by professor Stig Lundqvist of the Royal
Academy of Sciences
Translation from the Swedish text
Your Majesty, Your Royal Highnesses, Ladies
The 1973 Nobel Prize for physics has been awarded to Drs. Leo Esaki, Ivar Giaever and Brian Josephson for their discoveries of tunnelling phenomena in solids.
The tunnelling phenomena belong to the most direct consequences of the laws of modern physics and have no analogy in classical mechanics. Elementary particles such as electrons cannot be treated as classical particles but show both wave and particle properties. Electrons are described mathematically by the solutions of a wave equation, the Schrödinger equation. An electron and its motion can be described by a superposition of simple waves, which forms a wave packet with a finite extension in space. The waves can penetrate a thin barrier, which would be a forbidden region if we treat the electron as a classical particle. The term tunnelling refers to this wave-like property - the particle "tunnels" through the forbidden region. In order to get a notion of this kind of phenomenon let us assume that you are throwing balls against a wall. In general the ball bounces back but occasionally the ball disappears straight through the wall. In principle this could happen, but the probability for such an event is negligibly small.
On the atomic level, on the other hand, tunnelling is a rather common phenomenon. Let us instead of balls consider electrons in a metal moving with high velocities towards a forbidden region, for example a thin insulating barrier. In this case we cannot neglect the probability of tunneling. A certain fraction of the electrons will penetrate the barrier by tunnelling and we may obtain a weak tunnel current through the barrier.
The interest for tunnelling phenomena goes back to the early years of quantum mechanics, i.e. the late twenties. The best known early application of the ideas came in the model of alpha-decay of heavy atomic nuclei. Some phenomena in solids were explained by tunnelling in the early years. However, theory and experiments often gave conflicting results, no further progress was made and physicists lost interest in solid state tunnelling in the early thirties.
With the discovery of the transistor effect in 1947 came a renewed interest in the tunnelling process. Many attempts were made to observe tunnelling in semiconductors, but the results were controversial and inconclusive.
It was the young Japanese physicist Leo Esaki, who made the initial pioneering discovery that opened the field of tunnelling phenomena for research. He was at the time with the Sony Corporation, where he performed some deceptively simple experiments, which gave convincing experimental evidence for tunnelling of electrons in solids, a phenomenon which had been clouded by questions for decades. Not only was the existence of tunnelling in semiconductors established, but he also showed and explained an unforeseen aspect of tunnelling in semiconductor junctions. This new aspect led to the development of an important device, called the tunnel diode or the Esaki diode.
Esaki's discovery, published in 1958, opened a new field of research based on tunnelling in semiconductors. The method soon became of great importance in solid state physics because of its simplicity in principle and the high sensitivity of tunnelling to many finer details.
The next major advance in the field of tunnelling came in the field of superconductivity through the work of Ivar Giaever in 1960. In 1957, Bardeen, Cooper and Schrieffer had published their theory of superconductivity, which was awarded the 1972 Nobel Prize in physics. A crucial part of their theory is that an energy gap appears in the electron spectrum when a metal becomes superconducting. Giaever speculated that the energy gap should be reflected in the current-voltage relation in a tunnelling experiment. He studied tunnelling of electrons through a thin sandwich of evaporated metal films insulated by the natural oxide of the film first evaporated. The experiments showed that his conjecture was correct and his tunnelling method soon became the dominating method to study the energy gap in superconductors. Giaever also observed a characteristic fine structure in the tunnel current, which depends on the coupling of the electrons to the vibrations of the lattice. Through later work by Giaever and others the tunnelling method has developed into a new spectroscopy of high accuracy to study in detail the properties of superconductors, and the experiments have in a striking way confirmed the validity of the theory of superconductivity.
Giaver's experiments left certain theoretical questions open and this inspired the young Brian Josephson to make a penetrating theoretical analysis of tunnelling between two superconductors. In addition to the Giaever current he found a weak current due to tunelling of coupled electron pairs, called Coopers pairs. This implies that we get a supercurrent through the barrier. He predicted two remarkable effects. The first effect is that a supercurrent may flow even if no voltage is applied. The second effect is that a high frequency alternating current will pass through the barrier if a constant voltage is applied.
Josephson's theoretical discoveries showed how one can influence supercurrents by applying electric and magnetic fields and thereby control, study and exploit quantum phenomena on a macroscopic scale. His discoveries have led to the development of an entirely new method called quantum interferometry. This method has led to the development of a rich variety of instruments of extraordinary sensitivity and precision with application in wide areas of science and technology.
Esaki, Giaever and Josephson have through their discoveries opened up new fields of research in physics. They are closely related because the pioneering work by Esaki provided the foundation and direct impetus for Giaever's discovery and Giaever's work in turn provided the stimulus which led to Jo- sephson's theoretical predictions. The close relation between the abstract concepts and sophisticated tools of modern physics and the practical applications to science and technology is strongly emphasized in these discoveries. The applications of solid state tunnelling already cover a wide range. Many devices based on tunneling are now used in electronics. The new quantum interferometry has already been used in such different applications as measurements of temperatures near the absolute zero, to detect gravitational waves, for ore prospecting, for communication through water and through mountains, to study the electromagnetic field around the heart or brain, to mention a few examples.
Drs. Esaki, Giaever and Josephson,
In a series of brilliant experiments and calculations you have explored different aspects of tunelling phenomena in solids. Your discoveries have opened up new fields of research and have given new fundamental insight about electrons in semiconductors and superconductors and about macroscopic quantum phenomena in superconductors.
On behalf of the Royal Academy of Sciences I wish to express our admiration and convey to you our warmest congratulations. I now ask you to proceed to receive your prizes from the hands of his Majesty the King.
From Nobel Lectures, Physics 1971-1980, Editor Stig Lundqvist, World Scientific Publishing Co., Singapore, 1992
Copyright © The Nobel Foundation 1973