arXiv subjects

D. O. Gericke

Publications and source records attributed to D. O. Gericke.

13 recordsLinked to original sources

arXiv2025

Probing the atomic dynamics of ultrafast melting with femtosecond electron diffraction

Melting is an everyday phase transition that is determined by thermodynamic parameters like temperature and pressure. In contrast, ultrafast melting is governed by the microscopic response to a rapid energy input and, thus, can reveal the strength and dynamics of atomic bonds as well as the energy flow rate to the lattice. Accurately describing these processes remains challenging and requires detailed insights into transient states encountered. Here, we present data from femtosecond electron diffraction measurements that capture the structural evolution of copper during the ultrafast solid to liquid phase transformations. At absorbed energy densities 2 to 4 times the melting threshold, melting begins at the surface slightly below the nominal melting point followed by rapid homogeneous melting throughout the volume. Molecular dynamics simulations reproduce these observations and reveal a weak electron lattice energy transfer rate for the given experimental conditions. Both simulations and experiments show no indications of rapid lattice collapse when its temperature surpasses proposed limits of superheating, providing evidence that inherent dynamics limits the speed of disordering in ultrafast melting of metals.

cond-mat.mtrl-sci↗

arXiv2023

An adaptive model for the optical properties of excited gold

We study the temperature-dependent optical properties of gold over a broad energy spectrum covering photon energies below and above the interband threshold. We apply a semi-analytical Drude-Lorentz model with temperature-dependent oscillator parameters. Our approximations are based on the distribution of electrons over the active bands with a density of states provided by density functional theory. This model can be easily adapted to other materials with similar band structures and can also be applied to the case of occupational nonequilibrium. Our calculations show a strong enhancement of the intraband response with increasing electron temperature while the interband component decreases. Moreover, our model compares well with density functional theory-based calculations for the reflectivity of highly excited gold and reproduces many of its key features. Applying our methods to thin films shows a sensitive nonlinear dependence of the reflection and absorption on the electron temperature. These features are more prominent at small photon energies and can be highlighted with polarized light. Our findings offer valuable insights for modeling ultrafast processes, in particular, the pathways of energy deposition in laser-excited samples.

cond-mat.mtrl-sci↗

arXiv2023

Platform for Probing Radiation Transport Properties of Hydrogen at Conditions Found in the Deep Interiors of Red Dwarfs

We describe an experimental concept at the National Ignition Facility for specifically tailored spherical implosions to compress hydrogen to extreme densities (up to $\sim$800$\times$ solid density, electron number density n$_e$$\sim$4$\times$10$^{25}$ cm$^{-3}$ ) at moderate temperatures (T$\sim$200 eV), i.e., to conditions, which are relevant to the interiors of red dwarf stars. The dense plasma will be probed by laser-generated x-ray radiation of different photon energy to determine the plasma opacity due to collisional (free-free) absorption and Thomson scattering. The obtained results will benchmark radiation transport models, which in the case for free-free absorption show strong deviations at conditions relevant to red dwarfs. This very first experimental test of free-free opacity models at these extreme states will help to constrain where inside those celestial objects energy transport is dominated by radiation or convection. Moreover, our study will inform models for other important processes in dense plasmas, which are based on electron-ion collisions, e.g., stopping of swift ions or electron-ion temperature relaxation.

physics.plasm-ph↗

arXiv2018

Melting and phase change for laser-shocked iron

Using the LCLS facility at the SLAC National Accelerator Laboratory, we have observed X-ray scattering from iron compressed with laser driven shocks to Earth-core like pressures above 400GPa. The data shows shots where melting is incomplete and we observe hexagonal close packed (hcp) crystal structure at shock compressed densities up to 14.0 gcm-3 but no evidence of a double-hexagonal close packed (dhcp) crystal. The observation of a crystalline structure at these densities, where shock heating is expected to be in excess of the equilibrium melt temperature, may indicate superheating of the solid. These results are important for equation of state modelling at high strain rates relevant for impact scenarios and laser-driven shock wave experiments.

physics.plasm-ph↗

arXiv2016

Theory of Thomson scattering in inhomogeneous media

Thomson scattering of laser light is one of the most fundamental diagnostics of plasma density, temperature and magnetic fields. It relies on the assumption that the properties in the probed volume are homogeneous and constant during the probing time. On the other hand, laboratory plasmas are seldom uniform and homogeneous on the temporal and spatial dimensions over which data is collected. This is partic- ularly true for laser-produced high-energy-density matter, which often exhibits steep gradients in temperature, density and pressure, on a scale determined by the laser focus. Here, we discuss the modification of the cross section for Thomson scattering in fully-ionized media exhibiting steep spatial inhomogeneities and/or fast temporal fluctuations. We show that the predicted Thomson scattering spectra are greatly altered compared to the uniform case, and may even lead to violations of detailed balance. Therefore, careful interpretation of the spectra is necessary for spatially or temporally inhomogeneous systems.

physics.plasm-ph↗

arXiv2013

Comparison of electron-ion energy transfer in dense plasmas obtained from numerical simulations and quantum kinetic theory

We evaluate various analytical models for the electron-ion energy transfer and compare the results to data from molecular dynamics (MD) simulations. The models tested includes energy transfer via strong binary collisions, Landau-Spitzer rates with different choices for the cut-off parameters in the Coulomb logarithm, rates based on Fermi's golden rule (FGR) and theories taking coupled collective modes (CM) into account. In search of a model easy to apply, we first analyze different approximations of the FGR energy transfer rate. Then we investigate several numerical studies using MD simulations and try to uncover CM effects in the data obtained. Most MD data published so far show no distinct CM effects and, thus, can be interpreted within a FGR or binary collision approach. We show that this finding is related to the parameter regime, in particular the initial temperature difference, considered in these investigations.

physics.plasm-ph↗

arXiv2013

Observation of inhibited electron-ion coupling in strongly heated graphite

Creating non-equilibrium states of matter with highly unequal electron and lattice temperatures allows unsurpassed insight into the dynamic coupling between electrons and ions through time-resolved energy relaxation measurements. Recent studies on low-temperature laser-heated graphite suggest a complex energy exchange when compared to other materials. To avoid problems related to surface preparation, crystal quality and poor understanding of the energy deposition and transport mechanisms, we apply a different energy deposition mechanism, via laser-accelerated protons, to isochorically and non-radiatively heat macroscopic graphite samples up to temperatures close to the melting threshold. Using time-resolved x ray diffraction, we show clear evidence of a very small electron-ion energy transfer, yielding approximately three times longer relaxation times than previously reported. This is indicative of the existence of an energy transfer bottleneck in non-equilibrium warm dense matter.

physics.plasm-ph↗

arXiv2011

The hydrogen equation of state at high densities

We use a two-fluid model combining the quantum Green's function technique for the electrons and a classical HNC description for the ions to calculate the high-density equation of state of hydrogen. This approach allows us to describe fully ionized plasmas of any electron degeneracy and any ionic coupling strength which are important for the modeling of a variety of astrophysical objects and inertial confinement fusion targets. We have also performed density functional molecular dynamics simulations (DFT-MD) and show that the data obtained agree with our approach in the high density limit. Good agreement is also found between DFT-MD and quantum Monte Carlo simulations. The thermodynamic properties of dense hydrogen can thus be obtained for the entire density range using only calculations in the physical picture.

physics.plasm-ph↗

arXiv2010

X-ray scattering as a probe for warm dense mixtures and high-pressure miscibility

We demonstrate the abilities of elastic x-ray scattering to yield information on dense matter with multiple ion species and on the microscopic mixing in dense materials. Based on partial structure factors from ab initio simulations, a novel approach for the elastic scattering feature is applied to dense hydrogen-beryllium and hydrogen-helium mixtures. The scattering signal differs significantly between single species, real microscopic mixtures, and two separate fluids in the scattering volume.

physics.plasm-ph↗

arXiv2009

Quantum statistical model of nonlinear inverse bremsstrahlung absorption in strongly coupled plasmas

A new approach for the calculation of collisional inverse bremsstrahlung absorption of laser light in dense plasmas is presented. Quantum statistical formalism used allows avoiding {\em ad hoc} cutoffs that were necessary in classical approaches. Thus, the current method remains reliable for strong electron-ion interactions. In addition, both the dynamic, field dependent response and hard electron-ion collisions, are consistently incorporated. The latter were treated in an average manner as a stopping power that in turn was cast into a form of a friction force. Here, for the first time a link between the stopping power and the problem of collisional laser absorption is drawn. This allows the theories developed for the stopping power calculation, such as the quantum T-matrix approach, to be applied to the problem of collisional laser absorption. The new approach accommodates the low- and high-frequency limits explained in the text and is valid for arbitrary laser field intensities. A comparison with classical MD simulation is indicative of the validity of the new method in the wide parameter range tested.

physics.plasm-ph↗

arXiv2009

Heavy Ion-Beam Driven Isentropic Compression Experiments

A new design for heavy-ion beam driven isentropic compression experiments is suggested and analysed. The proposed setup utilises long stopping ranges and the variable focal spot geometry of the high-energy uranium beams delivered at the GSI Helmholtzzentrum für Schwerionenforschung and Facility for Antiproton and Ion Research accelerator facilities in Darmstadt, Germany, to produce a planar ramp loading of various samples. In such experiments, the predicted high pressure amplitudes (\unit[$<10$]{Mbar}) and the predicted short timescales of compression (\unit[$<10$]{ns}) will allow testing the time dependent material deformation phenomena at unprecedented extreme conditions.

physics.plasm-ph↗

arXiv2009

Nonlinear collisional absorption of laser light in dense, strongly coupled plasmas

We present a new theoretical approach for collisional absorption of laser energy in dense plasmas which accommodates arbitrary frequencies and high intensities of the laser field. We establish a connection between laser absorption by inverse Bremstrahlung and the stopping power. This relation is then applied to include strong correlations beyond the mean field approach. The results show an excellent agreement with molecular dynamics simulations up to very high coupling strength.

physics.plasm-ph↗

arXiv2009

Coupled Mode Effects on Energy Transfer Rates in Two Temperature Plasmas

We investigate the effects of collective modes on the temperature relaxation rates in fully coupled electron-ion systems. Firstly, the well-understood limit of weakly coupled plasmas is considered and the coupled mode formula within the random phase approximation is derived starting from the Lenard-Balescu kinetic equation. We show how the frequency integration can be performed by standard methods without applying additional approximations. Due to the well-defined approximation scheme, the results can serve as a benchmark for more approximate theories and numerical simulations in this limit. The coupled mode electron-ion transfer rates show a considerable reduction compared to the Fermi-Golden Rule approach for certain parameters and very small changes for other systems. We demonstrate how these coupled mode effects are connected to the occurrence of ion acoustic modes and under which conditions they occur. Interestingly, coupled mode effects can also occur for plasmas with very high electron temperatures; a regime, where the Landau-Spitzer approach is believed to give accurate results. Finally, we extend the approach to systems with strongly coupled ions by applying static local field corrections. This extension can substantially increase the coupled mode effects.

physics.plasm-ph↗