# R. RedmerUniversity of Rostock · Institut für Physik

R. Redmer

Univ.-Prof., Dr. rer. nat. habil.

## About

273

Publications

26,277

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8,642

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Citations since 2017

Additional affiliations

January 2010 - present

January 2009 - present

January 2005 - present

**Friedrich-Schiller-Universität Jena**

Description

- X-ray Diagnostics

Education

June 1991

April 1986

July 1982

## Publications

Publications (273)

We study state-of-the-art approaches for calculating x-ray Thomson scattering spectra from density functional theory molecular dynamics (DFT-MD) simulations based on a modified Chihara formula that expresses the inelastic contribution in terms of the dielectric function. We compare the electronic dynamic structure factor computed from the Mermin di...

The small semi-major axes of Hot Jupiters lead to high atmospheric temperatures of up to several thousand Kelvin. Under these conditions, thermally ionised metals provide a rich source of charged particles and thus build up a sizeable electrical conductivity. Subsequent electromagnetic effects, such as the induction of electric currents, Ohmic heat...

The small semi-major axes of Hot Jupiters lead to high atmospheric temperatures of up to several thousand Kelvin. Under these conditions, thermally ionised metals provide a rich source of charged particles and thus build up a sizeable electrical conductivity. Subsequent electromagnetic effects, such as the induction of electric currents, Ohmic heat...

Extreme conditions inside ice giants such as Uranus and Neptune can result in peculiar chemistry and structural transitions, e.g., the precipitation of diamonds or superionic water, as so far experimentally observed only for pure C─H and H 2 O systems, respectively. Here, we investigate a stoichiometric mixture of C and H 2 O by shock-compressing p...

We investigate shock-compressed copper in the warm dense matter regime by means of density functional theory molecular dynamics simulations. We use neural-network-driven interatomic potentials to increase the size of the simulation box and extract thermodynamic properties in the hydrodynamic limit. We show the agreement of our simulation results wi...

We investigate the thermopower and Lorenz number of hydrogen with Kohn-Sham density functional theory (DFT) across the plasma plane toward the near-classical limit, i.e., weakly degenerate and weakly coupled states. Our results are in concordance with certain limiting values for the Lorentz plasma, a model system which only considers electron-ion s...

We calculate the intermediate scattering function of warm dense aluminum by using density functional theory molecular dynamics simulations. From this data set, we derive the static and dynamic ion-ion structure factors. By applying a generalized collective modes model, we can fit the excitation spectra of the ion system and thereby extract the disp...

We investigate shock-compressed copper in the warm dense matter regime by means of density functional theory molecular dynamics simulations. We use neural-network-driven interatomic potentials to increase the size of the simulation box and extract thermodynamic properties in the hydrodynamic limit. We show the agreement of our simulation results wi...

The low-density limit of the electrical conductivity $\sigma(n,T)$ of hydrogen as the simplest ionic plasma is presented as function of temperature T and mass density n in form of a virial expansion of the resistivity. Quantum statistical methods yield exact values for the lowest virial coefficients which serve as benchmark for analytical approache...

The nonmetal-to-metal transition in dense fluid helium is discussed, which has been, in analogy to metallization of hydrogen, predicted as first-order plasma phase transition using chemical models for the equation of state and plasma composition. However, recent ab initio simulations performed for dense fluid helium indicate that this transition is...

We study ionization and transport processes in partially ionized multicomponent plasmas. The plasma composition is calculated via a system of coupled mass action laws. The electronic transport properties are determined by the electron-ion and electron-neutral transport cross sections. The influence of electron-electron scattering is considered via...

We study ionization and transport processes in partially ionized multicomponent plasmas. The plasma composition is calculated via a system of coupled mass-action laws. The electronic transport properties are determined by the electron-ion and electron-neutral transport cross sections. The influence of electron-electron scattering is considered via...

The miscibility gap in hydrogen-water mixtures is investigated by conducting Gibbs-ensemble Monte Carlo simulations with analytical two-body interaction potentials between the molecular species. We calculate several demixing curves at pressures below 150 kbar and temperatures of 1000 K ≤ T ≤ 2000 K. Despite the approximations introduced by the two-...

Thermal evolution models suggest that the luminosities of both Uranus and Neptune are inconsistent with the classical assumption of an adiabatic interior. Such models commonly predict Uranus to be brighter and, recently, Neptune to be fainter than observed. In this work, we investigate the influence of a thermally conductive boundary layer on the e...

The ultrafast synthesis of ε-Fe3N1+x in a diamond-anvil cell (DAC) from Fe and N2 under pressure was observed using serial exposures of an X-ray free electron laser (XFEL). When the sample at 5 GPa was irradiated by a pulse train separated by 443 ns, the estimated sample temperature at the delay time was above 1400 K, confirmed by in situ transform...

We explore the performance of the Gibbs-ensemble Monte Carlo simulation technique by calculating the miscibility gap of H2−He mixtures with analytical exponential-six potentials. We calculate several demixing curves for pressures up to 500 kbar and for temperatures up to 1800K and predict a H2−He miscibility diagram for the solar He abundance for t...

Materials at high pressures and temperatures are of great interest for planetary science and astrophysics, warm dense-matter physics and inertial confinement fusion research. Planetary structure models rely on an understanding of the behaviour of elements and their mixtures under conditions that do not exist on Earth; at the same time, planets serv...

SiO2 is one of the most fundamental constituents in planetary bodies, being an essential building block of major mineral phases in the crust and mantle of terrestrial planets (1–10 ME). Silica at depths greater than 300 km may be present in the form of the rutile-type, high pressure polymorph stishovite (P42/mnm) and its thermodynamic stability is...

Materials at high pressures and temperatures are of great interest for planetary science and astrophysics, warm dense matter physics, and inertial confinement fusion research. Planetary structure models rely on our understanding of the behaviour of elements (and their mixtures) at exotic conditions that do not exist on Earth, and at the same time p...

Mixtures of sodium and potassium differ substantially from the pure elements, while retaining the high compressibility, which is important to the complex behavior of dense alkali metals. We present powder x-ray diffraction of mixtures of Na and K compressed in diamond anvil cells to 48 GPa at 295 K. This reveals two stoichiometric intermetallics: a...

A realistic description of partially ionized matter in extreme thermodynamic states is critical to model the interior and evolution of the multiplicity of high-density astrophysical objects. Current predictions of its essential property, the ionization degree, rely widely on analytical approximations that have been challenged recently by a series o...

A realistic description of partially-ionized matter in extreme thermodynamic states is critical to model the interior and evolution of the multiplicity of high-density astrophysical objects. Current predictions of its essential property, the ionization degree, rely widely on analytical approximations that have been challenged recently by a series o...

Recent dynamic compression experiments [M. D. Knudson et al., Science 348, 1455 (2015); P. M. Celliers et al., Science 361, 677 (2018)] have observed the insulator-metal transition in dense liquid deuterium, but with an approximately 95-GPa difference in the quoted pressures for the transition at comparable estimated temperatures. It was claimed in...

Recent dynamic compression experiments [M. D. Knudson et al., Science 348, 1455 (2015); P. M. Celliers et al., Science 361, 677 (2018)] have observed the insulator-metal transition in dense liquid deuterium, but with an approximately 95 GPa difference in the quoted pressures for the transition at comparable estimated temperatures. It was claimed in...

The brightness of Neptune is often found to be in accordance with an adiabatic interior, while the low luminosity of Uranus challenges this assumption. Here we apply revised equation of state data of hydrogen, helium, and water and compute the thermal evolution of Uranus and Neptune assuming an adiabatic interior. For this purpose, we have develope...

The structure of the fluid carbon phase in the pressure region of the graphite, diamond, and BC8 solid phase is investigated. We find increasing coordination numbers with an increase in density. From zero to $30$ GPa, the liquid shows a decrease of packing efficiency with increasing temperature. However, for higher pressures, the coordination numbe...

The Kubo-Greenwood (KG) formula is often used in conjunction with Kohn-Sham (KS) density functional theory (DFT) to compute the optical conductivity, particularly for warm dense mater. For applying the KG formula, all KS eigenstates and eigenvalues up to an energy cutoff are required and thus the approach becomes expensive, especially for high temp...

Simulations of H–He mixtures using the van der Waals density functional – CORRIGENDUM - Volume 85 Issue 2 - Manuel Schöttler, Ronald Redmer

Celliers et al . (Reports, 17 August 2018, p. 677), in an attempt to reconcile differences in inferred metallization pressures, provide an alternative temperature analysis of the Knudson et al . experiments (Reports, 26 June 2015, p. 1455). We show that this reanalysis implies an anomalously low specific heat for the metallic fluid that is clearly...

New facilities explore warm dense matter (WDM) at conditions with extreme densities (exceeding ten times condensed matter densities) so that electrons are degenerate even at temperatures of 10–100 eV. Whereas in the nondegenerate region correlation effects such as Debye screening are relevant for the ionization potential depression (IPD), new effec...

The electrical σ and thermal conductivity λ of liquid iron are calculated with spin-polarized density-functional-theory-based simulations over a significant pressure and temperature range using the Kubo-Greenwood formalism. We show that a paramagnetic state is stable in the liquid up to high temperatures at ambient pressure and that the discrepancy...

We investigate a pump‐probe X‐ray Thomson scattering (XRTS) experiment that might be carried out at a free electron laser facility to study warm‐to‐hot states of dense matter. Ultrashort and intense X‐ray pulses with different energies (1,560–1,830 eV) heat a 1 µm thick Al target isochorically and create homogeneous and uncompressed warm‐to‐hot sta...

New facilities explore warm dense matter (WDM) at extreme conditions where the densities are very high (e.g., carbon up to density of 50 g cm$^{-3}$) so that electrons are degenerate even at 100 eV temperature. Whereas in the non-degenerate region correlation effects such as Debye screening and its improvements are relevant for the ionization poten...

We present thermodynamic material and transport properties for the extreme conditions prevalent in the interiors of massive giant planets and brown dwarfs. They are obtained from extensive ab initio simulations of hydrogen-helium mixtures along the isentropes of three representative objects. In particular, we determine the heat capacities, the ther...

We present thermodynamic material and transport properties for the extreme conditions prevalent in the interiors of massive giant planets and brown dwarfs. They are obtained from extensive \textit{ab initio} simulations of hydrogen-helium mixtures along the isentropes of three representative objects. In particular, we determine the heat capacities,...

With NASA’s Juno mission having arrived at its target and ESA’s JUICE mission in planning, the interest in state-of-the-art models for the interior structure and dynamics of Jupiter is increasing. This chapter reports on the related attempts within the Special Priority Program PlanetMag of the German Science Foundation and provides an up-to-date re...

We show results on the high-pressure equation of state of hydrogen–helium mixtures obtained from finite-temperature density functional theory molecular dynamics simulations using the van der Waals density functional. We discuss the calculation of non-ideal entropies based on different methods and show how nuclear quantum corrections influence the f...

We use finite-temperature density functional theory coupled to classical molecular dynamics simulation to calculate the miscibility gap of hydrogen-helium mixtures. The van der Waals density functional (vdW-DF) theory is used, which leads to lower demixing temperatures compared to computations using the Perdew-Burke-Ernzerhof functional. Our calcul...

We used molecular dynamics simulations based on density functional theory to study the thermophysical properties of warm dense helium. The influence of different exchange-correlation (XC) functionals was analyzed. We calculated the equation of state at high pressures up to several Mbar and temperatures up to 100 000 K in order to reconstruct recent...

The validity of the widely used linear mixing approximation for the equations of state (EOS) of planetary ices is investigated at pressure-temperature conditions typical for the interior of Uranus and Neptune. The basis of this study are ab initio data ranging up to 1000 GPa and 20 000 K calculated via density functional theory molecular dynamics s...

We use ab initio simulations based on density functional theory to calculate the electrical and thermal conductivities of electrons in partially ionized water plasmas at densities above 0.1 g/cm³. The resulting conductivity data are then fitted to analytic expressions for convenient application. For low densities, we develop a simple and fully anal...

We compute electrical and thermal conductivities of hydrogen plasmas in the non-degenerate regime using Kohn-Sham Density Functional Theory (DFT) and an application of the Kubo-Greenwood response formula, and demonstrate that for thermal conductivity, the mean-field treatment of the electron-electron (e-e) interaction therein is insufficient to rep...

We perform molecular dynamics simulations based on finite-temperature density functional theory in order to determine self-consistently the static and dynamic ion structure factor and the electronic form factor in lithium. This comprehensive data set allows for the extraction of the dispersion relation for collective excitations, the calculation of...

We perform molecular dynamics simulations based on finite-temperature density functional theory in order to determine self-consistently the static and dynamic ion structure factor and the electronic form factor in lithium. This comprehensive data set allows for the extraction of the dispersion relation for collective excitations, the calculation of...

We study the interaction of intense ultrashort laser pulses with cryogenic He jets using 2d/3v relativistic Particle-in-Cell simulations (XOOPIC). Of particular interest are laser intensities $(10^{15}-10^{20})$ W/cm$^2$, pulse lengths $\le 100$ fs, and the frequency regime $\sim 800$ nm for which the jets are initially transparent and subsequently...

A Monte Carlo code has been developed for the interaction of initially neutral systems with intense ultra-short laser pulses. Photoionization creates immediately a partially ionized, non-equilibrium plasma for which the relevant scattering processes are included. In this way the absorption of the laser energy in the plasma and the evolution of the...

Hydrogen, the simplest element in the universe, has a surprisingly complex phase diagram. Because of applications to planetary science, inertial confinement fusion and fundamental physics, its high-pressure properties have been the subject of intense study over the past two decades. While sophisticated static experiments have probed hydrogen's stru...

Supplementary Figures 1-7, Supplementary Note 1, Supplementary Methods and Supplementary References.

The strikingly low luminosity of Uranus (Teff ~ Teq) constitutes a long-standing challenge to our understanding of Ice Giant planets. Here we present the first Uranus structure and evolution models that are constructed to agree with both the observed low luminosity and the gravity field data. Our models make use of modern ab initio equations of sta...

Analytic free energy models for three solid high-pressure phases—diamond, body centered cubic phase with eight atoms in the unit cell (BC8), and simple cubic (SC)—are developed using density functional theory. We explicitly include anharmonic effects by performing molecular dynamics simulations and investigate their density and temperature dependen...

We construct thermodynamic potentials for two superionic phases of water [with body-centered cubic (bcc) and face-centered cubic (fcc) oxygen lattice] using a combination of density functional theory (DFT) and molecular dynamics simulations (MD). For this purpose, a generic expression for the free energy of warm dense matter is developed and parame...

The description of the interior structure and evolution of the Solar System giant planets continues to be a serious challenge. The most prominent example is Saturn for which simple homogeneous evolution models yield ages between 2 and 3 billion years (Gyr), i.e. much shorter than the age of the Solar System of Gyr. It has long been suggested that H...

Dense cryogenic hydrogen is heated by intense femtosecond infrared laser pulses at intensities of 1015-1016 Wcm-2. Three-dimensional particle-in-cell (PIC) simulations predict that this heating is limited to the skin depth, causing an inhomogeneously heated outer shell with a cold core and two prominent temperatures of about 25 and 40 eV for simula...

We have performed finite-temperature density functional theory
molecular dynamics simulations on dense methane, ammonia, and water mixtures (CH4:NH3:H2O) for various compositions and temperatures (2000 K ≤ T ≤ 10 000 K) that span a set of possible conditions in the interiors of ice-giant exoplanets. The equation-of-state, pair distribution function...

We report four structures for the 1:1 water-ammonia mixture showing superionic behavior at high temperature with the space groups P4/nmm, Ima2, Pma2 and Pm, which have been identified from evolutionary random structure search calculations at 0~K. Analyzing the respective pair distribution functions and diffusive properties the superionic phase is f...

Warm dense matter is characterized by solid state densities and electron temperatures of several electronvolts. Understanding the dynamics of WDM as well as measuring the parameters of WDM are important for the modeling of giant planets, shock-wave experiments, and inertial confinement fusion experiments. The new facilities FLASH and XFEL (Hamburg)...

Abrupt insulator-to-metal transition is experimentally observed in dense liquid D2 by application of magnetic shock waves of several hundred GPa to a cryocell containing D2 (22 K, density of 0.167 g/cm3) at the Sandia Z machine (Albuquerque, New Mexico, USA) for pulsed power research.

We have studied the light-matter interaction of ultra-short, intense optical laser fields with thin carbon foils via particle-in-cell simulations. Especially, the influence of additional impact ionization on the density and temperature of the generated plasma and on the corresponding Thomson scattering spectra was investigated. We predict a pump-pr...

Eighty years ago, it was proposed that solid hydrogen would become metallic at sufficiently high density. Despite numerous investigations, this transition has not yet been experimentally observed. More recently, there has been much interest in the analog of this predicted metallic transition in the dense liquid, due to its relevance to planetary sc...

The spectrum of x-ray Thomson scattering is proportional to the dynamic structure factor. An important
contribution is the ion feature which describes elastic scattering of x rays off electrons. We apply an ab initio
method for the calculation of the form factor of bound electrons, the slope of the screening cloud of free electrons, and the ion-ion...

Thomson scattering is a promising tool to infer warm dense matter (WDM) properties [1]. WDM is characterized by densities near to solid-density up to compressed matter well above solid-density and electron temperatures of several electron volts. In this plasma region, the transition from ideal plasmas to degenerate and strongly coupled plasmas occu...

Warm dense matter is of interest for modeling the interiors of planets and Brown Dwarfs. Corresponding pump‐probe experiments are performed at free electron laser facilities such as FLASH, LCLS or the future XFEL in Hamburg. X‐ray Thomson scattering is of special interest to extract the plasma parameters. In order to explain or predict the X‐ray Th...

We employ a combination of density functional theory (DFT), molecular dynamics (MD), and a variety of advanced postprocessing methods to construct an analytic thermodynamic potential (free energy) for ices VII and X. In particular, the temperature-dependent part of the free energy function is constructed using entropy data obtained via the spectrum...

We present new equations of state (EOS) for hydrogen and helium covering a
wide range of temperatures from 60 K to 10$^7$ K and densities from $10^{-10}$
g/cm$^3$ to $10^3$ g/cm$^3$. They include an extended set of ab initio EOS data
for the strongly correlated quantum regime with an accurate connection to data
derived from other approaches for the...

We have developed a model for analysing x-ray Thomson scattering data from high-density, millimetre-scale inhomogeneous plasmas created during ultra-high pressure implosions at the National Ignition Facility in a spherically convergent geometry. The density weighting of the scattered signal and attenuation of the incident and scattered x-rays throu...

We investigate subpicosecond dynamics of warm dense hydrogen at the XUV free-electron laser facility (FLASH) at DESY (Hamburg). Ultrafast impulsive electron heating is initiated by a ≤300-fs short x-ray burst of 92-eV photon energy. A second pulse probes the sample via x-ray scattering at jitter-free variable time delay. We show that the initial mo...

We give an introduction into the method of quantum molecular dynamics simulations which combines density functional theory with classical molecular dynamics. This method has demonstrated its predictive power in determining the thermophysical properties of matter under extreme conditions as found, e.g., in astrophysical objects like giant planets an...

The controversy with respect to the role of electron-electron collisions in
the dynamic conductivity of dense plasmas is resolved. In particular, the dc
conductivity is analyzed in the low-density, non-degenerate limit where the
Spitzer theory is valid and electron-electron collisions lead to the well-known
reduction of the result for a Lorentz pla...

Warm Dense Matter (WDM) occupies a loosely defined region of phase space intermediate between solid, liquid, gas, and plasma, and typically shares characteristics of two or more of these phases. WDM is generally associated with the combination of strongly coupled ions and moderately degenerate electrons, and careful attention to quantum physics and...

We investigate the effects of dynamical screening on the static transport properties. We review the low density expansion for the electron-electron and electron-ion correlation functions and give results for a 6-moment approach within linear response theory. The expansion coefficients are given. The convergence of thermoelectric transport coefficie...

We perform ab initio calculations for the equation of state of dense liquid hydrogen and deuterium using quantum molecular dynamics simulations based on finite-temperature density functional theory. This extensive data set allows us to determine specific density-temperature-pressure tracks such as the cold curve, precompressed and principal Hugonio...