Dr. Alexander Goncharov [Geophysical Laboratory, CIW, USA]

Title: Optical Properties & Conductivity of Planetary Material at Extreme Pressure-Temperature Conditions

Time: 2:00-3:00 PM, July 30, 2015, Thursday

Place: Conference room 201, Build 6, HPSTAR (Shanghai)

Host: Dr. Xiao-Jia Chen

Abstract:

Understanding the transformation of hydrogen from an insulating to a metallic state is a cornerstone of condensed matter physics and critically important in natural systems of gas giant planets. Likewise, the noble gases are elements of broad importance across science and technology, and are primary constituents of planetary and stellar atmospheres, where they segregate into droplets or layers that affect the thermal, chemical, and structural evolution of their host body. Using a novel experimental technique involving rapid heating and in situ laser and emission spectroscopy, we study the optical properties of hydrogen at high pressure and temperature as it undergoes this transition up to 150 GPa, and dense helium, neon, argon, and xenon at 4,000 to 15,000 K and pressures of 15-52 GPa. The data for hydrogen evince a transformation from transparent insulator to opaque conductor with a threshold temperature that decreases with increasing pressure. We observe spectral behavior distinct from a metal, ruling out predictions of an abrupt first-order phase transition from insulator to metal state in fluid hydrogen at these pressures, instead finding an intermediate semiconducting or semimetal state that suggests a broad transition to metallic conditions. A Smith-Drude optical model is found to describe well the present measurements and prior theory on warm dense hydrogen, providing a simple framework to examine the electronic and optical properties of hydrogen at extremes.  In the noble gases, the thermal activation and frequency dependence of conduction reveal an optical character dominated by electrons of low mobility, as in an amorphous semiconductor or poor metal, rather than free electrons as is often assumed for such wide band gap insulators at high temperatures. White dwarf stars and giant planets having helium outer atmospheres cool slower and may have different color than if atmospheric opacity were controlled by free electrons. Helium rain in Jupiter and Saturn becomes conducting at conditions well correlated with increased solubility in metallic hydrogen, while a deep layer of insulating neon may inhibit core erosion in Saturn.

Biography of the Speaker:

Dr. Alexander F. Goncharov was awarded combined BSc and MSc degrees in physics from Moscow Institute of Physics and Technology, Russia in 1979. After that, he moved to Institute for Spectroscopy, Russian Academy of Science and obtained a PhD in Physics in 1983. He is working in Institute of Solid State Physics of Chinese Academy of Sciences. He is also a senior staff scientist of Geophysical Laboratory of the Carnegie Institution for Science since 2003. At the Carnegie, he pioneered a suit of spectroscopy measurements in diamond anvil cells at very high pressures in excess of 200 GPa (2 million atmospheres) at high temperatures above 6000 K. He had research positions in Lawrence Livermore National Laboratory of USA, Max-Planck-Institute für Festkörperforschung of Germany and Institute of Crystallography, Russian Academy of Sciences of Russia. He is recognized expert in a multitude of experimental techniques in diamond anvil cells, including optical and synchrotron spectroscopy, and thermal properties at high-temperature and high-pressure extreme conditions. His research interests cover condensed-matter physics, simple molecular solids, the chemistry and physics of the Earth’s mantle and core, and high-pressure materials science. He has published nearly 200 papers in journals like Nature, Science, and Physical Review Letters with over 5000 citations.