Non-Equilibrium Warm Dense Matter
Research on properties of matter as in the centre of planets or stars, created by short- and long pulse lasers, x-rays, ion or electron beams, in diamond anvil cells or for fusion energy production.
Density response of solid and fluid carbon under high pressure
Carbon is a very interesting element that can have up to four covalent bonds with neighboring atoms, thus featuring many different solid phases as well as unusual properties in the fluid phase. Here, Kushal Ramakrishna and me present calculations for the inelastic structure factor of diamond, lonsdaleite, and the BC8 phase of carbon at high pressure. The fluid structure of carbon up to 10Mbar is investigated here.
Structure of dense matter in non-equilibrium
Processes in non-equilibrium matter can be generated using laser irradiation and can then be measured using scattering of x-rays with a resolution of femto seconds. Here, David A. Chapman and me develop the theoretical methods to describe the structure and thus scattering of x-rays in non-equilibrium plasmas and warm dense matter. The theory includes in particular correlations and quantum effects for general electron Wigner distribution functions.
Diamonds from plastic under high pressure
If one compresses a plastic foil to 1.5 million bar and 6000K, the carbon contained in the plastic segregates and immediately freezes into diamonds. There have been loads of speculations by scientist about this happening, now however a group led by D. Kraus (HZDR) was able to observe this process in situ. Here, are all the details. It is likely, that such a phase separation occurs naturally inside planets like Uranus or Neptune and other ice giants. Some news outlets found this very interesting as well: CNN, Guardian, Telegraph, The Times, Neue Zürcher Zeitung, Die Welt, Bild der Wissenschaft ...
The stopping power near the Bragg peak in fully ionized carbon plasmas
The stopping of particles beams in matter is a very interesting process of relevance for medical applications, materials processing as well as for clean energy production via nuclear fusion. We (Witold Cayzac and colleagues) have shown in this article in Nature Communications, which processes dominate the stopping of nitrogen ions in fully ionized carbon plasmas. This has enabled us, for the first time, to distinguish experimentally between the predictions of different stopping theories and to falsify some of those theories.
Ion acoustic modes in warm dense matter and the Born-Oppenheimer approximation
Ion acoustic modes in liquids and plasmas are equivalent to phonons in solids. They play an important role for energy transfer processes during temperature relaxation, influence the stopping of fast particles, and allow to determine the speed of sound in the medium as well as the temperature (via detailed balance). Our theoretical understanding of ion acoustic modes is crucial for our capabilities to model and interpret the x-ray scattering signal in experiments and to investigate new states of matter. Here, we (P. Mabey und Kollegen aus Oxford, Kollegen von SLAC/SIMES) investigated ion acoustic modes using first principle simulations.
Electron-phonon coupling, energy transfer, and the two-temperature model in laser heated metals
Laser irradiation of metals creates non-equilibrium states with high energy electrons. The relaxation in the system and the coupling to the phonons is a complicated process that sometimes is modelled within a two-temperature model. We (R. Ernstorfer and colleagues from FHI Berlin) found that a much better description is given by including the non-thermal behaviour of the phonons.
Hexagonal diamond (Lonsdaleite) found in shock experiments
Shock-compressed carbon changes phase from graphite to diamond. Under certain conditions, hexagonal diamond -lonsdaleite- can be produced. Lonsdaleite is a metastable phase of carbon featuring a crystal lattice structure in between graphite and diamond. Calculations predict lonsdaleite to be harder than diamond. Naturally occuring londsaleite is possibly generated in meteorite impacts. A collaboration under principal investigator Dominik Kraus (HZDR, University Berkeley) has just published these new insights. There is a nice summary here.
Gitter- und Elektronenstruktur in warmem Diamant bei p=130GPa (links), in warmem Lonsdaleite bei p=200GPa (mitte) und in flüssigem Kohlenstoff bei p=150GPa (rechts).
We are using first principle simulations like density functional molecular dynamics and quantum statistical Green's function methods to describe new and exotic states of matter in equilibrium and non-equilibrium and during various relaxation processes. We give predictions for x-ray spectra that may be obtained by scattering experiments.
Molecular hydrogen as in Jupiter's outer mantle, warm dense aluminium created by laser generated shock waves, metallic hydrogen as in Jupiters core.