Presentation Speech by professor Stig
Lundqvist, Chalmers University of Technology
Translation from the Swedish text
Your Royal Highnesses, Ladies and
The 1972 Nobel Prize for physics has been awarded to Drs John Bardeen, Leon N. Cooper and J. Robert Schrieffer for their theory of superconductivity, usually referred to as the BCS-theory.
Superconductivity is a peculiar phenomenon occurring in many metallic materials. Metals in their normal state have a certain electrical resistance, the magnitude of which varies with temperature. When a metal is cooled its resistance is reduced. In many metallic materials it happens that the electrical resistance not only decreases but also suddenly disappears when a certain critical temperature is passed which is a characteristic property of the material.
This phenomenon was discovered as early as 1911 by the Dutch physicist Kamerlingh Onnes, who was awarded the Nobel Prize for Physics in 1913 for his discoveries.
The term superconductivity refers to the complete disappearance of the electrical resistance, which was later verified with an enormous accuracy. A lead ring carrying a current of several hundred ampères was kept cooled for a period of 2 1/2 years with no measurable change in the current.
An important discovery was made in the thirties, when it was shown that an external magnetic field cannot penetrate a superconductor. If you place a permanent magnet in a bowl of superconducting material, the magnet will hover in the air above the bowl, literally floating on a cushion of its own magnetic field lines. This effect may be used as an example for the construction of friction-free bearings.
Many of the properties of a metal change when it becomes superconducting and new effects appear which have no equivalent in the former's normal state. Numerous experiments have clearly shown that a fundamentally new state of the metal is involved.
The transition to the superconductive state occurs at extremely low temperatures, characteristically only a few degrees above absolute zero. For this reason practical applications of the phenomenon have been rare in the past and superconductivity has been widely considered as a scientifically interesting but exclusive curiosity confined to the low temperature physics laboratories. This state of affairs is rapidly changing and the use of superconducting devices is rapidly increasing. Superconducting magnets are often used for example in particle accelerators. Superconductivity research has in recent years resulted in substantial advances in measuring techniques and an extensive used in the computer field is also highly probable. Advanced plans for the use of superconductivity in heavy engineering are also in existence. By way of an example, it may be mentioned that the transport of electric energy to the major cities of the world with the use of superconductive lines is being planned. Looking further ahead one can see, for example, the possibility of building ultrarapid trains that run on superconducting tracks.
Superconductivity has been studied experimentally for more than sixty years. However, the central problem, the question of the physical mechanism responsible for the phenomenon remained a mystery until the late fifties. Many famous physicists tackled the problem with little success. The difficulties were related to the very special nature of the mechanism sought. In a normal metal the electrons more around individually at random, somewhat similar to the atoms in a gas, and the theory is, in principle, fairly simple. In superconductive metals the experiments suggested the existence of a collective state of the conduction electrons-a state in which the electrons are strongly coupled and their motion correlated so that there is a gigantic coherent state of macroscopic dimension containing an enormous number of electrons. The physical mechanism responsible for such a coupling remained unknown for a long time. An important step towards the solution was taken in 1950 when it was discovered simultaneously on theoretical and experimental grounds that superconductivity must be connected with the coupling of the electrons to the vibrations of the atoms in the crystal lattice. The conduction electrons are coupled to each other via these vibrations. Starting from this fundamental coupling of the electrons Bardeen, Cooper and Schrieffer developed their theory of superconductivity, published in 1957, which gave a complete theoretical explanation of the phenomenon of superconductivity.
According to their theory the coupling of the electrons to the lattice oscillations leads to the formation of bound pairs of electrons. These pairs play a fundamental role in the theory. The complete picture of the mechanism of superconductivity appeared when Bardeen, Cooper and Schrieffer showed that the motion of the different pairs is very strongly correlated and that this leads to the formation of a gigantic coherent state in which a large number of electrons participate. It is this ordered motion of the electrons in the superconductive state in contrast to the random individual motion in a normal crystal that gives superconductivity its special properties.
The theory developed by Bardeen, Cooper and Schrieffer together with extensions and refinements of the theory, which followed in the years after 1957, succeeded in explaining in considerable detail the properties of superconductors. The theory also predicted new effects and it stimulated intense activity in theoretical and experimental research which opened up new areas. These latter developments have led to new important discoveries which are being used in a number of interesting ways especially in the sphere of measuring techniques.
Developments in the field of superconductivity during the last fifteen years have been greatly inspired by the fundamental theory of superconductivity and have strikingly verified the validity and great range of the concepts and ideas developed by Bardeens, Cooper and Schrieffer.
Drs. Bardeen, Cooper and Schrieffer,
You have in your fundamental work given a complete theoretical explanation of the phenomenon of superconductivity. Your theory has also predicted new effects and stimulated an intensive activity in theoretical and experimental research. The further developments in the field of superconductivity have in a striking way confirmed the great range and validity of the concepts and ideas in your fundamental paper from 1957.
On behalf of the Royal Academy of Sciences I wish to convey to you the warmest congratulations and I now ask you to receive your prizes from the Hands of His Royal Highness the Crown Prince.
From Nobel Lectures, Physics 1971-1980, Editor Stig Lundqvist, World Scientific Publishing Co., Singapore, 1992
Copyright © The Nobel Foundation 1972