Scientists in History: Explore Superconductivity
Science History: Learn how the personality, passion, and politics of scientists altered the history of superconductivity at The Emergent Universe, an online interactive science museum about emergence.
Passion, Personality, & Politics
Personality: The Race for Cold
The major players in the race to liquefy the gasses hydrogen and helium, James Dewar and Heike Kamerlingh Onnes, could not have been more different, and their personal approaches played a key role in the outcome of this race. Dewar was colorful, passionate and irascible, a consummate showman who thrived on public demonstration of his achievements. Dewar relied heavily on his own talents, as his fiery personality alienated other scientists, and he felt technicians to “be a waste.” In contrast, Kamerlingh Onnes was systematic and tenacious, a man with a grand vision whose iron will and social instincts enabled him to build a focused institute of collaborating scientists and highly skilled technicians – the first institute of its kind and the beginning of “big science.”
The race began once oxygen was liquefied in 1877. Dewar jumped in the next year, liquefying oxygen for a public audience in 1878, while Kamerlingh Onnes joined the race in 1882, when he became head of the physics department at Leiden. By 1895, while Kamerlingh Onnes was still building his institute, Dewar, working quickly with just a few assistants, had already begun investigating using evaporation to cool hydrogen to lower and lower temperatures. Dewar’s single-minded effort paid off: in an 1898 public demonstration, Dewar achieved glory, reaching the very cold temperature of -253ºC (20K) and liquefying hydrogen! In contrast, Kamerlingh Onnes, with his systematic institutional approach and highly stringent equipment standards, was unable to liquefy hydrogen for several more years, until 1906.
The next task was helium. Suprisingly, even with his 8 year head start on the problem, Dewar failed to ever liquefy helium – in fact, his entire low-temperature program eventually came to halt when his much-maligned assistant angrily quit. Yet, this “impossible” feat of liquefaction was accomplished by Kamerlingh Onnes’ lab in 1908, just 2 years after they had liquefied hydrogen. Why was it that Kamerlingh Onnes succeeded where Dewar failed?
Because the liquefaction of helium was a triumph of technical detail and Kamerlingh Onnes had surrounded himself with the very best technicians. At the temperatures required to liquefy helium ( -269ºC, or 4ºK), air freezes, meaning that any leaks in the apparatus or impurities in the helium will freeze, blocking valves and ruining the experiment – problems that constantly plagued Dewar. Under these conditions, Kamerlingh Onnes’ systematic institutional approach and high standards – standards that ensured a leak-free system – were critical. It was the only approach able to tame helium, and it would be another 15 years, until1923, before any other lab in the world would have the technical ability to reproduce Kamerlingh Onnes’ success. Thus, it was the technical superiority of Kamerlingh Onnes’ institute, and their ability to liquefy helium, that paved the way for the discovery of superconductivity.
Politics: Stalin and the two Levs
In the early 1930s, two exceptionally brilliant and prolific young physicists, Lev Landau and Lev Shubnikov, joined the same institute in Kharkov, Ukraine (then part of the Soviet Union). The two Levs rapidly became close friends, becoming known as “Lev the Stout and Lev the Thin,” and spent their time in deep conversation, investigating many complex problems in physics, including the mystery of superconductivity.
During this time, Shubnikov, who was recently returned from the Kamerlingh Onnes Institute, established the first Soviet low-temperature laboratory having liquid helium capabilities. Shubnikov, who knew how to make very pure samples, conducted some of the best experiments on the response of superconducting metals and alloys to magnetic fields. At the same time, Landau began publishing on superconducitivity and also developed a theory of phase transitions.
But in early 1937, Stalin’s purges expanded, plunging Soviet academia into fear and suspicion. Then, in August of that year, Lev Shubnikov was arrested. And nine months later, Lev Landau, who had by then left Kharkov for Piotr Kapitza’s institute in Moscow, was also arrested.
In 1939, after a year in Soviet prison, Lev Landau was nearly dead. Then, finally, one of Piotr Kaptiza’s courageous pleas on Landau’s behalf, this one directly to Stalin’s second-in-command, was successful, and Landau was released. Kapitza had argued that Landau was the only scientist who could possibly explain his new results on liquid helium, and, indeed, on Landau’s release, he succeeded in explaining these helium results ( work for which he would eventually win a Nobel Prize). Then, in 1950 Landau and Vitaly Ginzburg, extended Landau’s theory of phase transitions to successfully describe the transition to superconductivity. Landau, who had nearly died in prison, became one of the most influential theorists of the twentieth century.
But what of Shubnikov? Executed in prison in 1937, his story stops. And, because it was forbidden to reference Shubnikov, his work was also forgotten. It remained so until Shubnikov was posthumously exonerated in 1957. A young student of Landau’s, Alexei Abrikosov, extended the Landau-Ginzburg theory of superconductivity to explain Shubnikov’s alloy results, thus “discovering” what are now called “type II superconductors,” work for which Abrikosov would win the Nobel Prize. We can never know what Shubnikov, whose experiments were 20 years ahead of their time, would have discovered had he survived Stalin to continue his experimental investigations and collaborations with Landau. But, in Abrikosov’s words, “[Shubnikov’s] data gave me real inspiration…I am sure that, given the opportunity, he would have discovered that…there exist [type II] superconductors.”
Perseverance: John Bardeen
In 1937, John Bardeen, a modest man of few words, became interested in explaining the mysterious phenomenon of superconductivity. In 1941 he presented his first (incorrect!) theory. World War II intervened, however, and Bardeen did not return to the problem of superconductivity until 1950, when he received a phone call from an experimentalist, Bernard Serin. Serin had found a surprising result: that the collective vibrations of the metal ions (called phonons) were important for superconductivity. So Bardeen revamped his previous theory to consider the effect of the phonons on the electron energies, but he still couldn’t predict the superconducting state. Bardeen was in good company — Herbert Fröhlich had independently tried a similar approach and also failed. In fact, the list of physicists who tried to solve superconductivity and failed would grow to include nearly every outstanding theorist, from Einstein and Bohr to Feynman and Heisenberg.
In 1951, Bardeen wanted the freedom to continue pursuing superconductivity, so he left Bell Labs, where he had invented the transistor (for which he would receive a Nobel Prize). Once at the University of Illinois, he reviewed all the available experimental data on superconductivity and described, in a phenomenological way, the most critical results. At the same time, he explored new theoretical methods that he felt were likely to be relevant to this problem.
During this time, Fröhlich had looked beyond the effect phonons have on a single electron’s energy and investigated how they affect the interaction between electrons. He found that phonons could induce an attraction between two electrons – but his conclusions were qualified, as he had not included the strong repulsive force that also exists between electrons. Subsequently, Bardeen and his first postdoctoral researcher, David Pines, used an extension of Pines’ approach to electron interactions to confirm that the phonon-induced attraction could, for some electrons, outweigh their basic repulsion. Bardeen and Pines suggested that this effective net attraction between electrons could bring about superconductivity.
Bardeen then hired his second postdoctoral researcher, Leon Cooper, whose expertise in quantum field theory methods enabled him to solve a model problem in which 2 electrons having an attractive interaction are surrounded by a field of free electrons. Cooper found that the 2 attractive electrons could pair up to form a lower energy state, a key feature of superconductivity. But the superconducting state involves many electrons, not just two. How could these ideas be generalized? The breakthrough, which involved recognizing that the superconducting state could be described as being made up of overlapping pairs of electrons, was made by Bardeen’s graduate student Robert Schrieffer in 1957. The resultant theory of Bardeen, Cooper and Schrieffer successfully explained superconductivity, and it earned the trio a Nobel Prize in 1972.
Passion: The Race for High Temperatures
In 1983, when IBM gave Karl Müller license to pursue any line of research, he chose a rather unlikely program – looking for high critical-temperature (Tc) superconductors within the ceramic metal oxide family. While this idea was not entirely without precedent, it went against the conventional wisdom that the highest Tc’s would be found in intermetallic compounds. Yet intermetallic Tc’s had stalled out at Tc= 23K a decade earlier. So Müller, who had more experience with ceramic oxides than with superconductivity, chose a path less traveled.
Müller and a young materials scientist, Georg Bednorz, explored numerous oxide compounds, only to find that at low temperatures they became insulators, the exact opposite of a superconductor. Then, in late 1985, the pair decided to explore an La-Ba-Cu-O compound which had been shown to remain conducting well below room temperature. In a serendipitously brilliant move, Bednorz developed his own synthesis procedure. It generated not only the target compound, but also a second compound – which became superconducting at 11K. By refining the amount of Ba in this new compound, Bednorz and Müller raised Tc to an unheard of 35K.
Because there had previously been many erroneous reports of high Tc’s, Bednorz and Müller’s cautiously titled paper (published in November 1986) was ignored – by all but two groups. Then, at a meeting that December, Paul Chu rocked the scientific community by announcing that he had reproduced Müller’s results – only to be followed by Koichi Kitazawa, who’s team had also confirmed them. An entirely new class of superconductors with unbelievably high Tc’s had been discovered! Bednorz and Müller would be awarded the Nobel prize for this work the very next year.
With Tc at 35K, the dream of affordable superconducting applications, one’s that would become feasible at liquid nitrogen temperatures (Tc > 77K), suddenly seemed within reach. The race to find superconductivity above 77K was on, and everyone with any relevant expertise jumped into the fray. Paul Chu’s team immediately demonstrated that pressure could raise Tc for Müller’s compound to 40K, and they tried to synthesize a more compressed compound by using smaller, but electronically similar, atoms. In late January, they hit the jackpot, discovering a Y-Ba-Cu-O compound with a Tc of 93K! Competition at the time was so fierce that Chu submitted his publication with the element Y systematically replaced by Yb, changing it back only immediately before publication. While one can criticize Chu’s action, the fact that the fake formula was leaked, causing many groups to investigate the superconductivity of Yb-Ba-Cu-O, does validate Chu’s fears. Over the next few years a flurry of frenetic activity, accompanied by much wild speculation within the popular press, saw Tc increased in related compounds, reaching 138K in 1993.