Absolute Zero and the Conquest of Cold (29 page)

At a seminal meeting of low-temperature researchers from many countries, held at the Royal Society in 1935, Fritz London summed up all the post-1911 theorizing by asking the scientists to take a dazzling imaginative leap—to stop thinking about superconductiv
ity in terms of yesterday's classical physics and to instead consider superconductivity solely in terms of quantum physics and wave motions. Conceive of a superconductor, London pleaded, not as a collection of unrelated atoms but as one huge atom—and the problem will be more easily attacked. The big atom's interior order—the pattern synonymous with superconductivity—could be described, London said, by a single wave function. In other words, the superconducting state had to be produced by the electrons of this giant atom behaving coherently, or in unison.

Just when this notion was being put forth, a third ultra-low-temperature puzzle was discovered. This time it was made by a leading physicist who was unable to attend the meeting in 1935 at the Royal Society, though he desperately wanted to be there: Pyotr Kapitsa.

Previously, the Russian émigré had been the director of the Mond Laboratory in Cambridge, using equipment built expressly for him there at the suggestion of his teacher and mentor, Ernest Rutherford. The British had gone to this trouble because Kapitsa had demonstrated theoretical brilliance and practical ingenuity. For instance, he had invented a faster process for liquefying helium, and was working on a device that employed discharges of electricity to produce intense magnetic fields. By 1930 Kapitsa had accomplished so much that he had been elected a Fellow of the Royal Society, the first foreigner so honored in the previous two hundred years, and was doing important research on both magnetism and low temperatures. Each year he would take a trip back to Russia with his wife to visit their relatives, but when he did so in 1934, the couple were prevented from returning to Great Britain.

Two years of negotiating ensued until, in 1936, the Kapitsas, the Soviet government, and Rutherford completed a three-way deal. Kapitsa's wife agreed to go briefly to England to fetch the couple's two children and bring them back to the Soviet Union with her. In exchange for having his family reunited, Kapitsa accepted the direc torship of a new laboratory in the U.S.S.R., and Rutherford arranged for some of the Mond's equipment to be shipped to Moscow for use at the Institute for Physical Problems.

Once his machinery had arrived, in very short order Kapitsa succeeded in identifying and describing a third, new, and entirely unexpected aspect of matter in the region of ultracold. The discovery was galvanized into existence by a paper written by W. H. Keesom and his daughter Anna Petronella, which suggested that at the lambda point, the thermal conductivity of helium II increased over that of helium I by a factor of 3 million. This meant that helium II became a better conductor of heat than copper or silver, the best normal-temperature metallic conductors of heat.

Kapitsa, fascinated by helium II, used his imagination to make sense out of some odd things happening in research labs. Helium II had not been behaving like all other earthly liquids. It had escaped from containers dense and impermeable enough to prevent the leakage of any other fluid, even helium I. This ability of helium II had resulted in contamination of other fluids, making a shambles of experiments in several laboratories. Also, if a container of helium II was placed in a bath of helium II and filled to a level higher than the bath, the levels inside and outside the container would gradually equalize. Creeping up and over walls, defying friction and gravity, it seemed to refuse to adhere to normal physical rules of flow. "Helium," Kapitsa later wrote, "moves faster than a bullet."

Taking these Houdini-like effects as his starting point, Kapitsa tried to determine the parameters of helium II's escape artistry. Researchers at Cambridge and Leiden were also working on the problem, and the three groups kept in touch with one another by mail, telephone, and personal meetings, carrying on what Kurt Mendelssohn characterized as "ruthlessly searching discussions into the validity of each other's methods." In articles published beginning in 1938, Kapitsa seized the theoretical high ground and made order out of what had been perplexing chaos by describing what
helium II was doing as exhibiting
superfluidity,
.
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He attributed superfluidity to changes in the viscosity that were intimately related to what had initially prevented Onnes and Dana from publishing their data in 1922: the sharp rise in helium II's specific heat, later identified by Keesom and Petronella as being 3 million times greater than that of helium I. This fantastic ability of helium II to conduct heat and its ability to move about as though nothing could stand in its way, Kapitsa suggested, were aspects of the same phenomenon.

He devised an experiment that demonstrated the effects of these behaviors. Inside a large dewar of liquid helium II, he placed a smaller one also filled with liquid helium, and in that "bulblet" Kapitsa inserted a capillary tube with one end sealed inside and the other open to the helium vapor. The outer dewar was necessary to keep the inner one at the proper low temperature. Kapitsa set up a weathervane sort of instrument near the open end of the capillary and applied heat to the bottom of the bulblet. A submerged jet of invisible liquid helium issued from the top and turned the weathervane. The experiment went on for hours, with the vane spinning and the bulblet of liquid helium as full at the end as it had been before the start. Kapitsa figured out that the heat transformed some of the superfluid to normal fluid, which produced the submerged jet. He concluded that helium II had no entropy and a viscosity 10,000 times lower than that of liquid hydrogen, that is, an almost unmeasurably small viscosity, virtually none at all.

"At first sight," wrote Russian physicist Lev Landau of Kapitsa's weathervane experiment, liquid helium's properties "seem completely absurd, like the story of the giraffe which evoked the exclamation, 'There ain't no such animal!'"

No viscosity.

No inner magnetic field.

No electrical resistance.

The trio of unusual phenomena at the far edge of the ultracold was complete: superconductivity, superdiamagnetism, and superfluidity.

The discovery of this trio of phenomena meant the final eclipse of the old clockwork universe that obeyed Newtonian laws of motion. No adequate explanation of the new phenomena could be made by means of the old laws. Fortunately, though, this trio of puzzles surfaced at a time when quantum physics had matured enough to provide cogent attempts to explain superconductivity, superfluidity, and superdiamagnetism. Kapitsa was fond of saying that trying to detect the quantum nature of physical processes at room temperature was like trying to investigate the physical laws governing the collision of billiard balls on a table aboard a ship going through rough seas. And Landau would explain to his classes the advantages of lowering temperatures into the arena of the ultracold, where all sorts of processes slowed down and became more amenable to study. "As the temperature falls," Landau said, "the energy of the atomic particles decreases, the conditions in which classical mechanics are valid are eventually violated, and classical mechanics has to be replaced [as a tool for understanding] by quantum mechanics."

A protégé of Niels Bohr, and a man acknowledged as one of the great teachers of physics in the twentieth century, Landau was Kapitsa's closest colleague. An irascible perfectionist who liked to deny he had been a child prodigy even though he published a brilliant paper in quantum mechanics at the age of nineteen, Landau was arrested and imprisoned in the 1930s, charged with anti-Soviet activity and with being a Nazi spy, though he was Jewish. Kapitsa wrote directly to Stalin seeking Landau's release. "I beg you to give orders that his case should be very carefully considered"; Kapitsa acknowledged that his colleague's character was "bad," that he was "not easy to get on with [and] enjoys looking for mistakes in others
[which] has made him many enemies," but denied that Landau could ever do anything seriously dishonest. When this did not produce results, he wrote to the foreign minister and then to the head of the secret police, demanding the release of Landau and personally guaranteeing that his colleague would "not engage in any counterrevolutionary activities." He also threatened that if Landau was not freed, he, Kapitsa, would resign. Landau later wrote that Kapitsa's activism on his behalf required "superb courage, great humanity, and crystalline integrity." Released from prison, Landau returned to his laboratory at the Institute for Physical Problems, where he shortly began to examine helium II and its strange antics in a new way.

Landau suggested that helium II be considered one large molecule, akin to a crystal. At absolute zero, Landau believed, helium II was 100 percent superfluid. As the temperature rose, "elementary excitations"appeared on the superfluid, particles known as phonons (quantized sound waves) and rotons (which move in exotic ways, such as in the reverse direction of their momentum). These particles, Landau guessed, comprised the normal fluid. From these ideas, Landau was able to formulate equations for the motion of the two fluids, normal and superfluid. He was also able to define viscosity as "the ability of a liquid to oppose movement" and the effective absence of viscosity as the inability to oppose movement. He reasoned that if anything were able to retard the flow of helium II into the capillary from the larger dewar in Kapitsa's experiment, the kinetic energy of the liquid would be reduced, its temperature would rise, and it would behave like (or become) helium I. But since the capillary walls did not impede the movement or change the energy level of the phonons, the fluid remained in the form of helium II, and only after heating did it exit the top of the capillary and turn the weathervane. According to Landau, the velocity of helium II as it penetrated into the capillary was low enough to permit the unimpeded flow through certain walls, and against gravity. In other words, helium II was not faster than a bullet, as Kapitsa had contended; precisely the opposite was true. Helium II was the slowest yet the most persistently moving and unstoppable substance on Earth.

From the mid-1930s onward, scientists seemed to have most of the pieces of the deep-freeze puzzle spread out on a table in rough order, but they could not make the final assemblage. Along with the pieces having to do with magnetism, specific heat, and viscosity, there was another, provided also by Fritz London. He postulated that the difference between a metal in the normal state and in the superconducting state had to do with an "energy gap" involving something called the "Fermi surface," named after Italian physicist Enrico Fermi.
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London believed that if one could figure out the microscopic mechanism of the energy gap that differentiated the two states, that would explain superconductivity and diamagnetism; he was also certain that the explanation would involve electron interaction at the Fermi surface.

Once London had identified the energy gap, Mendelssohn contends, the path to assembling the pieces was clear, but mapping that path was a "formidable task requiring a superb knowledge of electronic phenomena in metals, great mastery of mathematical technique, and, above all, brilliant but controlled imagination."

One man might not possess all of these talents, but a trio of men did: John Bardeen, Leon Cooper, and Robert Schrieffer. Their summons to the puzzle board began in 1950, when a telephone call reawakened Bardeen's interest in superconductivity. In the fifteen years since London had directed attention to the energy gap, nuclear technology had been developed for the purpose of separating the isotopes of uranium and was now being turned to more benevolent uses. Looking into newly separated isotopes of mercury, two sets of researchers independently deduced an important mathematical relationship regarding superconductivity: the temperature at which a metal became superconducting varied inversely with the square root of its molecular weight. In 1950 E. Maxwell at the National Bureau of Standards and Bernard Serin at Yale University both arrived at the inverse-square-root formulation and prepared articles about it; Serin also telephoned his friend John Bardeen.

Bardeen was one of the prodigies of American science. The son of a medical-school dean and an artist, he graduated from high school at the age of fifteen and became one of the youngest students at Princeton's Institute for Advanced Studies in the mid-1930S. At Princeton and Harvard he studied the behavior of electrons in metals—and learned of the Londons' work in that area as it related to superconductivity. During the war Bardeen worked on magnetic fields given off by ships, and in the postwar era he teamed with Walter Brattain and William Shockley at Bell Labs to invent the transistor, for which the trio would win the Nobel Prize in 1956. Serin called Bardeen in 1950 because he knew Bardeen had come to believe that electron interaction at the Fermi surface of a metal could explain the onset of superconductivity,
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and Serin's experiments and mathematical formulations provided additional evidence for this possibility.

Bardeen tried to shape from the various clues a comprehensive theory that solved all the superconductivity puzzles, but he could not do so for a few years. Then he was able to invite to his home base, the Illinois Institute for Advanced Studies, Leon Cooper, who had recently received his doctorate from Columbia University in quantum physics. Because not enough office space existed at the institute, grad students and postdoctoral fellows were crowded into offices on Floor 3½ of a neighboring building, an enclave they labeled the Institute for Retarded Studies. In 1956 Bardeen asked Cooper to make room in his office for Robert Schrieffer, a doctoral candidate from the Massachusetts Institute of Technology (MIT). Bardeen had decreed that Schrieffer should spend a year in a laboratory as preparation for his doctorate, but when Schrieffer caused an explosion in the lab while welding metal in a hydrogen atmosphere, he was asked to concentrate on theory.

By then Bardeen had extended his guesswork beyond the energy gap described by Fritz London, adding the idea that superconductivity as a "phase transition" must be produced by a change involving the spin of the electrons. Phase transitions are the transformations that occur when a gas becomes a liquid or a liquid becomes a solid. In this case, the transition was from normal conductance to the superconducting state. While Cooper was traveling on a subway, the revelation came to him. Just above the temperature for the onset of superconductivity, electrons acted normally—they repelled one another, and one result was resistance to an electrical current. But when the temperature reached down to the transition there would be an interaction among electrons made possible by phonons., Two previously isolated electrons with oppositely directed spins would bond into a "Cooper pair." This bonding would be accompanied by some extra residual attraction that influenced all the other electrons, until none of them repelled one another. This was the superconducting state. Raising the temperature above the transition point would again break the Cooper pairs, making them repel one another, which produced electrical resistance.