Tunneling
During the 1920s, the French physicist Louis de Broglie introduced the notion that particles can have wave properties. The equations that he, Erwin Schrödinger, and other physicists developed to describe these properties led to some startling new interpretations of particle behavior. One of these properties has come to be known as tunneling.
In classical physics, a charged particle is not allowed to pass through certain regions where the energy is too high for it to overcome. Quantum mechanical wave theory offers a somewhat different perspective on this phenomenon. It says that the particle has some small, but non-zero, chance of passing through that energy barrier provided that the barrier is thin enough. The name given to the phenomenon suggests that the particle may " tunnel under" the apparently insurmountable energy barrier.
No opportunity to test this prediction was available before the 1950s. Then, research on semiconductors resulted in the development of materials in which tunneling can be observed. The first convincing evidence for tunneling was obtained by the Japanese physicist, Leo Esaki, in 1957. Esaki constructed a pn diode in which both semiconductors were heavily doped. The doping resulted in an abundance of electrons in the n semiconductor and an abundance of holes in the p semiconductor. Esaki then connected the two semiconductors by means of a very thin (100 Å) insulating film. He found that a current flowed across the apparently resistant barrier, providing evidence of the occurrence of tunneling.
The next step in tunneling research came about in 1960 when the Norwegian physicist, Ivar Giaever, examined the nature of tunneling in superconducting materials. By carefully selecting the metals used in the diode and the temperatures at which they were maintained, Giaever was able to determine fundamental properties of the superconducting elements. One consequence of his research was the collection of evidence to support the recently announced BCS theory of superconductivity.
The Nobel Prize in physics was awarded in 1973 to Esaki, Giaever, and Brian Josephson for their work on tunneling. Josephson's contribution to that effort came in 1962 when he was still a graduate student. Josephson's study of tunneling led him to predict two kinds of effects, one that would be observed with AC current, and the other with DC current. In the former (AC) case, Josephson predicted the appearance of an oscillating current between the two elements of the diode that would have a frequency of 483.6 megahertz per microvolt drop across the gap. In the latter (DC) case, Josephson predicted that a current would flow across the gap between two superconductors, even if there were no voltage drop across the gap. The two predicted phenomena were soon observed by Anderson and Rowell (the DC effect) and by Shapiro (the AC effect).
The work of Esaki, Giaever, and Josephson not only led to a vastly improved understanding of the fundamental structure of matter, but also has been used in the development of a variety of instruments used in research and industry. Highly sensitive devices known as superconducting quantum interference devices (SQUIDS), for example, are now widely used in instruments such as magnetometers, voltmeters, low-temperature thermometers, and high-speed computers.
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