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Tuesday, October 2, 2012

John Bardeen


John Bardeen (1908-1991) was the first person to win the Nobel Prize twice in the same discipline. The first award was made for his part in the discovery of the transistor, and the second for his part in developing the theory of superconductivity.
John Bardeen was born in Madison, Wisconsin, on May 23, 1908. He was the son of Althea Harmer and D. Charles R. Bardeen, who was professor of anatomy and dean of the medical school at the University of Wisconsin. John Bardeen graduated from Madison Central High School in 1923 and earned bachelor and masters degrees in electrical engineering from the University of Wisconsin in 1928 and 1929 respectively.
Early Work and Doctoral Studies
In 1929 and 1930 Bardeen worked as a research assistant in electrical engineering, investigating geophysical and other sorts of problems with professor Leo J. Peters. In 1930 Peters and Bardeen took positions with Gulf Research and Development Corporation in Pittsburgh, where they worked on some early applications of geophysics to petroleum prospecting.
Bardeen resigned from Gulf in 1933 to resume his formal studies. He earned his doctorate at Princeton University in 1936, with a mathematical thesis on the work function of metals. His advisor at Princeton was Eugene Wigner. Between 1935 and 1938 Bardeen was a member of the Society of Fellows at Harvard University, where he investigated further problems in the physics of metals with Percy Bridgman and J. H. Van Vleck. (It is worth noting that Van Vleck, who had first taught quantum mechanics to Bardeen in 1928 and 1929 at the University of Wisconsin, also gave Walter H. Brattain [one of the other inventors of the transistor] his first course in quantum mechanics when Brattain was a graduate student at the University of Minnesota.)
First Efforts at Theory of Superconductivity
From 1938 to 1941 Bardeen was an assistant professor of physics at the University of Minnesota. During this time he made his first efforts at devising a theory of superconductivity.
In a superconducting medium, electrical resistance drops to zero below the critical temperature, and currents once begun flow indefinitely. (The phenomenon was first observed in 1911 by Kammerligh Onnes for the element mercury, for which the critical temperature is 4.2 K.-Kelvin). In 1933 it was discovered that a fundamental property of superconductors is that they exclude magnetic fields from their interiors. Fritz and Heinz London were able to describe this property of superconductors in terms of macroscopic electrodynamic potentials, and in the same year Fritz London suggested that superconductivity was a quantum effect manifested on the macroscopic scale. It was more than 20 years, however, before a microscopic quantum theory of superconductivity was developed.
Bardeen's first attempt at a theory of superconductivity was based on the idea of a gap in the energy levels available to electrons. Electrons in superconducting states would be unable to absorb energy quanta unless the quanta were large enough to carry them over the energy gap into states representing normal conductivity, and they would consequently be trapped in superconducting states. Bardeen suggested that the energy gap would arise from interactions of the electrons in a conductor with static displacements of the crystal lattice, but his theory was unsuccessful.
In 1941 Bardeen left the University of Minnesota for a position with the Naval Ordnance Research Laboratory that lasted the duration of World War II. His concerns during the war were with underwater ordnance and minesweeping.
Nobel Prize in Physics
Bardeen was hired in the fall of 1945 by Bell Telephone Laboratories. Here he became a member of William Shockley's semiconductor research division, playing a major part in the invention of the point-contact transistor. It was Bardeen who determined why Shockley's first design for a semiconductor amplifier would not work; the energy states of a semiconductor favored the formation of a layer of charge on its surface, and this charge screened the interior from the influence of an electric field that was required by Shockley's design. Walter H. Brattain, another member of Shockley's group, investigated the properties of the surface states, and from his experiments grew a practical semiconductor amplifier, the transistor. The transistor was first demonstrated on December 16, 1947, and Bardeen, Brattain, and Shockley were awarded the Nobel Prize in Physics in 1956 for their discovery.
Bardeen's interest in superconductivity was reawakened in 1950 by the discovery of the isotope effect; it was found that the critical temperature for a superconductor depended on the square root of its atomic mass. Bardeen concluded that the interaction of electrons with ions in a crystal lattice must play an important part in superconductivity, but he was still unable to explain the phenomenon. The Ginzburg-Landau equations, which gave a phenomenological description of the ordering of conduction electrons in a superconductor but did not explain the causes of that ordering, also appeared in 1950.
More Research on Superconductivity
Bardeen left Bell Laboratories in the fall of 1951 for a professorship at the University of Illinois. In 1955 he renewed his research on the phenomenon of superconductivity, this time with the aid of his graduate student J. R. Schrieffer and of Leon N. Cooper. In 1956 Cooper discovered that an attractive potential between pairs of electrons could give rise to a gap in the energy levels available to electrons, and hence to a condensation of electrons in superconducting states. The attraction between electrons is not direct, in fact, but arises from a dynamic interaction of pairs of electrons with the crystal lattice. An electron may produce a vibration of an ion in the lattice, and this vibration will be experienced by a second electron as an attraction towards the first electron. Bardeen, Cooper, and Schrieffer discovered that the pairing of the electrons is such that (for a state in which no current flows in the superconductor) an electron with a given momentum and spin will be paired with an electron having the opposite momentum and spin. The two electrons are not close together, and in order for the pairing to occur all pairs of electrons must have the same net momentum. Hence the superconducting state is stable against perturbations since one Cooper pair cannot be destroyed without destroying all of them. As well as the vanishing resistance of a super-conductor, the theory of Bardeen, Cooper, and Schrieffer justified the equations of Ginzburg and Landau and London's description of the magnetic properties of a superconductor.
Although a magnetic field is excluded from the interior of a superconductor, it is possible for a magnetic field to destroy superconductivity; the cost in energy to exclude magnetic flux from the superconductor may be greater than the energy gained in the transition to the superconducting state, and the superconductor reverts to its normal condition. For type I superconductors, the fields necessary to destroy superconductivity are quite small. In some type II superconductors fairly strong magnetic fields can be tolerated, and alloys designed to have type II superconducting properties can be used to make practical superconducting electromagnets.
For their successful model of superconductivity, Bardeen, Cooper, and Schrieffer were awarded the Nobel Prize in physics in 1972. Subsequent refinements of their work have produced ever better agreement of theory and experiment.
Honors and Awards
Among many other honors, John Bardeen was elected to the National Academy of Sciences in 1954. He married Jane Maxwell in 1938, and they had three children. After 1975 he served as emeritus professor at the University of Illinois. He died in Boston on January 30, 1991, as the result of heart failure following surgery that had revealed the presence of lung cancer.
In 1994, The Minerals, Metals and Materials Society established the John Bardeen Award which recognizes individuals who have made outstanding contributions and shown leadership in the field of electronic materials.


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