Once more, scientists have ventured into the frontier of very cold matter. At a press conference today, physicists announced that they had created a new state of matter known as a Fermi condensate. This discovery provides an exciting new way of understanding the mysterious physics of high-temperature superconductors.
Last November, two teams of researchers announced that they had created a type of matter known as a Bose-Einstein condensate (BEC)--a cluster of particles that acts like a single, enormous, quantum-mechanical object--out of a class of particles known as fermions (ScienceNOW, 13 November, 2003). A fermion, by nature, can't act the same way as its neighbor, so it was a major achievement to get fermions to pair up, condense into a BEC, and march in quantum-mechanical lockstep.
Now one of the two groups, led by Deborah Jin of the Joint Institute for Laboratory Astrophysics (JILA) in Boulder, Colorado, has taken the next step. Instead of using magnetic fields to bind the fermions relatively tightly together into "molecules"--the necessary condition for a fermionic BEC-the researchers use the fields to make the fermions slightly repulsive to each other. Even though the fermions can't bind, they still pair up in a sense, each partner affecting its counterpart's motion. That means the fermions can condense. This quantum-mechanical condensation is more like what happens in a superconductor, where repulsive electrons form "Cooper pairs," than what happens in a BEC. Although the two areas are related, BECs and superconductors are guided by different theories, and the crossover is murky. "It's a very new regime," says Jin. "The theory isn't clear."
Eric Cornell, a physicist who works in another research group at JILA, says that for many years, scientists thought that a condensate was either BEC-like or purely superconductor-like. "There was no compromise," he says, adding that Jin's work proves this idea false. Indeed, Jin's preliminary data show that there's a smooth transition between the BEC and superconducting regimes. Jin adds that the work may illuminate the physics of high-temperature superconductors, although practical applications are quite a distance away. In any case, says Cornell, the experiment is "a technological and scientific tour de force."
Jin's lab site