Physicists aren’t exactly known for their clarity, but you can’t get much clearer than “superconductivity.” Superconductivity is a phenomenon that occurs when materials are cooled below a critical temperature, allowing electrical current to flow through them without resistance. Superconductivity has many technological implications. If it could be harnessed fully, superconductivity would allow for hyper-efficient machines that could sustain electrical currents indefinitely.
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When certain metals are cooled below a certain temperature they exhibit superconductivity. This point is known as the metal’s critical temperature. Unfortunately, cooling a potential superconductor to its critical temperature is not easy. However, since the 1980s high temperature superconductors have been identified. These materials (typically ceramics) exhibit superconductivity at a critical temperature above 77 K. Materials can be cooled to this point via liquid nitrogen. Unfortunately, the brittle nature of most ceramics makes them poor building material for superconducting wires.
Superconductivity is made possible thanks to the pairing of electrons in superconductors. These electron pairs are called Cooper Pairs. In metals, negatively charged, detached electrons distort the metal’s rigid lattice of positive ions. This distortion ends up attracting more electrons and allows the electrons to form Cooper Pairs. At the critical temperature, all electrons in the metal form Cooper pairs. Once paired, all electrons behave as a unit, allowing them to flow through a metal without resistance.
BCS Theory was developed in 1957 by Bardeen, Cooper, and Schrieffer to explain the behavior of superconductors. BCS Theory described the coupling of electrons through lattice distortion and vibration as a phonon-mediated interaction. Phonons are special quantum particles that describe the energy of a vibrating lattice. BCS Theory also formalized the isotope effect: a superconductor’s critical temperature is inversely proportional to the mass of the isotope in the superconductor. This effect was first proposed following experiments involving superconductive mercury but was not reproduced until BCS performed the experiments.
The Meissner Effect is a secondary phenomenon of superconductivity. This effect describes the exclusion of magnetic fields within a superconductor. The London Equation accounts for the Meissner Effect by stating that a magnetic field decays exponentially from the surface of a superconductor to its interior. The distance a magnetic field penetrates into a superconductor before it entirely decays is known as the London Penetration Depth.
Ginzburg-Landau (GL) Theory is another theory describing superconductor behavior. GL Theory initially described Type I superconductors (typically pure metals, not alloys), including the famous high-temperature superconductor Yttrium Barium Copper Oxide (YBCO). GL Theory introduces a value for superconductors known as coherence length. Coherence length refers to the minimum change that can be made to a superconductor’s electron density without destroying the material’s superconductive state.
Quizbowl is about learning, not rote memorization, so we encourage you to use this as a springboard for further reading rather than as an endpoint. Here are a few things to check out:
We know how to induce superconductivity at low temperatures. What about room temperature?
Phonon-electron interaction is crucial to inducing superconductivity. Visit this website to learn a little more about what exactly a phonon is.
Check out this video for a visual overview of the physics of superconductors.
No, superconductors don’t drive trains. But they can run on tracks.
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