6 research outputs found
Carbon ionization at Gbar pressures: an ab initio perspective on astrophysical high-density plasmas
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 of experiments. Here, we propose a
novel ab initio approach to calculate the ionization degree directly from the
dynamic electrical conductivity using the Thomas-Reiche-Kuhn sum rule. This
Density Functional Theory framework captures genuinely the condensed matter
nature and quantum effects typical for strongly-correlated plasmas. We
demonstrate this new capability for carbon and hydrocarbon, which most notably
serve as ablator materials in inertial confinement fusion experiments aiming at
recreating stellar conditions. We find a significantly higher carbon ionization
degree than predicted by commonly used models, yet validating the qualitative
behavior of the average atom model Purgatorio. Additionally, we find the carbon
ionization state to remain unchanged in the environment of fully-ionized
hydrogen. Our results will not only serve as benchmark for traditional models,
but more importantly provide an experimentally accessible quantity in the form
of the electrical conductivity.Comment: accepted for publication in Physical Review Researc
The Speed of Sound in Methane under Conditions of the Thermal Boundary Layer of Uranus
We present the first direct observations of acoustic waves in warm dense
matter. We analyze wavenumber- and energy-resolved X-ray spectra taken from
warm dense methane created by laser-heating a cryogenic liquid jet. X-ray
diffraction and inelastic free electron scattering yield sample conditions of
0.30.1 eV and 0.80.1 g/cm, corresponding to a pressure of
13 GPa and matching the conditions predicted in the thermal boundary
layer between the inner and outer envelope of Uranus. Inelastic X-ray
scattering was used to observe the collective oscillations of the ions. With a
highly improved energy resolution of 50 meV, we could clearly distinguish
the Brillouin peaks from the quasi-elastic Rayleigh feature. Data at different
wavenumbers were used to obtain a sound speed of 5.90.5 km/s, which
enabled us to validate the use of Birch's law in this new parameter regime.Comment: 7 pages, 4 figures with supplementary informatio