Getting Warmer, but Still a Puzzle
It took half a century for the best minds in physics to come up with a reasonable explanation for superconductivity--the ability of electric currents to float through certain materials completely untouched, without an ounce of energy loss.
The theory that won the Nobel Prize for this feat in 1972 explained the strange phenomenon only at extremely cold temperatures. But that wasn’t a problem, because all superconductors known at the time only worked at extreme cold.
Then 10 years ago, an IBM scientist in Germany discovered something the theory said couldn’t possibly exist--a so-called high temperature superconductor, or HTSC.
It was as if life had been discovered at 1,000 degrees, said UCLA physicist Steve Kivelson. “If you found life [at that temperature], it would be totally different. So it’s something basically, fundamentally, new.”
Physicists reacted with the closest thing to complete chaos to hit that staid community in a long time. Although the phenomenon defied explanation, it also promised almost unlimited electric power at a reasonable price. So while experimentalists set off on searches for new superconducting materials that could be made into wires and cables and motors, the theorists were scratching their heads.
Ten years later, they are still searching for explanations. They are also more convinced than ever that the answer to the mystery of HTSC will rewrite the book on the physics of materials. “It really challenges our fundamental assumptions about how the world works,” said AT&T; Bell Labs physicist Robert Cava.
Almost certainly, a Nobel Prize awaits the physicist who cracks the puzzle. Moreover, a solution should point the way to practical applications for almost any technology that relies on electricity or magnetism--which is almost every technology.
For now, however, high temperature superconductors have left physicists scrambling to explain two seeming impossibilities: First, the new materials are ceramics, and ceramics weren’t supposed to carry electricity at all--much less without resistance. Second, no material was supposed to become superconducting at such “warm” temperatures (warm meaning hundreds of degrees Fahrenheit below zero--but still dozens of degrees hotter than traditional superconductors).
On the face of it, there is no way an electric current can sneak through a maze of atoms without the jostling that normally saps the current and dissipates the energy away as heat. But according to BCS theory (so named after its inventors John Bardeen, Leon Cooper and Robert Schrieffer), it can happen if a substance gets cold enough to freeze into a “super-ordered” state, just as cold-enough water freezes into ice.
In general, as matter gets colder, atoms become more orderly. More order means less random vibration, or heat. The electrons in superconductors are so ordered that all behave as one--like a line of Rockettes. Such precision choreography depends on electrons forming partnerships--called Cooper pairs (after Cooper). The pairs then schlump together in synchrony into a single atomic state--behaving as if they were one atom.
The seemingly insoluble problem was: How do electrons form pairs? Electrons are particles of negative electric charge and strongly repel each other; nowhere in nature had opposite electric charges merged into a single entity.
According to BCS theory, supercold electrons can overcome their mutual repulsion when vibrations in the material create a kind of pucker that pulls them together--like a wake behind the boat that sucks in unwary swimmers.
However, the theory cannot explain high temperature superconductivity. At such warm temperatures, physicists agree, there is too much random jostling about for pairs to form and stay together. There has to be some other, stronger way for the pairing to take place.
“At the end of the day, you have electron pairs,” Kivelson said. But no one agrees on how that happens.
Proposed theories are hotly contested and are all over the physics map. They encompass everything from slight modifications of the standard BCS theory to starting over from scratch.
Schrieffer is tackling the problem for the second time in his career, an opportunity he relishes. “It’s wonderful,” he said. “You have one child, and then you realize it’s not the end.”
Schrieffer thinks the old theory could work for the new materials with a few modifications. Grasping for ways to explain the elusive idea he has in mind, he carves the air with his hands, drawing air pictures of electron pairs created when their spins get stirred up, creating “a trail of wrong-pointing spins.”
Princeton physicist Phil Anderson, who also won a Nobel Prize in physics, is convinced that a fundamental new theory of particles is behind the strange phenomenon. He believes that the electrons split into two different particles--a “spinon” that carries the particle’s spin, and a “holon” that carries its charge. Spinons and holons comprise electrons the same way elementary particles called quarks comprise protons and neutrons.
Theorist Douglas Scalapino of UC Santa Barbara believes the electrons attract each other because they spin in opposite directions. Yet, he says, “some important piece of the puzzle is still missing. . . . It’s as if [the material] is a fabric that’s under some terrific tension, and we don’t understand what that tension is.”
Stanford’s Robert Laughlin sees the answer to the riddle of the universe in high temperature superconductors. Empty space, he says, “is a kind of superconductor that pervades the universe.”
One reason the theorists are stymied, says UCLA’s Kivelson, is in part because “we’ve been trying to solve this problem with pure thought.” Experiments have been notoriously unreliable. The new materials are brittle, fragile, inconsistent and easily contaminated.
Until the past year, Kivelson said, no two laboratories could be counted on to make the same ceramic stuff. “You couldn’t do the same experiments on the same material.” The confusion meant that theorists could interpret the results more or less any way they saw fit.
Plus, nothing like the new ceramics exists naturally on Earth. Therefore, little is known about their properties. “You don’t go out and find a piece of [one of the new ceramics] under a rock,” Kivelson said. “We don’t even understand their normal [non-superconducting] properties.”
Not helping matters is the fact that the field is bitterly contentious. “We’re like the blind men and the elephant,” Kivelson said, “but it’s worse because we’re not talking to each other so we don’t know what we’re looking at.”
Fortunately, the last year has seen a vast improvement in the quality and consistency of lab experiments. At long last, theorists believe that they are on the brink of progress. “There are some facts,” said Kivelson. “There is hope.’
Anderson is even more optimistic. “My feeling is, we’re getting into the home stretch.”
In the end, of course, experiments will decide. “In physics, all decisions are ultimately made by experiment,” said UC Berkeley physicist Marvin Cohen. “That’s a wonderful thing. In this field, you go out on a limb, and the experimentalists come after you with a saw.”
Chaos and Order
It took nearly 50 years for the best minds in physics to come up with a reasonable explanation for superconductivity- the ability of electric currents to float through certain materials completely untouched, without energy loss. Then, 10 years ago, a scientist in Germany discovered something the theory said couldn’t possibly exist-a so-called high temperature superconductor.
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Time Line
1911: Dutch physicist Heike Kamerlingh Onnes discovers superconductivity in mercury at 4.2 degrees above absolute zero, which is minus 460 degrees Fahrenheit.
1957: John Bardenn, Leon Cooper and Robert Schrieffer come up with a theory of how superconductivity works.
1972: Theory wins Nobel Prize in physics.
1973: Highest temperature for superconductors is stuck at 23.3 degrees above absolute zero; most theories predict that’s as warm as superconductors can get- too cold to be practical.
1986: Karl Alex Muller and Johannas Georg Bednorz of IBM Zurich publish a finding of a newceramic superconductor at 30 degrees above absolute zero.
1987: University of Houston’s Paul Chu achieves superconductivity at 90 degrees above absolute zero, making practical applications possible. The March meeting of the American Physical Society becomes the “Woodstock of Physics.”
1988: Highest critical temperature for superconductors climbs to 125 degrees above absolute zero.
1996: Highest temperature for material under pressure stands at 164 degrees above absolute zero. No theoretical explanation in sight.
