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How does superconductivity work?



3_Foto Schwebe-Experiment

Photo: Karlsruhe Institute of Technology

Superconductivity occurs in certain metals (e.g. mercury and lead) when they are exposed to extremely low temperatures – around absolute zero. Superconductors have zero electrical resistance and are surrounded by an inherently stable magnetic field. In 1957 Bardeen, Cooper and Schreiffer proposed the BCS theory to explain the phenomenon. Superconducting metals have a crystal-lattice structure, in which electrons hold the atoms together. But the electrons can also detach themselves and, leaving behind a positively charged atom, move around the lattice relatively freely. That’s how an electric current travels through the material. Under normal conditions, the electrons come across a lot of obstacles as they move. This is known as electrical resistance. Some of those obstacles are the other electrons. Two electrons will repel each other because they are both negatively charged. Another problem is the thermal motion of the atoms, which means the lattice is constantly shaking. But if the metal is cooled, the vibrations slow down and the lattice becomes a kind of soft mattress. Electrons then roll into depressions in this mattress and team up in what are known as Cooper pairs. These traveling companions can now journey through the lattice entirely unimpeded. To describe the phenomenon in full, we would have to turn to quantum theory and that would be going too far for our purposes. One thing it is interesting to note, however, is that the BCS theory only applies to conventional, low-temperature superconductors. High-temperature superconductors, which have a lot of exciting potential for various applications, are still a mystery because we don’t yet know exactly how the Cooper pairs form in them.

Dr. Susann Beetz of the Ideas 2020 Team answered this question.