In memoriam : J. Robert Schrieffer, Illinois alumnus and Nobelist who devised the BCS wave function for superconductors
Professors Nigel Goldenfeld, Eduardo Fradkin, and Gordon Baym for Illinois Physics
Schrieffer was the “S” in the famous BCS theory of superconductivity, one of the towering achievements of 20th century theoretical physics, which he co-developed with his Ph.D advisor Professor John Bardeen and postdoctoral colleague Dr. Leon N. Cooper. At the time that Schrieffer began working with Bardeen and Cooper, superconductivity was regarded as one of the major challenges in physics. Since the discovery of the hallmark feature of superconductivity in 1911—the zero resistance apparently experienced by a current in a metal at temperatures near absolute zero—a long list of famous theoretical physicists had attempted to understand the phenomenon, including Albert Einstein, Niels Bohr, Richard Feynman, Lev Landau, Felix Bloch, Werner Heisenberg and John Bardeen himself (who was awarded the Nobel Prize for his co-invention of the transistor at around the time that Schrieffer began working with him in 1956).
The difficulty was that, unlike all other states of matter that had been understood using classical or quantum physics, superconductivity appeared to involve the balance of such small energies that it seemed impossible to be able to compute them accurately enough to explain the phenomenon. Most efforts to explain superconductivity worked with the properties of individual electrons. On top of that, experiments suggested that the electrons in a superconductor somehow were affected by the isotopic mass of the atoms in the material. In 1955, Bardeen, working with David Pines, had shown that electrons in a metal could experience a force that was not just their repulsive charged interactions, but also an attractive force that arose through their interaction with sound waves in the material. This led Cooper to propose a way out of the first difficulty: his idea was that an attractive interaction between two electrons, combined with the quantum mechanics associated with their obeying Fermi statistics, could lead to a new and subtle bound state of the electrons, loosely analogous to the way in which an electron and a proton are bound to form a hydrogen atom. Cooper’s insight suggested that one must somehow incorporate a cooperative interaction between electrons, and not treat them only individually, but it only applied to two electrons. How to extend this to the huge number of electrons in a real material was the remaining problem, and it was this problem that Schrieffer solved.
According to his own account, while riding the subway during an American Physical Society meeting in New York City between Jan 30 and Feb 2, 1957, Schrieffer realized that he could combine Cooper’s pair idea with the variational approach used by Sin-Itiro Tomonaga (later to win the Nobel Prize for the co-discovery of the theory of quantum electrodynamics) to address the so-called pion-nucleon problem in high-energy physics. On the train, Schrieffer wrote out the wave function of all the interacting electrons using quantum mechanics, together with the grand canonical distribution from statistical mechanics in order to conserve the number of electrons in the metal. The next day, he was able to use the variational method to solve what is now known as the gap equation for superconductivity, showing that Cooper’s pair idea could be extended to the entire set of electrons in the metal.
Later, Schrieffer would explain the physical intuition behind his discovery in terms of couples doing the frug, a popular dance at the time in which the dance partners are far apart but between them are many other couples. “Now, these dancing couples are essentially totally covering the dance floor. There is very little space not covered by people. So when they dance they have to do a highly intricate step of moving into a space that, at that instant, happens to be vacant. And this is enormously complicated choreography, so that one doesn't trip, if you like, or hit someone else. And the electrons can't hit each other, or at least they can't occupy the same space at the same time. Fine. So they're all dancing together. By dancing, if you like, they lower their energy or make themselves happier or whatever analogy you like to make. The wave function is just... symbols which record the dance the electrons are making.”
Schrieffer would go on to a distinguished career in theoretical condensed matter physics, with postdoctoral work at the University of Birmingham (UK) and the Niels Bohr Institute (Denmark), and holding faculty positions at the University of Chicago, the University of Illinois (1959–1961), the University of Pennsylvania, the University of California, Santa Barbara and Florida State University, where he helped establish the National High Magnetic Field Laboratory. At the University of California, he was the second director of the National Science Foundation’s Institute for Theoretical Physics and played a major role in developing it as a resource for the entire physics community. Schrieffer also led the physics community through his service in the American Physical Society, of which he was president in 1996.
Bob (as he was known to his friends and colleagues) was a good friend of the Department of Physics at Illinois, interacting closely with the condensed matter group over the years, and helping many of us with our careers, both through scientific and personal interactions. His last visit to Illinois was in October 2007, on the occasion of the 50th anniversary celebration of the BCS work.
Schrieffer received many awards and honors during his career, including the Nobel Prize in Physics in 1972 and the National Medal of Science (1983). He was a member of the US National Academy of Sciences, the American Academy of Arts and Sciences and the American Philosophical Society.
In 1960, Schrieffer married Anne Grete Thomsen, whom he had first met while a postdoc in Denmark, and they had three children.