Ronald E. Cohen — Page 2

arXiv1996

Prediction of Phase Transition in CaSiO$_3$ Perovskite and Implications for Lower Mantle Structure

First principles linear response calculations are used to investigate the lattice dynamics of what is thought to be the third most abundant phase in the lower mantle, CaSiO_3 perovskite. The commonly assumed cubic structure (Pm3m) is found to be dynamically unstable at all pressures, exhibiting unstable modes along the Brillouin zone edge from the M-point to the R-point. Based on these results, we predict that the ground state structure of CaSiO_3 perovskite is a distorted phase with lower than cubic symmetry. We predict that a phase transition occurs in CaSiO_3 perovskite within the earth's lower mantle from the low temperature distorted phase to the cubic phase at high temperature. The predicted phase transition provides a possible explanation of some of the seismological observations of reflective features within the lower mantle.

physics.geo-ph↗

arXiv1995

Insulator-metal transition in solid hydrogen: Implication of electronic structure calculations for recent experiments

Electronic structure calculations for compressed molecular hydrogen are performed to provide more insight into the diversity of phenomena recently observed experimentally. We perform full-potential LAPW calculations and analyze them in terms of a molecular tight-binding model. We show that $σ_g$ and $σ_u$ bands overlap occurs at rather low pressure, while an insulating state persists at specific orientations up to 200 GPa, in accord with previous work, and is due to opening of hybridization gaps at the Fermi level. We also present calculations of electronic properties such as plasma frequencies and electron-vibron coupling.

mtrl-th↗