Metals and Insulators

Basic physics: electricity is electrons going from one place to another. "Flowing" is the metaphor often used, and it happens because a material that conducts electricity -- a metal-- has loose electrons, a sea of them, so to speak. When you plug in the copper wire of your TV, microwave or toaster, the loose electrons in copper form a "current."

Why doesn't the current flow into your hand and let you experience an electric "shock"? Basic physics again: the hard plastic encasing the prongs of the plug and the rubbery plastic wrapped around the wire are insulators -- a material in which all the electrons are tightly bound to the atoms, not free to roam and make electricity. This fundamental difference is one of the basic properties that characterize a material. A metal, generally speaking, can't change to an insulator and vice-versa, which is one reason why metal-ammonia solutions are so interesting.

"What is it," asks Michael Klein, "that takes a system from being an insulator, in which it is a poor conductor of electrical current, to a metallic state, which is characterized, among other things, as having high electrical conductivity?" Klein directs the Laboratory for Research on the Structure of Matter at the University of Pennsylvania. Among several projects he is pursuing at the Pittsburgh Supercomputing Center, he and colleagues Glenn Martyna and Zhihong Deng have done calculations investigating the properties of metal-ammonia solutions. Their results provide, for the first time, a detailed picture of the transition between insulating and metallic states in these fascinating solutions.

Electron Density in Cesium-Ammonia Solutions

These three images show the results of modeling cesium in ammonia at concentrations of approximately 0.5 percent, 2 percent and 9 percent. Each "simulation cell" contains ammonia molecules (orange & white stick figures) cesium ions (pink balls) and electron density shown as violet-gray contours. The outermost contour contains 95 percent of the electron density. The first cell contains 512 ammonia molecules and two cesium atoms, the second 256 ammonias and four cesiums, and the third 256 ammonias and 24 cesiums.

Researcher: Michael L. Klein, University of Pennsylvania.
Hardware: CRAY Y-MP C90
Software: User developed code.
Keywords: electronic states, quantum chemistry, metal-ammonia solutions, Car-Parinello, density-functional theory, metal-insulator transition, cesium-ammonia solutions, materials science, structure of matter.

Related Material on the Web:
Projects in Scientific Computing, PSC's annual research report.

References, Acknowledgements & Credits