When a new family of high-temperature superconductors was discovered nearly two years ago, theorists immediately began scrawling equations in notebooks and consuming vast amounts of computer time trying to explain how these materials could conduct electricity without any losses due to resistance.
These copper- and oxygen-based ceramic superconductors promise a host of commercial applications, ranging from smaller, faster computers to high-speed, magnetically levitated trains. Their chief advantage is that they can be cooled with liquid nitrogen, rather than liquid helium, which has to be cooled to a much lower temperature and is therefore much more expensive to use.
It had immediately seemed clear that these new superconductors represented a departure from the conventional “BCS” theory of superconductivity in metals, which had won its creators--John Bardeen, Leon Cooper and Robert Schrieffer--a Nobel prize in 1972. A second Nobel for theory--for ceramics--seemed a definite possibility.
“There’s no question that a race is under way” to develop a new theory, said physicist Thomas F. George of the State University of New York at Buffalo.
But even more important than a prize is the effect such a theory would have on research. Experimental chemists and physicists have been floundering in their laboratories, putting together literally tens of thousands of ceramic compounds by a process of trial and error in a search for materials that are superconducting at still higher temperatures.
A comprehensive theory of superconductivity, both experimenters and theorists believe, would impose order on that search by telling researchers what kinds of materials and structures were most promising.
Theorists have produced theories, lots of theories.
“For every theorist, there is at least one theory,” said Caltech theorist William A. Goddard III.
Last week, five more theorists, Goddard among them, presented their ideas at the American Chemical Society meeting in Los Angeles. Goddard’s theory has received the most attention, in part because it is the only one that is calculated from elementary laws of physics, rather than by tweaking the BCS theory. But Goddard also received attention because his theory predicts that the new materials could never become superconducting at room temperature, as researchers had fervently hoped.
Instead, he said, the maximum superconducting temperature of the copper oxide materials is about minus 54 degrees Fahrenheit, just above the temperature of dry ice. Many of the other theories do not predict a maximum temperature, leaving open the possibility of room temperature superconductors.
Goddard’s theory also predicts that one type of device made from superconductors, called a Josephson junction and potentially useful in supercomputers and other electronic devices, will not work as well when made from the new superconductors as when made from existing superconductors.
The goal of this and other new theories is to explain the dance of the electrons, which carry the electric current.
In a normal conductor, each electron is like a solo dancer in the middle of a vast floor of other dancers, doing its own thing and bumping into other dancers. For the electrons, some electrical current is converted into heat with each bump.
But in a superconductor, electrons pair up like ballroom dancers, and the movement of each pair is identical to that of the millions of other pairs.
The problem is, electrons have negative charges and thus resist being paired. To explain how electrons in superconductors overcome this mutual repulsion, Goddard and other theorists have postulated a veritable “zoo” of fundamental particles called magnons, phonons, excitons and other exotic names that could bind the electrons together.
The problem, said George, who organized the superconductor symposium last week, is that it is very difficult to choose from among the competing theories. One advantage of Goddard’s theory is that it makes some very specific predictions that can be tested experimentally.
“Unless a theory can predict something that wasn’t known before, it’s not very useful,” Goddard said.