Resistance becomes lower under pressure

The property of matter often changes drastically as external knobs such as temperature (goes lower) and pressure (gets raised) are tuned (http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.108.026403).

Wustite (FeO) is a prototype for the iron-bearing minerals found in the Earth. Though FeO is insulating at ambient conditions, in the late 1980s researchers observed it undergo a transition to a metallic state when compressed by shock waves. The nature of this transition has, however, been unclear.

In a paper in Physical Review Letters, Kenji Ohta of Osaka University, Japan, and colleagues report their combined theoretical and experimental attack on the problem. The research team measured high-temperature resistivity and structural x-ray diffraction patterns of FeO in a diamond anvil cell to simulate conditions in Earth’s mantle. At a temperature of 1900 kelvin and pressure of 70 gigapascals, Ohta et al. were able to watch as FeO in a rocksalt atomic structure became metallic without any structural changes.

To understand these findings, Ohta et al. performed density-functional calculations of electrical conductivity as a function of temperature and pressure. The results suggest that their observations are consistent with a new kind of insulator-metal transition involving fluctuations between a high-spin state to a low-spin state in the FeO. For geophysicists, this makes the picture of conductivity deep in the Earth richer: both insulating and metallic phases must be added to the phase diagram, with potential implications for thermal and electrical conductivity, and in turn models of the planetary magnetic field. –David Voss