Tempo di lettura stimato: 8 minuti

You can find the first part here!

A trace of wondering surrounding the discovery of superconductivity persists even today. Books about it begins like fairy tales: ”In Leiden in 1911, Kamerlingh Onnes…”. Only the “Once upon a time is missing”.

However, we are romantic spirits, Billy, so we can create a poetic and engaging framework to the infinite masterpiece of superconductivity. Then…

Once upon a time, Kamerlingh Onnes was studying the reduction of resistance at low temperatures in Leiden, focusing on mercury. The most challenging endeavour for him was how to reproduce a temperature near to absolute zero (-273 °C). Even in the most rigorous Siberian winter, temperature gets only very rarely below -60 °C. Leiden was the only place in the world at that time where anyone knew how to create such an intense cold. After a lot of attempts and disappointments, Onnes’s mission regarding the resistivity of solid mercury at cryogenic temperatures was finally accomplished by using liquid helium as a refrigerant. On April 8, 1911, 16:00 hours, Onnes noted “Kwik nagenoeg nul” on his notebook, which in elfin Dutch translates as “[Resistance of] mercury almost zero.”. He observed that at the temperature of 4.19 K, resistivity abruptly disappeared, becoming thousands of times less in amount relative to the best conductor at ordinary temperature. Onnes later reversed the process and found that at 4.2 K resistance became again relevant in the material. The scientist, proud and excited, immediately understood the importance of his discovery and tested it performing an experiment. Onnes introduced an electric current into a superconductive ring and removed the battery that generated it. Upon measuring the electric current, Onnes found that its intensity did not diminish with time. The current persisted due to the superconductive state of the conductive medium, since no resistance was obstructing it. Eureka! Onnes published articles about the phenomenon and, for his research, he was awarded the Nobel Prize in Physics in 1913.

By 1913, physicists from all over the world were looking forward to making pilgrimages to Leiden to observe Onnes’s remarkable result. However, in 1914 other events were occurring in Europe that temporarily eclipsed the discovery of superconductivity. Although Holland remained neutral in the Great War, all researches at Leiden were forced to stop until 1919, when the Leiden group started to work on superconductivity again, discovering a number of new superconducting elements, for example tin.

Some years later, Leiden was not the reign of liquid-helium any longer because this special skill became available also in other countries. The German Walther Meissner rapidly became a Master of liquid-helium and in 1933 he started an experiment designed to determine whether the current in a superconductor flows on its surface or in its bulk. A very small coil was used to measure the magnetic field between two cylinders of tin, carrying current in parallel. Meissner chose his valiant pupil Robert Ochsenfeld to carry out the experiment. The result was dramatic and unexpected: even when the tin cylinders were not carrying any current, the magnetic field between them increased when they were cooled into their superconducting state. Such an increase just outside the sample was due to an expulsion of the applied field inside the material, resulting in a denser and not uniform distribution of the field lines in the immediate external region. This was a piece of evidence that superconductors were not only perfect conductors (zero resistance), but also perfect diamagnets (repulsion of magnetic flux density). This latest discovery was curious, but nobody could understand its real and huge potential in practical use, yet: magnetic levitation. However, the discovery of the Meissner effect was a crucial turning point: superconductivity acquired the status of thermodynamic phase transition.
In 1934, a couple of German brothers, Heinz and Fritz London, were studying the electromagnetic behaviour of superconductors, in Oxford. They had fled from Germany because of the Nazis’ race laws, looking for a more peaceful and safer atmosphere. So far, superconductivity had introduced new experimental knowledge, that seemed to clash with the ancient Sacred Commandments of Physics: Maxwell’s laws. Thus, they work hard to adapt Maxwell’s laws on the base of the two main properties of superconductors, in the light of the principle of coherence and symmetry that governs the world. They ended up with the so called London equations, where an important role is played by the “London penetration depth”, which defines the thickness of the superficial layer in which supercurrents arise, guaranteeing that Meissner effect takes place.

The Second World War interrupted research in superconductivity just as the First World War had done in the past. During these dark period, scientists’ attention was focused, by choice, by force or by misunderstanding, on other physical issues, whose purposes were not always so noble. Light shone again on superconductivity in the ‘50s, when real progress began once again to be made. This was a greatly fruitful decade: new important characters came out, whose contribution had been exceedingly precious, not only for the new results themselves, but also because the interconnections between the different perspectives became clearer and clearer.

At the beginning, the phenomenon still resisted any true microscopic understanding, but some pieces of the puzzle did begin to come together, particularly in the phenomenological model of Vitaly Ginzburg and Lev Landau. The Ginzburg-Landau theory managed to combine important elements of the Londons’ picture of superconducting electrodynamics with Landau’s earlier analysis of second-order phase transitions, based on symmetry observations. In the meantime, a young theorist in Landau’s group in Moscow, Alexei Abrikosov, proposed a new class of superconducting materials, that he then called “Type II superconductors”. One day, during his researches, he discovered that not all superconductors are characterized by a critical surface that clearly separates the normal behaviour by the superconducting state, where Meissner effect occurs. Some of them exhibit a particular third intermediate state which admits the penetration of flux lines, not continuously, but in a regular arrangement of discrete entities: the future Abrikosov vortex lattice. Scientists will soon understand that the interaction of such a peculiarity with the Meissner effect is able to produce the result we are looking for.

Another team was about to take part to the endeavour: John Bardeen, already well-known for his work leading to the discovery of the transistor, Leon Cooper and John Robert Schrieffer. Bardeen had to leave them for some months in 1956, when he had to go to Stockholm to be awarded the Nobel Prize for the transistor, but he joined again his two companions in 1957. They focused on the microscopic aspect of superconductivity and worked furiously, because they were afraid to be scooped by Richard Feynman. In fact, he was a fearsome rival since he was working on the same topic; the hints he caught and his intuition were rapidly bringing him near the solution, becoming a threat for the coveted award: prestige, glory, and, why not, the Nobel Prize.
However, finally it was not Feynman, but Bardeen, Cooper, and Schrieffer who produced the microscopic theory: an elegant formulation which takes into account all of the puzzling phenomena of superconductivity and succeeds in making new predictions, shortly after confirmed by experiment. They won the Nobel Prize in 1972 (the second for Bardeen) for what will soon become popular as the BCS theory.

Almost 50 years after its discovery, superconductivity could, at long last, reasonably be said to have been explained, at least in its main aspects.

And everyone lived happily ever after…

The following part on Monday 19/03/18! Stay tuned Billy: you can subscribe here to our newsletter!

Pubblicato da Giulia Maffeis

Crede che esistano due cose infinite: l'universo e il suo amore per la fisica, ma riguardo la prima nutre ancora dei dubbi.