Place: United States of America
Subject: biography, physics
US physicist whose work on semiconductors, together with William Shockley (1910-1989) and Walter Brattain (1902-1987), led to the first transistor, an achievement for which all three men shared the 1956 Nobel Prize for Physics. Bardeen also gained a second Nobel Prize in 1972, which he then shared with John Robert Schrieffer (1931- ) and Leon Cooper for a complete theoretical explanation of superconductivity.
Bardeen, who is the only person thus far to be awarded two Nobel Prizes for physics, was born in Madison, Wisconsin on 23 May 1908. He began his career with a BS in electrical engineering at the University of Wisconsin awarded in 1928. For the next two years, he was a graduate assistant working on mathematical problems of antennae and on applied geophysics, but it was at this stage that he first became acquainted with quantum mechanics. He moved to Gulf Research in Pittsburgh, where he worked on the mathematical modelling of magnetic and gravitational oil-prospecting surveys, but the lure of pure scientific research became increasingly strong, and in 1933 he gave up his industrial career to enrol for graduate work with Eugene Wigner (1902-1995) at Princeton, where he was introduced to the fast-developing field of solid-state physics. His early studies of work functions, cohesive energy, and electrical conductivity in metals were carried out at Princeton, Harvard, and the University of Minnesota. From 1941-45, he returned to applied physics at the Naval Ordnance Laboratory in Washington, DC, where he studied ship demagnetization and the magnetic detection of submarines.
In 1945 Bardeen joined the Bell Telephone Laboratory, and his work on semiconductors there led to the first transistor, an achievement rewarded with his first Nobel Prize in conjunction with Walter Brattain and William Shockley. He moved to the University of Illinois in 1951, where in 1957, along with Bob Schrieffer and Leon Cooper, he developed the microscopic theory of superconductivity (the BCS theory) that was to gain him a second Nobel Prize in 1972. After 1975, he was an emeritus professor at Illinois, concentrating on theories for liquid helium-3, which have analogies with the BCS theory.
The electrical properties of semiconductors gradually became understood in the late 1930s with the realization of the role of low concentrations of impurities in controlling the number of mobile charge carriers. Current rectification at metal-semiconductor junctions had long been known, but the natural next step was to produce amplification analagous to that achieved in triode and pentode valves. A group led by William Shockley began a programme to control the number of charge carriers at semiconductor surfaces by varying the electric field. John Bardeen interpreted the rather small observed effects in terms of surface trapping of carriers, but he and Walter Brattain successfully demonstrated amplification by putting two metal contacts 0.05 mm/0.002 in apart on a germanium surface. Large variations of the power output through one contact were observed in response to tiny changes in the current through the other. This so-called point contact transistor was the forerunner of the many complex devices now available through silicon-chip technology.
Ever since 1911, when Heike Kamerlingh Onnes first observed zero electrical resistance in some metals below a critical temperature, physicists had sought a microscopic interpretation of this phenomenon of superconductivity. The methods that proved successful in explaining the electrical properties of normal metals were unable to predict the effect. At very low temperatures, metals were still expected to have a finite resistance due to the scattering of mobile electrons by impurities. Bardeen, Cooper, and Schrieffer overcame this problem by showing that electrons pair up through an attractive interaction, and that zero resistivity occurs when there is not enough thermal energy to break the pair apart.
Normally electrons repel one another through the Coulomb interaction, but a net attraction may be possible when the electrons are imbedded in a crystal. The ion cores in the lattice respond to the presence of a nearby electron, and the motion may result in another electron being attracted to the ion. The net effect is an attraction between two electrons through the response of the ions in the solid. The pairs condense out in a kind of phase transition below a temperature Tc (typically of the order of a few kelvin), and the involvement of the ions of the lattice in the interaction is confirmed by the dependence of Tc on the isotopic mass in the metal - that is, Tc varies with the variation of atomic vibration frequencies in the solid. The requirement of a finite energy to break up a so-called Cooper pair leads to a small energy gap below which excitations are not possible.
The BCS theory is amazingly complete, and explains all known properties associated with superconductivity. Although applications of superconductivity to magnets and motors were possible without the BCS theory, the theory is important for strategies to make Tc as high as possible - if Tc could be raised above liquid-nitrogen temperature, the economics of superconductivity would be transformed. In addition, the theory was an essential prerequisite for the prediction of Josephson tunnelling with its important applications in magnetometers, computers, and determination of the fundamental constants of physics.
Both of Bardeen's achievements have important consequences in the field of computers. The invention of the transistor led directly to the development of the integrated circuit and then the microchip, which has made computers both more powerful and more practical. Superconductivity enables the basic arithmetic and logic operations of computers to be carried out at much greater speeds and may lead to the development of artificial intelligence.
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