Call Them Irresistible
By
Tim Folger
Superconductors, physicists say, will someday change the world. Before then it would be nice if somebody, somewhere, understood how they work.
Walk into a typical physics lab and you'll confront a mad profusion of electrical cables, lasers, computers, vacuum pumps, and who knows what else. Enter Robert Cava's lab, though, and the first thing you'll see is a six-inch-long dart with green plastic fins stuck into a chart tacked onto the side of a locker. A closer look reveals the chart to be that familiar chemistry-class prop, the periodic table of the elements. The dart has pierced a little square
slightly to the right of center, the one belonging to element 29--copper.
Throwing darts at the chart, Cava explains, is how he chooses the raw ingredients for his experiments.
He is joking, but he's not straying all that far from the truth. Cava, a materials scientist at AT&T Bell Laboratories' sprawling research complex in Murray Hill, New Jersey, studies superconductors. Unlike all other substances known to us, superconductors have the seemingly magical ability to conduct electricity without resistance. Physicists have hooked up batteries to superconductors, then removed the batteries from the circuit and watched the current continue to flow as if the batteries were still in place--in one case for as long as four years, at which point the physicists simply tired of the experiment. Prolonging it would not have been advisable. Calculations suggest that superconducting currents, like some Energizer Bunny from hell, can persist for millions of times longer than the universe has thus far existed.
Such extraordinary materials, naturally, could make for an extraordinary material life. Our electricity could flow through superconducting power lines without any loss of energy. Levitating high- speed trains could float on powerful superconductor-generated magnetic fields. Computers, medical scanners, motors, and generators with superconducting components could be made shockingly smaller and more powerful. The potentially immense practical payoff of these resistance-free materials is one reason so many physicists, like Cava, can't resist studying them these days.
Just a few years ago the field was populated by a mere handful of researchers. Certainly others were aware of superconductivity, but the phenomenon had been observed only at temperatures near absolute zero, the point at which atoms themselves nearly freeze in place. In 1987, however, physicists discovered materials that became superconductors at dramatically higher temperatures. Suddenly it seemed possible that this arcane laboratory
phenomenon might break out into the real world. Superconductors were front-page news.
There haven't been many headlines since. It's not that the new superconductors were a bust exactly, but they have turned out to be more problematic than anyone had anticipated. Even today, as some important practical applications are at last beginning to emerge from the lab, champions of the technology are more cautious than they were eight years ago. As it turns out, the new superconductors are odd, brittle compounds, and shaping them into useful products--like a wire--has proved an enormous challenge. But even more fundamental than the practical problem is the theoretical one: no one really understands how superconductors work. Physicists cannot agree on a theory that explains the mysterious phenomenon of superconductivity, let alone specify how an experimentalist might search for new superconducting compounds. And this takes us back to Cava's little joke.
"Our job is to find new superconducting materials," says Cava. "How do we figure out what to do? This is our answer"-- he points to the dart. "There is no real theoretical basis for understanding what's going to happen when we mix different things together. There are no rules at all that direct you. Only your own experience. So in the end, 99 percent of what we try doesn't do what we want."
Cava's predicament typifies the murky state of superconductor research today. It's a contentious field, with theorists falling for the most part into two surprisingly hostile camps, both convinced, like religious zealots, that the truth is theirs. Although to an outsider their debates sometimes recall the hairsplitting disputes of medieval scholastics, many physicists consider superconductivity the problem of their field.
"It is one of the deepest and most fundamental problems in physics," says Philip Anderson, a Nobel laureate physicist at Princeton and one of the leading theorists on superconductivity. "Perhaps it doesn't have the glamour or general interest that cosmology has, or quantum gravity, but in terms of fundamentally understanding how the world works, this is very deep and very difficult."
It wasn't always like this. At one time physicists thought they had superconductors all figured out.
For most of this century the study of superconductivity was an unhurried affair, carried out in the quiet backwaters of physics. The very first superconductor was discovered in the Netherlands, in 1911, by Heike Kamerlingh Onnes, a physicist interested in the properties of materials at extremely low temperatures. At the time he was the only physicist in the
world who could liquefy helium, and the only one able to closely approach absolute zero. He used his exclusive supply of liquid helium to chill mercury down to about -452 degrees Fahrenheit, or 7 degrees above absolute zero. One of the properties he then measured was the metal's electrical resistance.
Kamerlingh Onnes found that as he lowered the temperature, mercury's resistance decreased steadily until, at about 7 degrees above absolute zero, it suddenly plunged and vanished entirely. He was at a loss to explain what he had seen, but not for a name: supergeleiding, or "superconductivity."
Over the next couple of decades Kamerlingh Onnes and other physicists observed the phenomenon in many metals besides mercury. In all cases, it occurred only at temperatures barely above absolute zero. But because liquid helium, the required coolant, was expensive and difficult to handle, superconductors remained a laboratory curiosity--strange and fascinating, but of no practical importance. On the theoretical side, also, superconductivity
remained tantalizing but elusive. More than 40 years would pass after its discovery before a trio of physicists would come up with what seemed to be a definitive explanation of how it worked.
The three theorists--John Bardeen, Leon Cooper, and Robert Schrieffer--won a Nobel Prize in 1972 for what is now known as the BCS model of superconductivity. (Bardeen also shared an earlier Nobel for his role in inventing the transistor.) Their theory, published in March 1957, when all three were at the University of Illinois, essentially described the behavior of electrons in metals at very low temperatures.
The atoms in all metals are arranged in stacks of orderly, Levittown-like arrays. Unlike the electrons around an isolated atom in free space, the electrons in a crystal lattice of a metal, like copper, no longer belong to one atom. Instead the electrons become communal property, forming a sort of sea that engulfs the metal lattice. At normal temperatures heat from the environment keeps this atomic lattice constantly vibrating. When a battery, for example, starts a current flowing in the electron sea of a conventional conductor, the electrons in the current encounter resistance as they are slowed or deflected by the electric fields of the atoms in the vibrating lattice. Physicists knew that very low temperatures would quench these thermal vibrations, but they couldn't see exactly how that would lead to superconductivity.
What Bardeen, Cooper, and Schrieffer proposed was that a two-step process underlies superconductivity. First, at low enough temperatures the disordered electron sea starts to break apart as the electrons team up in loosely bound pairs. This pairing at first seems to defy reason. Electrons, after all, are negatively charged and should repel one another. But a more subtle process comes into play when things get very cold.
Picture a single electron traveling through a metal lattice at low temperatures. When it passes between two neighboring atoms in the lattice, the BCS model says, the negatively charged electron distorts the lattice slightly; the positively charged atoms of the lattice get pulled, just a bit, toward the passing electron. This distortion creates a region of positive charge in the wake of the passing electron. A second electron, attracted to this little zone of positive charge, follows in the wake of the first much as one race car might be pulled along by the slipstream of another.
The chain stops there, though. Electron triplets never form; the pairing process is delicate--the distortion of the lattice that bonds the pairs is fleeting--and nature doesn't seem to be able to forge stable links between more than two electrons at a time. BCS theory predicts that pairing can happen only at very low temperatures. Heat the lattice to more than about 70 degrees above absolute zero (to -389 degrees Fahrenheit) and thermal energy breaks up the pairs as effectively as a gunshot scatters a flock of birds; the extra heat makes
the electrons so energetic that they simply part company.
Once the electrons have all paired up, the stage is set for act 2 of the BCS drama, which features one of quantum mechanics' bizarre descriptions of particle behavior. Physicists lump all the particles in the universe into two broad families: fermions and bosons. Electrons, protons, and neutrons--that is, just about all the familiar components of matter-- are fermions. Photons--particles of light--are examples of bosons. All these particles have a property that physicists call spin, which describes the tiny amount of momentum generated by a spinning particle. Fermions have exactly half the amount of spin of bosons.
Quantum mechanics holds that fermions and bosons have very different properties. Bosons are gregarious, like inebriated conventioneers. They can all join together in a unified state of energy, as photons do in a laser beam. Fermions, and hence electrons, are loners. No two can occupy exactly the same energy state. Like dancers in a raucous nightclub, the electrons in a metal at normal temperatures are a jumble of distinct energies. Disorder prevails.
But at low temperatures, a radical change takes place in the electrons' behavior. The wild dance becomes an ordered waltz. After electrons pair off, they act like a different type of particle altogether. Since the spin of two fermions equals the spin of one boson, paired electrons essentially become bosons. They can now all occupy a single energy level, and they tumble into this "ground state" like rush-hour commuters filling a train. Physicists describe the transition as a "phase change," one equivalent to water suddenly turning to ice. The entire superconductor at this point very closely resembles a giant atom. All the electrons move in unison; individual electrons no longer get deflected randomly in the lattice. Just as electrons whirl eternally around a nucleus, the electron pairs course ceaselessly throughout a superconductor.
The BCS model seemed to explain superconductivity satisfactorily. It even predicted a firm upper temperature limit for the phenomenon, a barrier that seemed to relegate superconductors to the status of a permanent physics freak-show attraction. Only liquid helium could chill materials to temperatures as low as -389 degrees, and besides costing five dollars a quart, liquid helium requires bulky refrigeration equipment.
In the decades following the formulation of the BCS theory, a few physicists toiled away at searching for mixtures that might break the theoretical barrier. But no one came close. The best anyone could come up with, in 1974, was a mixture of niobium and germanium that became superconducting at about 41 degrees above absolute zero, or -418 degrees.
Then suddenly in 1987, in a series of stunning, almost accidental discoveries, a number of labs around the world smashed through the old BCS barrier. New materials appeared that became superconducting at -235 degrees, or 224 degrees above absolute zero. That is, of course, still cold by any normal standard, but in the world of superconductors it was a milestone: it meant that instead of liquid helium, physicists could use liquid nitrogen to chill their exotic compounds. Liquid nitrogen is readily available, easy to handle, and at 30 cents a quart, as physicists like to say, cheaper than beer.
The discoveries elated physicists. By 1988 some were predicting that in a few years they might develop room-temperature superconductors, which wouldn't require any coolants.
Obviously, that didn't happen. For one thing, these new superconductors were strange--they were ceramics, which to materials scientists means not just things like clay but also nearly any type of solid that is neither purely a metal nor a carbon compound. Normally ceramics are insulators. They don't carry electric currents at all. ("Your toilet bowl doesn't conduct electricity," Cava points out. "Nor do your dinner plates.") Another problem is that
ceramics are brittle, and some of the most important applications for
superconductors--electromagnets, generators, and motors--require flexible wires.
Besides these practical problems, theorists now found themselves nearly back at square one. Although physicists still believe that the BCS model explains the old superconductors, it can't explain the newer ones: the chief problem is that superconductivity in these compound persists at temperatures well beyond where theory predicts it should break down. Theorists, though, perhaps even more than nature, abhor a vacuum, so there has been no
shortage of ideas to explain what's happening. Of all of them, two have emerged as the major challengers to the old BCS theory. Supporters in each camp cite numerous experiments to support their ideas, claiming the other side should wake up and recognize that the battle has been fought and won.
"There are many people who are quite sure that they are not confused, but they are in fact totally confused," says Philip Anderson of Princeton. Anderson thinks some of the people working on the problem have let their egos get in the way of good science. "Some of the elder statesmen have been the worst," he says. "There are people who think that they own the field of superconductivity." After reflecting for a moment he adds, "I suppose a lot of
people think that I play that role."
Anderson does seem to take a proprietary view of the field. Perhaps that's because he feels the problem at hand has broad implications for physics beyond understanding superconductivity. He thinks that if physicists can figure out how superconductors work, they will gain new insight into how electrons behave in solids.
Anderson has hammered out a theory for the new superconductors with Sudip Chakravarty, a physicist at UCLA. Like most other theorists, they are focusing their attention on ceramic compounds of copper and oxygen--in particular, a powdery black superconductor called yttrium- barium-copper-oxide.
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