30 July 2008

Princeton scientists discover new electronic states of matter

Contact: This email address is being protected from spambots. You need JavaScript enabled to view it.

TALLAHASSEE, Fla. — Using very high magnetic fields, a team of scientists has discovered a new way that electrons behave in materials – a discovery that could lead to new kinds of electronic devices.

Writing in the July 25 issue of the journal Science, a team led by Princeton Professor N. Phuan Ong has shown that electrons in the common element bismuth display a highly unusual pattern of behavior – a dance of sorts – when subjected to a powerful magnetic field at ultra-low temperatures.

Ong and his collaborators performed the experiment at the National High Magnetic Field Laboratory's DC Field Facility.

Normally, electrons in bismuth come in three different varieties. But in the experiment described by the researchers, the electrons in the magnetized, supercold sample simultaneously assumed the identity of all three classes of electrons, following a strict choreography that could only stem, they say, from the strange rules of quantum physics. Quantum mechanics is the area of physics that governs the behavior of objects in the microscopic world. The experiment documented the first "phase transition" – a term used to describe an abrupt change in the behavior of a material – ever observed in a Group V element, one of the categories in the periodic table.

"If you can imagine, it's as if we were looking at passengers scrambling through Grand Central Station in New York, watching them run in different directions. All of a sudden, the whistle blows and we see them run to the same train. This is a simple example of a sudden transition to collective behavior," Ong said.

By witnessing what physicists call a "collective state," the team saw what Ong described as one of the wonders of nature. "It's a manifestation of quantum mechanics," he said.

It had been known that, in the complicated environment of a crystalline solid such as bismuth, its electrons move more rapidly than they do in conventional materials. Although the maximum speed of electrons in bismuth is small compared with photons moving at the speed of light, the electrons mimic accurately the behavior of elementary particles accelerated to very high speeds. In bismuth, this "relativistic" property makes them likely candidates for the quantum behavior the scientists observed.

"This is exciting because this was predicted, but never shown before, and it may eventually lead to new paradigms in computing and electronics," said Thomas Rieker, program director for materials research centers at the National Science Foundation.

If scientists are able to document the behavior of the electrons in bismuth and therefore predict their path through a material, they may be able to manipulate those properties for electronic systems powering futuristic "quantum" computing devices.

"In the quest to develop ever smaller and faster transistors, physicists and engineers are attempting to harness the quantum behavior of electrons," Ong said. "Research in bismuth and another material, graphene, may uncover further new results that will expand the tool kit of quantum researchers."

Electrons are the lightest elementary particles with an electric charge. In the past, understanding the rules governing the way electrons move through materials has allowed scientists to make major advances, from the development of medical imaging to the invention of the transistor.

"The modern era of computing and telecommunications rests on advances in solid state physics," Ong said. "We can't yet know what we will learn from this but the past tells us that understanding the behavior of electrons points us in important new directions." 

The experiment also involved Mag Lab affiliate and University of Florida Professor Art Hebard; and Robert Cava, the Russell Wellman Moore Professor of Chemistry and department chair, physics graduate students Lu Li and Joseph Checkelsky and postdoctoral fellow Yew San Hor, all of Princeton. Scientists from the University of Michigan also participated.

To obtain the results, the scientists balanced a crystal of high-purity bismuth at the tip of a tiny gold cantilever and measured the minute flexing of the cantilever as the magnetic field changed.
The research was supported by the National Science Foundation through a Materials Science and Engineering Center grant to the Princeton Center for Complex Materials.

This news was adapted and published with the permission of its author, Kitta MacPherson, and the Princeton University Office of Communications.