Kristian Fossheim - Superconductivity: Discoveries and Discoverers: Ten Physics Nobel Laureates Tell Their Story

Kristian Fossheim 1

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Abstract

Physics is a science which aims at answering the big mysteries in Nature. Physicists have always been attracted by the greatest challenges. But sometimes even the most demanding problems reveal themselves little by little. On the 8th of April 1911 a discovery was made through an apparently simple experiment in a glass flask of very special design in a physics laboratory in Leiden, Holland. The experiment set in motion a series of events with few parallels in the history of science. But physics was far from ready for the advent of superconductivity, the enigmatic phenomenon which Heike Kamerlingh Onnes and his student Gilles Holst had just observed. Today, more than a hundred years later, after great scientific research efforts and big investments, and after many impressive scientific and technical breakthroughs, a cloud of mystery still hovers over aspects of superconductivity. Nature continues to play her elusive game with the best minds of physics.

Physics is a science which aims at answering the big mysteries in Nature. Physicists have always been attracted by the greatest challenges. But sometimes even the most demanding problems reveal themselves little by little. On the 8th of April 1911 a discovery was made through an apparently simple experiment in a glass flask of very special design in a physics laboratory in Leiden, Holland. The experiment set in motion a series of events with few parallels in the history of science. But physics was far from ready for the advent of superconductivity, the enigmatic phenomenon which Heike Kamerlingh Onnes and his student Gilles Holst had just observed. Today, more than a hundred years later, after great scientific research efforts and big investments, and after many impressive scientific and technical breakthroughs, a cloud of mystery still hovers over aspects of superconductivity. Nature continues to play her elusive game with the best minds of physics.

It seems right, after passing the 100 year milestone, to take stock of the intellectual property upon which we stand in this field, and from which basis scientists launch further expeditions into the remaining enigma. Physicists fascination with superconductivity prevails, and continues to attract new generations.

The year 1911 would turn out to be a great year in science history for an additional reason: The discovery of the atomic nucleus by Rutherford. Subsequently, the first model of atomic structure, the Bohr model, followed in 1913. The impact on science would be tremendous. 1911 will forever be a year hard to match in the annals of science.

It is not unusual in science history that an apparently simple observation opens a Pandoras box with wide-ranging consequences. H. K. Onnes studied electrical resistance in a metallic wire, hardly something that could change the world, you should think. In 1911 it was already known that electrical resistance in metals diminishes gradually and continuously as temperature is lowered more and more below the ambient. This fact had been carefully established by recent research, not only in Leiden. But Leiden had established itself as one of the central research arenas in the new field of low temperature physics, in a combination of curiosity driven search into new territory and development of fabulously sophisticated glass blown cooling devices. When H. K. Onnes and his team, in 1908, after many years of systematic efforts managed to condense the noble gas helium, the path was laid for unprecedented study of the low temperature properties of matter; gases, liquids and solids.

A problem which had been much debated at the time was what would be the ultimate low temperature behaviour of electrical resistivity on approaching zero degrees on the Kelvin scale. How low could the resistivity ultimately become? Would resistivity continue to decrease, and gradually vanish for all practical purposes? Or, would the current carriers eventually freeze, or stick to the atom like some thought, thus preventing the charge carriers from participating in electrical conduction, forcing resistivity to increase again?

What H. K. Onnes and coworkers discovered, was something entirely different from both of these alternatives, and completely surprising: Resistance- and hence resistivity- in solid frozen mercury metal filaments vanished abruptly at about 4.2 K degrees above absolute zero, or at about minus 269 degrees centigrade, and remained zero at all lower temperatures. This phenomenon was called superconductivity. The temperature where it happens defines a dividing temperature which, as it would later turn out, is characteristic of each metal, and is called the superconducting transition temperature, Tc. In the pure metals of the periodic table, Tc would typically be below 10 K. As years went by, most but not all metals were found to be superconducting at low enough temperatures. Famous examples of non-superconductors, paradoxically it seemed, were the best metals like gold, silver and copper. Soon also a great variety of metallic alloys were found to possess the superconducting property. But no explanation could be found at the time.

It would be wrong to say that the world of science stood in awe of the new discovery. When H. K. Onnes received the Nobel Prize in physics in 1913, superconductivity was not even mentioned. Rather, the emphasis was on Onnes great feat in low temperature science and technology leading to condensation of the highly volatile inert gas of helium. It would later turn out that yet another important property of superconductors had still to be discovered. Nature reveals its secrets only when the appropriate questions are asked through precisely designed experiments. As is often the case, the problem was to know which question to ask.

It would take another 22 years before that next step was achieved, in 1933, when the deeper nature of superconductivity was revealed in a magnetic experiment by Walther Meissner and Robert Ochsenfeld in Germany. Before discussing that experiment, let us first sidestep a little and recall some simple facts: The most common metals, like lead, tin and aluminium, are classified as very weak paramagnets. This important characteristic is due to the fact that although electrons have the ability to align their magnetic moments with an applied magnetic field, and thus reinforce an externally applied field, only a very tiny fraction, those with the highest kinetic energy, are allowed to do so in a metal. This is due to the lack of available quantum states for most electrons into which they can accomodate if their magnetic moment is turned parallel to the field. Therefore, the number of electrons oriented parallel and antiparallel to the field, respectively, are almost equal, and the magnetism of the gas of freely moving electrons in a metal is almost zero. This is what is characterized as weak paramagnetism.

The second aspect of superconductors, discovered in 1933, came just as unexpectedly as the sudden loss of electrical resistivity in 1911. A piece of metal was first held in the normal state above Tc, while its entire body was permeated by an externally applied magnetic field from the solenoid in which it was located. The resulting magnetic field inside the sample was then very nearly the same as that outside, as described above. The sample was then cooled through the critical temperature Tc. On passing Tc, it was recorded that the magnetic field inside was suddenly and completely expelled. Hence, by lowering the temperature by just a small fraction of a degree, the material changed its magnetic character completely, from weakly paramagnetic above Tc, to a state of complete screening, with no magnetic field in the body below Tc, i.e. perfect diamagnetism. This must have been caused by the sudden creation of an opposing field which exactly cancelled the applied field inside. This remarkable behaviour, never observed before, is referred to as the Meissner effect , a phenomenon which ranks among the greatest theoretical challenges ever encountered in the history of physics. It was demonstrated that this constituted a new thermodynamic state, and that it was not a consequence of infinite conductivity. The deeper nature of the Meissner effect as a realisation of the Higgs mechanism was discovered almost 30 years later by Anderson, as told by him in the Anderson chapter of this book. Further comments on the Higgs mechanism are given in Chap..