Organic Crystals Coaxed to Conduct Quickly

MINNEAPOLIS--A new organic crystal has a surprising ability to perform a weird quantum mechanical trick, researchers reported on 22 March at the annual meeting of the American Physical Society. The finding is encouraging news for those who hope to use organic materials to create semiconductors, material used to make the chips that power electronic devices.

In recent years, researchers have tried to concoct organic semiconductors, materials that should be cheaper, lighter, and more flexible than current silicon models. But they've hit a speed bump--organic materials transmit electric charges much more slowly than do inorganic materials. One otherwise promising material, pentacene, passes a charge along at about one-thousandth the speed of pure silicon crystals--at least at room temperatures.

But semiconductors work their very best at very low temperatures, so a team led by Bertram Batlogg of Lucent Technologies' Bell Labs in Murray Hill, New Jersey, tested pentacene to see how it would perform near absolute zero. They fashioned a single pentacene crystal into a slab just a few millionths of a meter thick. The team placed the slab in a magnetic field and ran a current through it lengthwise. The field shoved the electrons sideways, bumping them to the edges of the slab. This created a measurable voltage, called the Hall voltage, across the slab's width.

The researchers expected the voltage to increase proportionally with the magnetic field. But when the crystal was cooled to within 2°C of absolute zero, the voltage climbed in a complicated, jerky pattern of steps--the hallmark of the rarefractional quantum Hall effect. The effect arises when positive electrons form small gangs that behave like single particles with a total charge that is some fraction of an electron's charge. But this only happens if the electrons move freely and quickly through the material. Indeed, the researchers estimate that pentacene ushers charges around 10 times faster than crystals of pure silicon.

No one expected humble pentacene to perform this quantum mechanical trick, says Horst Stormer, a physicist at Columbia University who shared the 1998 Nobel Prize for discovering the fractional quantum Hall effect. "If it had been April 1st," he says, "I wouldn't have believed it."