CEMS Faculty

Research in my group is devoted to quantum mechanical studies of materials. The first principles methodology used is based on Density Functional Theory (DFT) and Pseudopotentials (PP). DFT-PP based methods do not rely on any ``a priori'' information about the material, except the fundamental constants of nature and the atomic numbers of the elements. Yet, they are predictive beyond the current limits of many kinds of experiments. This is particularly true of matter subjected to extremely high pressures and/or high temperatures (P,T). First principles molecular dynamics (MD) and lattice dynamics (LD) are powerful complementary methods we use to address thermodynamic properties of matter at the extreme conditions encountered throughout the solar system, such as those in planetary interiors and surfaces.

Our major effort is dedicated to the understanding of the thermo-chemical state of the Earth’s mantle, traditionally divided into upper mantle (UM) (down to 410 km depth), transition zone (TZ) (from 410 to 670 km), and lower mantle (LM) (from 670 to 2898 km). Although there is a consensus today regarding the mineralogy of the UM, also but less so of the TZ’s, no samples are available from the LM. This region is remotely probed by seismic tomography which offers 3D maps of seismic velocities and density. From these the elastic properties of the mantle can be extracted. Interpretation of this information in terms of temperature field and mineralogy depends entirely on the comparison of this information with independently determined elasticity of candidate mineral phases and their aggregates (rocks). We are pursuing high P,T elasticity calculations of possible LM aggregates to help unravel the thermo-chemical state of this region.

Hydrogen is the most abundant element in the universe and oxygen the most abundant element in the terrestrial planets. Not surprisingly solid water is one of the most abundant solids in the solar system. The P,T regime throughout the solar system invites a wealth of solid structures, some ordered, some only partially ordered, others fully disordered. To date more than twelve crystalline phases have been identified to the Mbar pressure regime, not all being fully ordered or stable. The stability field of these phases is not well characterized, the reason being the large amplitude of the hysteresis loops. Two other related phenomena are the negative Clapeyron slope for the melting transition of ice Ih, the low P form of ice, and its pressure induced amorphization. This rich phenomenology is apparently unmatched by other solids. We are currently investigating pressure induced amorphization in ice and some structural systematics in P,T space. Pressure induced amorphization was discovered in ice Ih under pressure, but it has been observed in several materials, some of which we have investigated in the past (silica and BAs).

Last, but not least, we are interested in investigating the magnetic state of materials, now at low T’s. Magnetism, to a great extent, is a phenomenon still better investigated experimentally. It is a low energy phenomenon that demands high accuracy from first principles methods. Nevertheless experiments can derive many insights and guidance from first principes theory in search of new effects. For instance, there are intriguing magneto-structural effects and we are interested in their exploration by using pressure. However, our main interest is the investigation of conductive materials where there exists a strong relationship between magnetism and transport. We are particularly interested in potentially novel half metal systems, such as CoS₂. This research is inspired by and assists the experimental effort on magnetic hetero-structures at the University of Minnesota.