Presentation Speech by Professor Stig Lundqvist of the Royal Academy of Sciences

Translation from the Swedish text

Your Majesties, Your Royal Highnesses, Ladies and Gentlemen,

This year’s Nobel Prize for Physics has been awarded to Professor Klaus von Klitzing for the discovery of the quantized Hall effect.

This discovery is an example of these unexpected and surprising discoveries that now and then take place and which make research in the sciences so exciting. The Nobel Prize is sometimes an award given to large projects, where one has shown great leadership and where one with ingenuity combined with large facilities and material resources has experimentally verified the correctness of theoretical models and their predictions. Or, one has succeeded through creation of new theoretical concepts and methods to develop theories for fundamental problems in physics that resisted all theoretical attempts over a long period of time. However, now and then things happen in physics that no one can anticipate. Someone discovers a new phenomenon or a new fundamental relation in areas of physics where no one expects anything exciting to happen.

This was exactly what happened when Klaus von Klitzing in February 1980 was working on the Hall effect at the Hochfelt-Magnet-Labor in Grenoble. He discovered from his experimental data that a relation which had been assumed to hold only approximately seemed to hold with an exceptionally high accuracy and in this way the discovery of the quantized Hall effect was made.

The discovery by von Klitzing has to do with the relation between electric and magnetic forces in nature and has a long history. Let us go back to 1820, when the Danish physicist H.C. Ørsted found that an electric current in a wire influenced a compass needle and made it change its direction. He discovered this phenomenon in a class with his students. No one had seen a relation between electric and magnetic forces before. More than 50 years later a young American physicist, E.H. Hall, speculated that the magnetic force might influence the charge carriers in a metallic wire placed in a magnetic field and give rise to an electric voltage across the wire. He was able to show that when sending an electric current through a strip of gold there was a small voltage across the wire in a direction perpendicular both to the current and the magnetic field. That was the discovery of the Hall effect.

The Hall effect is now a standard method frequently used to study semiconductor materials of technical importance, and the effect is described in all textbooks in solid state physics. The experiment is in principle very simple and requires only a magnetic field plus instruments to measure current and voltage. If one varies the magnetic field, the current and voltage will change in a completely regular way and no surprising effects are expected to happen.

von Klitzing studied the Hall effect under quite extreme conditions. He used an extremely high magnetic field and cooled his samples to just a couple of degrees above the absolute zero point of temperature. Instead of the regular change one would expect, he found some very characteristic steps with plateaus in the conductivity. The values at these plateaus can with extremely high accuracy be expressed as an integer times a simple expression that just depends of two fundamental constants: the electric elementary charge and Planck’s constant which appear everywhere in quantum physics.

The result represents a quantization of the Hall effect – a completely unexpected effect. The accuracy in his results was about one part in ten million, which would correspond to measuring the distance between Stockholm and von Klitzing’s home station Stuttgart with an accuracy of a few centimeters. The discovery of the quantized Hall effect is a beautiful example of the close interrelation between the highly advanced technology in the semiconductor industry and fundamental research in physics. The samples used by von Klitzing were relined versions of a kind of transistor we have in our radios. His samples, however, had to satisfy extremely high standards of perfection and could only be made by using a highly advanced technique and refined technology.

The quantized Hall effect can only be observed in a two-dimensional electron system. Two-dimensional electron systems do not occur in nature. However, the development in semiconductor technology has made possible the realization of a two-dimensional electron system. In the kind of transistor that von Klitzing used, some of the electrons are bound to the interface between two parts of the transistor. At sufficiently low temperature the electrons can move only along the interface and one has effectively a two-dimensional electron system.

von Klitzing’s discovery of the quantized Hall effect attracted immediately an enormous interest. Because of the extremely high accuracy the effect can be used to define an international standard for electric resistance. The metrological possibilities are of great importance and have been subject to detailed studies at many laboratories all over the world.

The quantized Hall effect is one of the few examples, where quantum effects can be studied in ordinary macroscopic measurements. The underlying detailed physical mechanisms are not yet fully understood. Later experiments have revealed completely new and unexpected properties and the study of two-dimensional systems is now one of the most challenging areas of research in physics.

Professor von Klitzing,

On behalf of the Royal Swedish Academy of Sciences I wish to convey our warmest congratulations and ask you to receive your prize from the hands of His Majesty the King.