Synchronized Swimming
Thursday, November 4, 2010 at 8:52AM
Vatche Sahakian in frosh, quantum

Three key conceptual ingredients underly the phenomenon of superconductivity, each one more beautiful than the other. When certain materials are cooled down to very low temperatures, it is found that - at some critical temperature - the material is able to conduct electricity without any loss of energy! Hence the term "super"conductivity. The importance of practical applications of this cannot be overstated. Its conceptual physics underpinnings are however even more dramatic. Here are the three key ideas involved in this remarkable phenomenon.

(1) The quantum world - i.e. the real world - looks very different from the world we are used to (see this and this previous posts for more). In reality, all matter is to be described by waves of probabilities - waves that behave very much like those on the surface of the ocean. What's waving is not water, but the likelihood that a particle of matter is here or there. And these probability profiles propagate, combine, and scatter much like water waves do. When one has a large number of constituents in a system, the wavelets of each particle add up incoherently and the quantum effects are washed away (no pun intended) at large macroscopic distances. Occasionally, one can however arrange to sync the wavelets of matter so that they reinforce each other and form a substantial probability tsunami that brings the freaky quantum world into our own safe and naive realm. This happens typically when you cool down a system of particles to low enough temperatures. However, the effect is most dramatic when the particles are bosons instead of fermions (see post on bosons and fermions). The result is what is known as a Bose condensate: a collection of quantum wavelets in sync and adding up to a macroscopic probability profile. This is for example the case for photons in a laser, electrons in superconductivity, and molecules in superfluidity (see post on superfluidity for a bit more).

(2) A typical metal can be visualized as follows: a lattice of atomic nuclei spread around in a regular pattern - forming the scaffolding of the material; and waves of electrons propagating through the lattice randomly. When a piece of metal is hooked up to a battery, the electrons are forced into motion in a certain direction, like water through a pipe. However, electrons are attracted by the nuclei and repel each other through the electromagnetic force. The result is that the flow of electrons is not a smooth one; it is interrupted by various obstacles - other electrons and nuclei in the material. This is a source of energy loss for electrical energy flow. Hence, current in a metal typically involves "resistance", a mechanism for loss of energy. When certain metals are cooled down to very low temperatures, there is the possibility for the electrons to form a coherent Bose condensate - generating an electrical flow more akin to soldiers walking in step and reinforcing the efficiency of the flow. But the electrons are not bosons, they are fermions… To realize the quantum syncing of electron wavelets as needed, we would want bosonic particles. Nature comes up with an amazing solution to this puzzle: if you bind two electrons together - as in an purely "electron atom" - the result is a boson. From my previous post about bosons and fermions, you may remember that a boson is associated with integer spin, and a fermion with half-integer spin. Electrons have spin 1/2; two bound together necessary generate integer spin, and hence a bosonic entity. So, pairs of bound electrons - known as Cooper pairs - can potentially form a Bose condensate at low enough temperatures, which then can carry electrical energy flow through the material at very high efficiency, all the way to 0% energy loss: a quantum effect involving the syncing of probability waves and manifesting itself through a dramatic macroscopic effect…

(3) But there is a problem with this story: electrons are attracted to the metal nuclei but repel each other electromagnetically… How the heck can you then bind two together? Once again, Nature kicks in with an amazingly intricate solution. At low enough temperatures, in certain materials the lattice of atomic nuclei deforms under the influence of the electrons flowing through it. It does so in a very special way. The accompanying figure shows a cartoon of the setup. The deformed nuclei mediate a pull between the two electrons, hence binding them through a precarious balance of forces! The mechanism is ingenuous, counter-intuitive, and delicate: all ingredients of Nature working at its best.

With all these factors colluding together, certain materials can transfer electrical energy or current with no energy loss - at low enough temperatures. But this is not the end of this amazing story. Superconductors exhibit other dramatic effects having to do with their interactions with magnets. For this, we will need to talk about two types of superconductors - enigmatically called type I and type II. But we'll need another post just to address this subject. Meanwhile, have also a look at the videos in my previous post to see superconductors in action.

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