Irresistible Part 2
It is no accident that the dart stuck in the table of elements in Cava's lab punctures copper. Most of the new high-temperature superconductors consist of sheets of copper and oxygen atoms layered between various other elements, usually yttrium and barium, a formula hit upon by experimenters eight years ago after much trial and error. The copper and oxygen atoms seem to be the crucial ingredients; no one really understands what role the yttrium and barium have. Cognoscenti usually just refer to these materials as the cuprates.
To explain how superconductivity operates in the cuprates, Anderson and Chakravarty propose a radically different way of looking at electrons. In the conventional model of solids, electrons sail through a lattice past other electrons, suffering the odd collision here, the repulsion of another electron there. None of these interactions seriously affects the integrity of the electron as a discrete particle. It still roams proud and free through the solid.
But in the copper-oxide sheets in the new superconductors, and perhaps in other materials as well, Anderson and Chakravarty believe that this description of electron behavior is fundamentally inaccurate. They maintain that it is unrealistic to assume that the electron is not strongly affected by other electrons and atoms in the solid. Such interactions, they calculate,
could be quite powerful--powerful enough that in some cases the electron separates into two more-fundamental particles, which they call the holon and the spinon. The holon keeps the electron's charge; the spinon keeps the electron's spin.
Holons and spinons require energy to remain separate, and thus exist only when the material is in its normal state. When the material cools and becomes superconducting, the holons and spinons recombine into electrons again; then, just as in BCS theory, the electrons form pairs. In effect, the electrons are transformed into bosons and can again all settle into one coherent energy state. But now there is a crucial departure from the BCS picture. In Anderson and Chakravarty's model, the electron pairs perform another quantum mechanical trick.
One of the strangest elements of quantum mechanics, demonstrated by many experiments, is that electrons and all other particles sometimes don't seem to be solid particles at all but instead behave like waves. The well-defined particle becomes a smear. In Anderson and Chakravarty's model, this waviness enables electron pairs to extend from one copper-oxygen layer to another, "tunneling through" an intermediate layer of atoms, which in many of the new superconductors consists of yttrium and barium. As the pairs cross these layers, they lose a little kinetic energy and become more stable. The pairs are harder to break up when heated, so superconductivity persists at higher temperatures. But at normal temperatures, most of the electrons in the lattice exist in the form of holons and spinons. Electrons at these higher temperatures can't form the pairs that lead to superconductivity, because they are bound up--or even split up--by their strong interactions in the copper-oxygen lattice.
Anderson and Chakravarty claim that a number of experiments support their theory. Two seem especially striking. First, says Chakravarty, several labs have reported that as you increase the number of copper-oxygen layers in a superconductor, the compound will remain superconducting at higher temperatures. This jibes with their theory, which says that the layers make the electron pairs more stable.
In a second group of more subtle experiments, researchers have shone infrared light on cuprate materials and measured the amount of reflection. These experiments found that the very same compound will reflect more light while it is superconducting than it will at higher temperatures. The results seem puzzling, but Anderson and Chakravarty say that their theory explains them naturally--they are a consequence of the state of the electrons in the material at different temperatures.
When light hits something, it jiggles electrons. The jiggled electrons immediately emit radiation, which we see as reflected light. But in Anderson and Chakravarty's model, at normal temperatures the electrons have split into holons and spinons. So the light jiggles fewer intact electrons, and less light gets reflected. When the temperature drops and the material becomes a superconductor, holons and spinons merge, the light jostles many electron pairs, so more light bounces back.
"That experiment to my mind is conclusive," says Anderson. "It isn't to everybody else, of course."
Anderson knows his audience.
"I don't think there's a chance in hell that Anderson's interlayer pairing has a thing to do with this," says Douglas Scalapino, a physicist at the University of California at Santa Barbara.
While Anderson and Chakravarty believe that a fundamental shift in our understanding of the electron is needed to explain the new superconductors, Scalapino and David Pines, a theorist from the University of Illinois, believe that less drastic measures will suffice.
Much simplified, their model seems quite similar to BCS theory. But they maintain that it's not the negative charge of the electron that distorts the lattice of the superconductor but the electron's tiny magnetic field. The magnetic field of the passing electron tugs on nearby atoms in the lattice, distorting the local magnetic field of the copper atoms. The slight magnetic distortion in the electron's wake attracts another electron. Again, once electrons pair up, they all fall into the same energy state, just as in the BCS picture. Pines's calculations show that this attraction holds the electrons together at higher temperatures than are possible in the older theory.
If this seems like a minor adjustment to the BCS theory, Anderson and Chakravarty agree. "You just take over the BCS equations from a textbook," says Chakravarty. "All you are doing is changing one thing. You're changing the attractive interaction. My commonsense point of view tells me that there is something very special about these materials, that you cannot just tweak textbook equations."
Nevertheless, Pines says that numerous experiments support their theory. "You've got a cast of thousands, and they all get the same answer," he says.
Many of those experiments test one of Pines and Scalapino's keypredictions. In their theory, as in Anderson and Chakravarty's, the wavelike nature of the electron becomes very important; pairs of electrons combine to produce a single traveling wave. It turns out that the subtle calculations of quantum mechanics show that the crests of a "pair wave" traveling in one direction through the copper-oxygen grid will always coincide with the troughs of a pair wave traveling perpendicular to the first pair. The two waves will cancel, and the superconducting current will vanish.
Dale Van Harlingen, an experimental physicist at the University of Illinois, put this odd prediction to the test. He attached a tiny C- shaped piece of lead to a pepper-grain-size square of an yttrium-barium- copper-oxide superconductor. The arms of the lead C touched two adjacent sides of the cuprate square. Then he lowered the temperature and started a current in the little piece of lead, which is a conventional BCS superconductor. The idea was that the current from the lead would enter the cuprate superconductor at right angles. If Pines and Scalapino are right, the perpendicular currents should cancel out in the cuprate because the electron pair waves will annihilate each other. That is exactly what Van Harlingen observed. As a control, he checked to see whether a current would flow through the cuprate if the current came through one side of the square only. It did.
Anderson and Chakravarty's theory, says Pines, may have seemed plausible a year or two ago, but he believes the tide has turned. "Matters have changed dramatically now, and there are some 40 to 50 experiments that support us."
Anderson and Chakravarty say that their own theory can be adjusted to comply with these experiments as well, but that the other camp's model can't explain the importance of the layers in the superconductors or their light-reflecting properties.
Who's right? And will it matter to physicists like Cava?
These are all really good people," says Robert Dynes, an experimental physicist at the University of California at San Diego. "They're not doing foolish things." Dynes, although he has done experiments that seem to support Anderson and Chakravarty's experiment, believes neither side fully understands superconductors yet.
"We don't know what's causing superconductivity," he says. "At least I don't. Some of these people think they do. No theory has comfortably described all these experiments. I'll probably offend a lot of my friends with that statement. But all the cards haven't fallen into place."
Dynes also says that more is at stake here than figuring out how superconductors work. "The thing that theorists like Phil and David Pines and--the list just goes on as long as your arm--the thing that's got them excited, and that causes all the strife, is the underlying belief that we're entering a new era in our understanding of electrons. It's a much, much broader problem. I think people really smell a new era."
The confusion over superconductivity, says Dynes, betrays a serious gap in physicists' understanding of the most basic properties of matter. Physicists like to think they understand why gases change to liquids, and liquids to solids--and why some materials conduct electricity and others don't. "We understand copper," he says. "We understand conduction in aluminum. Why don't we understand it in copper-oxides?"
The theorists may be stymied, but that hasn't stopped progress on the practical front. A handful of research teams have finally managed to wring something practical from the brittle new superconductors. In April, for example, a team at Los Alamos National Laboratory announced that it had made wires from yttrium-barium-copper-oxide. The researchers claim
that their wire--it actually looks more like a thin piece of tape--can carry nearly 100 times the current of any other superconducting wire, and more than 1,000 times as much current as a typical household copper wire. Whether the enormous technological potential of superconductors--the levitating trains, the superconducting power lines, and more--will ever
be realized depends largely on whether superconducting wires can be manufactured cheaply
and reliably.
The Los Alamos researchers feel they've made a promising start. "The question is," says Paul Arendt, one of the physicists who developed the tape, "is there a market? We think there is. Will people use these? We're certain they will."
One of the problems the Los Alamos team overcame, one that has slowed progress in the field for years, was to make sure that the superconducting crystals that make up their tap were properly aligned. If the crystal grains don't line up, the current is drastically reduced. To avoid that problem, the physicists first coated a thin, flexible strip of nickel alloy with a layer of perfectly aligned crystals of a mineral called zirconia. The zirconia acts like a buffer and prevents the nickel from contaminating the superconductor, which gets deposited next. It also acts like a template for it. When a laser vaporizes the cuprate superconductor, the vaporized grains line up naturally as they nestle into the aligned zirconia crystals. Finally the researchers wrap the coated nickel strip around a narrow tube filled with liquid nitrogen to chill the superconductor.
Despite this auspicious beginning, the Los Alamos team still has some years of work ahead of it. So far the tape it's made is only two inches long and a half-inch wide. Arendt says his team doesn't see any problems in making the tape longer.
"We're scaling up now," he says. "We're doing this on a shoestring, and we're hoping to get industrial investors."
Cava meanwhile presses on in his search for new materials. While a successful theory might help guide him and other experimentalists, Cava is more interested in working outside the boundaries set by the theorists.
"We are very driven by intuition and empiricism," he says of his lab staff. "It's a very successful method for finding new things. If you theorize too much, you can find only the things that you understand. If you do things beyond what you can theorize about, you find something that's a surprise. That's what we want. Surprises."
Cava recently found an unusual boron-carbon superconductor. Although it becomes a superconductor at temperatures well below that of the cuprates, it serves as a reminder that the periodic table may yet hold some surprises for him and that the theorists shouldn't get too cocky. "One of our big goals in here is to find the thing that's going to open everybody's eyes," he says.
He walks over to a drawer and pulls it open. The drawer is filled with nearly a hundred small jars that contain almost every element in the universe in one form or another--a sort of messy cosmic medicine cabinet. The small jars tumble about and clack together as the drawer slides out. Cava picks one up and muses: "There's a lifetime--ten lifetimes--of things to try that nobody has tried before."
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