NEWS
 
Carbon under extreme conditions: Phase boundaries and electronic properties from first-principles theory
A. Correa, S. A. Bonev and G. Galli

At high pressure and temperature, the phase diagram of elemental carbon is poorly known. We present predictions of diamond and BC8 melting lines and their phase boundary in the solid phase, as obtained from first-principles calculations. Maxima are found in both melting lines, with a triple point located at ~850 GPa and ~7,400 K. Our results show that hot, compressed diamond is a semiconductor that undergoes metalization upon melting. In contrast, in the stability range of BC8, an insulator to metal transition is likely to occur in the solid phase. Close to the diamond/liquid and BC8/liquid boundaries, molten carbon is a low-coordinated metal retaining some covalent character in its bonding up to extreme pressures. Our results provide constraints on the carbon equation of state, which is of critical importance for devising models of Neptune, Uranus, and white dwarf stars, as well as of extrasolar carbon-rich planets.

Broadband Dielectric Function of Nonequilibrium Warm Dense Gold
Y. Ping, D. Hanson, I. Koslow, T. Ogitsu, D. Prendergast, E. Schwegler, G. Collins, and A. Ng

We report on the first single-state measurement of the broadband (450–800 nm) dielectric function of gold isochorically heated by a femtosecond laser pulse to energy densities of 106-107 J/kg. A Drude and an interband component are clearly seen in the imaginary part of the dielectric function. The Drude component increases with energy density while the interband component shows both enhancement and redshift. This is in strong disagreement with predictions of a recent calculation of dielectric function based on limited Brillouin zone sampling.

UCRL-WEB-224160