration of the full electron system. We analyze the experimentally accessible energy-dependent dynamics at the front and back surface and find that the dominant process varies, depending on both position and on the energetic window. Overall, our work improves the understanding of the interplay of electronic nonequilibrium processes occurring in optically thick laser-driven systems with relevant implications for future electronic applications.

cond-mat.mtrl-sci↗

arXiv2026

Electronic Signature of Melting Onset in Polycrystalline Copper at Extreme Conditions

Ultrafast melting is fundamentally a structural transition of the ionic lattice, but this rearrangement also reshapes the electronic properties by changing the energy landscape and scattering mechanisms. Although the electrons react almost instantaneously, it is not a priori clear how much lattice disorder is required for a significant response. Here, we show that the onset of melting already produces a clear electronic signature in polycrystalline copper. Using single-shot terahertz time-domain spectroscopy on thin films excited over a wide range of laser fluences, we infer the transient conductivity during the first picoseconds after excitation. The data, supported by two-temperature molecular-dynamics simulations, show that before melting, electron transport is substantially limited by grain-boundary scattering and that melting strongly suppresses this channel. As melting begins at these interfaces, we observe a transient increase in the conductivity that directly marks the onset of the phase transition. More broadly, these results show that ionic and electronic relaxation stages are closely coupled in nonequilibrium laser-driven matter and that optical measurements can resolve distinct stages of melting.

cond-mat.mtrl-sci↗

arXiv2026

From Code to Figure: A FAIR-Aligned Data Provenance Chain for Reproducible Simulation Research in Numerical Physics

Computational physics increasingly depends on large simulation datasets generated by software that remains under active development for many years. In such settings, reproducibility requires not only well documented data but also explicit links between code versions, simulation inputs, generated outputs, analysis steps, and published figures. Here, we present an integrated workflow for reproducible and FAIR-aligned simulation research in numerical physics. We describe how version control, code review, automated testing, structured logging, metadata-rich output, and standardized post-processing can be combined to support traceability from software development to publication. The presented concepts demonstrated for one particular simulation framework are broadly applicable to computational physics and other data-intensive areas of scientific computing.

physics.comp-ph↗

arXiv2026

Competing thermalization pathways of photoexcited hot electrons

Photoexcited hot carriers in solids can drive processes, such as photocatalytic reactions on the surface, beyond those available in thermal equilibrium. Hot-electron-mediated reaction pathways are limited by the thermalization of the nonequilibrium electron distribution through microscopic scattering events. Commonly, thermalization is exclusively attributed to electron-electron scattering, whereas electron-phonon scattering is considered relevant mainly for the energy equilibration with the lattice. With a kinetic model based on full Boltzmann collision integrals, we demonstrate that each scattering mechanism alone can thermalize the electron distribution, albeit along different trajectories in phase space. We find an opposite dependence on the excitation strength of the respective thermalization times and show that both processes can become comparable for weak excitations, corresponding to a sample temperature increase of a few Kelvin. Our results unravel the contributions of electron-electron and electron-phonon scattering to the thermalization across the full range of experimental excitation strengths up to the melting regime, thus facilitating the prediction of thermalization times for hot-carrier-based applications.

cond-mat.mtrl-sci↗

arXiv2026

Interplay of electron-magnon scattering and spin-orbit induced electronic spin-flip scattering in a two-band Stoner model

This paper presents a theoretical investigation of electron-magnon scattering processes in the ultrafast demagnetization in itinerant ferromagnets. In the framework of a ferromagnetic model system, we compute the spin-dependent dynamics of electrons in itinerant Bloch states by including electron-magnon and electron-electron scattering processes on an equal footing. While the former process flips the electronic spin accompanied by the creation or destruction of a magnon, the latter exchanges electronic angular momentum with the lattice due to the influence of spin-orbit coupling. We show that, for a realistic choice of the electron-magnon interaction and deposited pulse energy, the interplay of these two different scattering mechanisms leads to the creation of magnons and a transfer of angular momentum to the lattice that constitutes an essentially non-equilibrium microscopic scenario for the ultrafast demagnetization process in itinerant ferromagnets.

cond-mat.mtrl-sci↗

arXiv2026

Roadmap for warm dense matter physics

This roadmap presents the state-of-the-art, current challenges and near future developments anticipated in the thriving field of warm dense matter physics. Originating from strongly coupled plasma physics, high pressure physics and high energy density science, the warm dense matter physics community has recently taken a giant leap forward. This is due to spectacular developments in laser technology, diagnostic capabilities, and computer simulation techniques. Only in the last decade has it become possible to perform accurate enough simulations \& experiments to truly verify theoretical results as well as to reliably design experiments based on predictions. Consequently, this roadmap discusses recent developments and contemporary challenges that are faced by theoretical methods, and experimental techniques needed to create and diagnose warm dense matter. A large part of this roadmap is dedicated to specific warm dense matter systems and applications in astrophysics, inertial confinement fusion and novel material synthesis.

physics.plasm-ph↗

arXiv2026

Thermalization of Optically Excited Fermi Systems: Electron-Electron Collisions in Solid Metals

Ultrafast optical excitation of metals induces a non-equilibrium energy distribution in the electronic system, with a characteristic step-structure determined by Pauli blocking. On a femtosecond timescale, electron-electron scattering drives the electrons towards a hot Fermi distribution. In this work, we present a derivation of the full electron-electron Boltzmann collision integral within the random-k approximation. Building on this approach, we trace the temporal evolution of the electron energy distribution towards equilibrium, for an excited but strongly degenerate Fermi system. Furthermore, we examine to which extent the resulting dynamics can be captured by the numerically simpler relaxation time approach, applying a constant and an energy-dependent relaxation time derived from Fermi-liquid theory. We find a better agreement with the latter, while specific features caused by the balance of scattering and reoccupation can only be captured with a full collision integral.

cond-mat.mtrl-sci↗

arXiv2025

Probing laser-driven surface and subsurface dynamics via grazing-incidence XFEL scattering and diffraction

We demonstrate a grazing-incidence x-ray platform that simultaneously records time-resolved grazing-incidence small-angle x-ray scattering (GISAXS) and grazing-incidence x-ray diffraction (GID) from a femtosecond laser-irradiated gold film above the melting threshold, with picosecond resolution at an x-ray free-electron laser (XFEL). By tuning the x-ray incidence angle, the probe depth is set to tens of nanometers, enabling depth-selective sensitivity to near-surface dynamics. GISAXS resolves ultrafast changes in surface nanomorphology (correlation length, roughness), while GID quantifies subsurface lattice compression, grain orientation, melting, and recrystallization. The approach overcomes photon-flux limitations of synchrotron grazing-incidence geometries and provides stringent, time-resolved benchmarks for complex theoretical models of ultrafast laser-matter interaction and warm dense matter. Looking ahead, the same depth-selective methodology is well suited to inertial confinement fusion (ICF): it can visualize buried-interface perturbations and interfacial thermal resistance on micron to sub-micron scales that affect instability seeding and burn propagation.

physics.optics↗

arXiv2025

Dynamic interfacial effects in ultrathin ferromagnetic bilayers

We investigate the magnetization dynamics of an ultrathin Co (1.5 nm) /Py (1.5 nm) bilayer system from femtosecond (fs) to nanosecond (ns) timescales. Magnetization dynamics in the fs timescales is characterized as a highly non-equilibrium regime due to an ultrafast reduction of magnetization by laser excitation. On the other hand, the dynamics in the ns timescales is characterized as a close-to-equilibrium regime involving the excitation of coherent magnons. We demonstrate that the interfacial interaction between the Co and Py layers in these two non-equilibrium regimes across the timescales is dynamic and simultaneously influences the magnetization loss in the fs timescales and the magnon dynamics in the ns timescales. On ultrafast (fs) timescales, comparison between time-resolved magneto-optical Kerr effect (TR-MOKE) measurements and temperature-based μT model simulations reveals that the bilayer exhibits demagnetization dynamics intermediate between those of its individual layers. When driven far from equilibrium by ultrashort laser pulse excitation, the magnetization dynamics of the individual Co and Py layers appear to remain decoupled and evolve independently in the initial stages of the ultrafast response. On the other hand, in the ns regime, the two individual layers of the bilayer precess together at the same frequency in a coupled manner as one effective single layer. Furthermore, by correlating the ultrafast demagnetization to precessional damping we attempt to bridge the two non-equilibrium regimes across fs to ns timescales. These results improve our understanding of magnetization dynamics across timescales in ultrathin exchanged-coupled ferromagnetic bilayers and provide valuable insights for the design of high-frequency and energy efficient spintronic device concepts.

cond-mat.mtrl-sci↗

arXiv2025

Ultrafast electron-phonon scattering in antiferromagnetic Dirac semimetals

Topological antiferromagnetic systems, which exhibit anisotropic band structures combined with complex relativistic spin structures in momentum space, have shown strong magnetoresistance effects driven by Dirac fermion characteristics. While these new antiferromagnets have been studied in transport experiments, very little is known about their spin-dependent electronic dynamics on ultrafast timescales and far-from-equilibrium behavior. This paper investigates theoretically the spin-dependent electronic dynamics due to electron-phonon scattering in a model electronic band structure that corresponds to a Dirac semimetal antiferromagnet. Following a spin conserving instantaneous excitation we obtain a change of the antiferromagnetic spin polarization due to the scattering dynamics for the site-resolved spin expectation values. This allows us to identify fingerprints of the anisotropic band structure in the carrier dynamics on ultrashort timescales.

cond-mat.mes-hall↗

arXiv2024

Signatures of ballistic and diffusive transport in the time-dependent Kerr-response of magnetic materials

We calculate the influence of diffusive and ballistic transport on ultrafast magnetization in thick metallic films. When only diffusive transport is present, gradients of magnetization in the material remain up to picosecond timescales. In contrast, in the extreme superdiffusive limit where ballistic transport dominates, the magnetization changes homogeneously in space. We calculate the measurable magneto-optical responses for a $\SI{40}{\nano\meter}$ Nickel film. Although the resulting Kerr rotation dynamics are found to be very similar in the two limits of transport, our simulations reveal a clear signature of magnetization gradients in the Kerr ellipticity dynamics, namely a strong probe-angle dependence for the case when diffusive transport allows gradients to persist. We then perform probe-angle dependent complex magneto-optical Kerr effect (CMOKE) measurements on an excited \SI{40}{\nano\meter} Nickel film. The angle dependence of the measured Kerr signals closely matches the simulated response with diffusive transport. Therefore we conclude that the influence of ballistic transport on ultrafast magnetization dynamics in such films is negligible.

cond-mat.mtrl-sci↗

arXiv2023

Influence of band occupation on electron-phonon coupling in gold

Electron-phonon coupling is a fundamental process that governs the energy relaxation dynamics of solids excited by ultrafast laser pulses. It has been found to strongly depend on electron temperature as well as on nonequilibrium effects. Recently, the effect of occupational nonequilibrium in noble metals, which outlasts the fully kinetic stage, has come into increased focus. In this work, we investigate the influence of nonequilibrium density distributions in gold on the electron-phonon coupling. We find a large effect on the coupling parameter which describes the energy exchange between the two subsystems. Our results challenge the conventional view that electron temperature alone is a sufficient predictor of electron-phonon coupling.

cond-mat.mtrl-sci↗

arXiv2023

Time-resolved spectral densities of non-thermal electrons in gold

Noble-metal nanoparticles for photocatalysis have become a major research object in recent years due to their plasmon-enhanced strong light-matter interaction. The dynamics of the hot electrons in the noble metal are crucial for the efficiency of the photocatalysis and for the selective control of reactions. In this work, we present a kinetic description of the non-equilibrium electron distribution created by photoexcitation, based on full energy-resolved Boltzmann collision integrals for the laser excitation as well as for the electron-electron thermalization. The laser-induced electronic non-equilibrium and the inherently included secondary electron generation govern the dynamics of non-thermal electrons. Applying our method to gold, we show a significant dependence of hot electron dynamics on kinetic energy. Specifically, the timescales of the relaxation as well as the qualitative behavior are depending on the evaluated energy window. During the thermalization processes there are cases of increasing electron density as well as of decreasing electron density. Studying the influence of excitation parameters, we find that the photon energy and the fluence of the exciting laser can be tuned to influence not only the initial excitation but also the subsequent characteristics of the time-resolved electronic spectral density dynamics. The electronic thermalization including secondary electron generation leads to time-dependent spectral densities which differ from their specific final equilibrium values for picoseconds after irradiation ended.

physics.app-ph↗

arXiv2023

Intertwined relaxation processes maintain athermal electron distribution in laser-excited dielectrics

We study the relaxation dynamics of laser-excited non-equilibrium electron distributions in the valence- and conduction band of a dielectric. We apply Boltzmann collision integrals to trace the influence of different scattering mechanisms on the energy- and particle density of electrons and holes. Our results show a two-timescale behavior of the equilibration process: Thermalization within each band towards a respective Fermi distribution as well as equilibration of the band-resolved temperatures occur within a few femtoseconds. In contrast, the equilibration of the respective chemical potentials, driven by scattering processes involving particle exchange like impact ionization and Auger recombination, requires timescales in the range of hundreds of femtoseconds. We evaluate the effect of our model assumptions for distinct material parameters on the extracted specific relaxation times. All these timescales, however, strongly increase, when the additional scattering channel with the cold phonon system is considered: Our simulations demonstrate that an athermal non-Fermi electron distribution can be maintained well up to the picosecond range.

cond-mat.stat-mech↗

arXiv2023

Laser-induced Creation of antiferromagnetic 180-degree domains in NiO/Pt bilayers

We demonstrate how the antiferromagnetic order in heterostructures of NiO/Pt thin films can be modified by optical pulses. We irradiate our samples with laser light and identify an optically induced creation of antiferromagnetic domains by imaging the created domain structure utilizing the X-ray magnetic linear dichroism effect. We study the effect of different laser polarizations on the domain formation and identify a polarization-independent creation of 180° domain walls and domains with 180° different Néel vector orientation. By varying the irradiation parameters, we determine the switching mechanism to be thermally induced and demonstrate the reversibility. We thus demonstrate experimentally the possibility to optically create antiferromagnetic domains, an important step towards future functionalization of all optical switching mechanisms in antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2022

Indirect optical manipulation of the antiferromagnetic order of insulating NiO by ultrafast interfacial energy transfer

We report the ultrafast, (sub)picosecond reduction of the antiferromagnetic order of the insulating NiO thin film in a Pt/NiO bilayer. This reduction of the antiferromagnetic order is not present in pure NiO thin films after a strong optical excitation. This ultrafast phenomenon is attributed to an ultrafast and highly efficient energy transfer from the optically excited electron system of the Pt layer into the NiO spin system. We propose that this energy transfer is mediated by a stochastic exchange scattering of hot Pt electrons at the Pt/NiO interface.

cond-mat.mtrl-sci↗

arXiv2021

Control of transport phenomena in magnetic heterostructures by wavelength modulation

We demonstrate the tuneablity of the ultrafast energy flow in magnetic/non-magnetic bilayer structures by changing the wavelength of the optical excitation. This is achieved by an advanced description of the temperature based $μ$T-model that explicitly considers the wavelength- and layer-dependent absorption profile within multilayer structures. For the exemplary case of a Ni/Au bilayer, our simulations predict that the energy flow from Ni to Au is reversed when changing the wavelength of the excitation from the infrared to the ultraviolet spectral range. These predictions are fully supported by characteristic signatures in the magneto-optical Kerr traces of the Ni/Au model system. Our results will open up new avenues to steer and control the energy transport in designed magnetic multilayer for ultrafast spintronic applications.

cond-mat.mes-hall↗

arXiv2021

Implementation of the electronic non-equilibrium in the two-temperature model

We investigate a temperature-based model, called extended two-temperature model (eTTM), that describes the electronic non-equilibrium and its effect on energy dissipation in metals after ultrashort laser excitation. We derive and discuss improvements in comparison to published versions of this model [E. Carpene, Phys. Rev. B 2006, 74, 024301; G. Tsibidis, Appl. Phys. A 2018, 124, 311]. The comparison of the results of the eTTM with results of the well-known two-temperature model (TTM) shows a delayed increase of the electronic temperature when being calculated with the eTTM. We find a good agreement in the non-equilibrium energy distribution after absorption of photons with results from a kinetic description using a Boltzmann collision term. The model provides a convenient tool for fast calculation of features of the non-equilibrium electrons. As an example we inspect the dynamics of high-energy electrons observable in photo-electron spectroscopy and demonstrate the advantage of the eTTM over the conventional two-temperature model.

cond-mat.mtrl-sci↗