NCSA NEWS

This is the hydrogen scientists know best--the forms they have measured, modeled, and analyzed for decades. But there are other manifestations of hydrogen that, until recently, have eluded investigation. Place this simple element in extremes of temperature and pressure, and it will display a range of personalities that are surprising and profound.

One of these other personalities, which is being investigated by NCSA physicist and Alliance Executive Committee member David Ceperley, is metallic hydrogen. Squeeze hydrogen at pressures 2 to 10 million times greater than normally found on Earth and this colorless, odorless, tasteless gas transforms into a metal. Conditions like this exist inside Jovian planets like Jupiter, where cold clouds of hydrogen gas turn to liquid metallic hydrogen under the pressure exerted by this gaseous giant. Extremes of temperature and pressure can exist on Earth--when hydrogen is ignited within some rocket engines, or during thermonuculear fusion.

Researchers have known about the metallic state of hydrogen for more than 60 years--ever since quantum mechanics predicted the rules governing electrons. But they lacked the ability to make quantitative predictions of this state--the kind of knowledge that sheds light on the distribution of mass in Jovian-like objects and may lead to more efficient ways to achieve thermonuclear fusion. It is knowledge that determines hydrogen's equation of state, which Ceperley believes he is now "tantalizingly close" to providing.

Recent research by Ceperley, along with former graduate student William Magro and graduate student Burkhard Militzer, indicates that hydrogen's transition into this metallic state is abrupt. Just as water skips the slush phase when it freezes into ice, hydrogen appears to undergo a similarly dramatic first-order transition from a metallic molecular fluid into a metallic atomic fluid when it reaches 2 million atmospheres. The nature of this transition has been debated for more than 20 years. If follow-up studies uphold these findings, scientists will have a broader understanding of hydrogen's state at high densities. Armed with this information, they can better understand hydrogen's role in the evolution of stars and planets or they can manipulate conditions to produce exotic and potentially useful materials. They may even extrapolate to predict the behavior of other related elements.

"With this we can make analogies to other materials and understand what causes an electron to switch from a 'bound' state in which it is an insulator and the 'liberated' state that makes it a metal," says Ceperley. "If our research is correct, [there is a critical point after which] you increase the pressure just a tiny bit and suddenly the molecular bonds break and electrons are free."