S. Mazevet

Ab initiomodel of optical properties of two-temperature warm dense matter

We present a model to describe thermophysical and optical properties of two-temperature systems consisted of heated electrons and cold ions in a solid lattice that occur during ultrafast heating experiments. Our model is based on ab initio simulations within the framework of density functional theory. The optical properties are obtained by evaluating the Kubo-Greenwood formula. By applying the material parameters of our ab initio model to a two-temperature model we are able to describe the temperature relaxation process of femtosecond-laser-heated gold and its optical properties within the same theoretical framework. Recent time-resolved measurements of optical properties of ultrafast heated gold revealed the dynamics of the interaction between femtosecond laser pulses and solid state matter. Different scenarios obtained from simulations of our study are compared with experimental data [Chen, Holst, Kirkwood, Sametoglu, Reid, Tsui, Recoules, and Ng, Phys. Rev. Lett. 110, 135001 (2013)].

Dynamic X-ray diffraction observation of shocked solid iron up to 170 GPa

Significance Iron is the main constituent of the core of rocky planets; therefore, understanding its phase diagram under extreme conditions is fundamental to model the planets’ evolution. Using dynamic compression by laser-driven shocks, pressure and temperature conditions close to what is found in these cores can be reached. However, it remains unclear whether phase boundaries determined at nanosecond timescales agree with static compression. Here we observed the presence of solid hexagonal close-packed iron at 170 GPa and 4,150 K, in a part of the iron phase diagram, where either a different solid structure or liquid iron has been proposed. This X-ray diffraction experiment confirms that laser compression is suitable for studying iron at conditions of deep planetary interiors difficult to achieve with static compression techniques. Investigation of the iron phase diagram under high pressure and temperature is crucial for the determination of the composition of the cores of rocky planets and for better understanding the generation of planetary magnetic fields. Here we present X-ray diffraction results from laser-driven shock-compressed single-crystal and polycrystalline iron, indicating the presence of solid hexagonal close-packed iron up to pressure of at least 170 GPa along the principal Hugoniot, corresponding to a temperature of 4,150 K. This is confirmed by the agreement between the pressure obtained from the measurement of the iron volume in the sample and the inferred shock strength from velocimetry deductions. Results presented in this study are of the first importance regarding pure Fe phase diagram probed under dynamic compression and can be applied to study conditions that are relevant to Earth and super-Earth cores.

Laser-driven quasi-isentropic compression experiments and numerical studies of the iron alpha-epsilon transition in the context of planetology

The iron alpha-epsilon transition is one of the most studied solid-solid phase transition. However, for quasi-isentropic compression, the dynamic and the influences of this transition on the high-pressure states of iron are still unknown. We present experimental results and numerical simulations to study these effects. Experiments performed at LULI2000 and the Janus laser facility (LNLL), using two different ramp shapes and different compression rates allowed to study the dynamic of the alpha-epsilon transition. We have observed the transition at particle velocity ranging from 0.25 km/s to 0.52 km/s depending on the compression rate. Depending on the ramp, either a shock formation was observed (high compression rate) at the transition or a flat plateau whose duration is function of compression rate. Increasing the compression rate leads to a smaller plateau duration. These results are important for reproducing Earth and Super-earth core conditions (2-15Mbar, 5- 15000K) on laboratory where the quasi-isentropic compression is the most promising experimental scheme.

Experiments and simulations on Al-Au mixtures and mixtures laws

We measured the thermodynamical and transport properties of aluminum-gold mixtures in the warm dense matter regime and for various concentrations. We compare these measurements with quantum molecular dynamics (QMD) simulations. We find that the calculated pressures and resistivities of both the mixtures and pure phases are in good agreement with the measurements. This further allows us to test the mixing rules usually employed to predict the properties of the mixed phases from the pure ones. We show, in this regime, that the partial densities mixing rule predicts the pressure of the mixture rather accurately but fails in its prediction of the optical conductivity. To improve this latter prediction, we find that we must invoke an isothermal-isobaric mixture rule to compute the pure phase contributions at the correct densities.