Materials Physics in Giant Planetary Interiors
Prof. Dr. Lars Stixrude, UCL Department of Earth Sciences, London´s Global University

Most planets are so large that their characteristic pressure (~1 Gbar) exceeds by orders of magnitude current experimental capability.  The behavior of materials in this regime is poorly understood, but likely to be rich, with important implications for our understanding of planetary formation and evolution.  The discovery of exo-planets and the development of high energy density experiments motivate a closer look.  We have been using density functional theory, combined with a variety of methods from statistical mechanics, such as molecular dynamics and lattice dynamics, to study materials in this extraordinary regime.  I will explore two systems up to ~1 Gbar and ~50 eV: Helium and Iron, focusing on the phenomena of temperature-induced metallization in the fluid state, and polymorphic phase transformations induced by changes in Fermi surface topology in the solid state.  As the second most abundant chemical element in the universe, helium makes up a large fraction of giant planets, including Jupiter and Saturn, and most extra-solar planets discovered to date.  We find that fluid helium undergoes temperature-induced metallization at high pressure: the band gap closes at 20,000 K at a density half that of zero-temperature metallization, resulting in electrical conductivities greater than the minimum metallic value.  Gap closure is achieved via increased structural disorder in the liquid with weakening of structural scattering and the pseudo-gap.  The change in electronic structure in Helium may have important implications for the miscibility of helium in hydrogen and for understanding the thermal histories of giant planets.  Iron is likely to be an abundant constituent of most planets because of its cosmic abundance and high melting point.  While current experiment and theory show a single high pressure phase: hexagonal close-packed up to a few Mbar, we find a sequence of phase transformations at much higher pressures: to fcc and then to bcc, that can be linked directly with electronic transitions: s to d transfer and 3p-3d hybridization.  The participation of core electrons in bonding and stabilization of a new phase is intriguing and suggests that this phenomenon may be widespread at high pressure.