NCSA NEWS

Copper. Aluminum. Iron. These are the elements we typically think of as metals. Their atomic structures are orderly--often arrayed in compact lattices that are as uniform as the squares on a checkerboard. So close are these atoms that the successive energy levels at which the electrons orbit the nuclei become almost indistinguishable, with the outer high-energy bands forming into a conduction band in which electrons move easily throughout the crystal. This delocalized state of electrons is what imparts the conductivity we associate with a metal.

The propensity of all materials metallize when subjected to sufficient pressure was first recognized in the 1930s by Hungarian physicist Eugene P. Wigner. Using the newly penned Schrodinger equation, which governs the particle and wavelike behavior of subatomic matter, he calculated how pressure and volume conspire to push electrons into a delocalized state. His predictions of the pressure required for this transition, though, were off by 10-fold. The fault laid not in the basic ideas but in the overly simplistic numerical methods then available to him. At that time, computer were nothing more than glorified adding machines. Laboratory experiments for replicating these extreme conditions, too, were decades away.

Recent gains in computing power and algorithms, coupled with new experimental methods, are opening to study this once inaccessible physical realm. Scientists knowledge of the pressures necessary for hydrogen's transition to a metal, for example, has grown steadily since Wigner's day. Now, physicists are zeroing in on how this transition occurs. They know that when they compress hydrogen, its electrons abandon their usual spherical orbits and lock into a herringbone lattice, with the alternating rows pointing in different directions. Greater compression is thought to force the electrons into a simple metallic lattice. What Ceperley and Militzer believe they will soon answer is what happens when both density and temperature are increased--a few thousand degrees or higher. "We are studying what happens in the liquid state," says Ceperley. "We're interested in those temperatures and pressures at which it is a real metal--the molecules are gone and you have just hydrogen atoms and free electrons."