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In short, says Douglas Scalapino, of the University of California at Santa Barbara, recent developments are something like the breaking of the four- minute mile. Beforehand, it had been considered nearly impossible; afterward, "you could go to any track meet and some guy was breaking it." The activity, says Cava, "is more exciting than a supernova. Astrophysicists can watch it, but when it happens, it happens and it's gone. In superconductivity, the events are still going on, and the physics is just beginning to pour in."
So are the scientific papers. Says Metallurgist Frank Fradin, director of Argonne's materials science division, who is also an associate editor of Physical Review Letters: "As of three weeks ago, we had 98 papers submitted on the subject, and only a small fraction of them will ever get published. Progress is so rapid that a result of two to three weeks ago is already out of date. We've had to institute a whole new system to speed up the publication process." One important discovery: at least a dozen different compounds, all subtly different from the one Chu found, appear to act as high-temperature superconductors.
While scientists know the chemical composition of the new class of superconductors, they are less certain about how they work. True, a theory exists that explains low-temperature superconductivity. It is known as BCS, from the initials of Author John Bardeen and his colleagues Leon Cooper and Robert Schrieffer, who shared the 1972 Nobel Prize for Physics for their effort. But BCS may not apply to the strange goings-on at higher temperatures.
Ordinary conductivity, the measure of a material's ability to transmit electrical current, is determined by events that take place at the atomic level. Atoms consist of a tiny dense nucleus that contains positively charged protons and chargeless neutrons. Around the nucleus whirl the negatively charged electrons, residing in shells with shapes determined by the electrons' energy levels.
In many atoms, particularly those of metallic conductors, the outer shell has a number of empty slots, and the electrons that it does contain are not bound as tightly to it as those in the inner shells. Just as the sun's gravitational pull is weaker on distant Pluto than on nearby Mercury, the hold of an atomic nucleus is also weaker on electrons in the outermost layers.
So when an electric current -- which is simply a stream of moving electrons -- flows in a conductor, electrons move from empty slot to empty slot in the outer shells of the atoms. A material like rubber, on the other hand, is an insulator: it consists largely of atoms with completely filled, stable outer shells. Thus when voltage is applied, electrons have no empty slots to move into, and no current flows.
But even the best of ordinary conductors have some resistance to the flow of electrical current. The reason: as current passes through, some of the electrons collide with other electrons, thus dissipating their energy in the form of heat. According to the BCS theory, these collisions are avoided in superconductivity. "What causes a material to become superconducting is a phase change,"* explains Bardeen, now a professor emeritus at the University of Illinois. "You can think of it as electrons condensing into a new state." That state involves the pairing of electrons and a kind of group discipline.