Why Every Dog Should Love Quantum Physics 6: Superconductors

If you're still not sure whether you should be teaching physics to your dog, here's another good reason: Superconductors.

The "super" in "superconductor" refers to the fact that these materials conduct electric current with absolutely zero resistance, better than the best ordinary metals. This has obvious applications in the green technology field (which dogs should definitely be interested in, as discussed in a previous installment)-- if you could remove the resistance of power lines, you would lose less energy on the way from the generating plant to your home, increasing the energy efficiency of the power grid, and lowering costs for everyone.

It's a lovely idea, but runs into a major snag: most materials becomes superconducting only at temperatures around that of liquid helium, which is just four degrees above absolute zero. The energy required for the necessary cooling systems is enormous, and would wipe out any efficiency gains. Normal superconductors are used only for limited high-tech applications, such as the Large Hadron Collider and other research instruments.

There are, however, "high temperature superconductors," materials that become superconducting at temperatures around that of liquid nitrogen. That's still very cold, but much more reasonable-- in fact, a local company is working on making transmission lines out of high-temperature superconductors.

The obvious question is "can we do even better?" Answering that question will require quantum physics.

Conventional low-temperature superconductivity is reasonably well understood, and is an essentially quantum phenomenon. Electrons inside a superconductor will "pair up" through interactions with the positively charged ions that form the structure of the material-- an electron will tug one ion a little out of place, which in turn pulls on another nearby electron, causing the two electrons to move together like a single particle. Electrons are "fermions," and no two electrons can have exactly the same quantum state, but two electrons together are a "boson," a different class of particle which will happily coexist in large numbers. At very low temperatures, the paired electrons will all "condense" into a single state, which then flows without resistance.

The theory of how a conventional superconductor works was figured out by John Bardeen, Leon Cooper, and Robert Schrieffer, and is colloquially known as BCS theory as a result. Bardeen, Cooper, and Schrieffer won the 1972 Nobel Prize for the theory, which is the cornerstone of modern understanding of low-temperature materials.

BCS theory does not, however, work well for high-temperature superconductors. There's still no really good theory of how those materials become superconductors, which makes it almost impossible to say what the limits of superconductivity are. Would it be possible to make a material that will superconduct at temperatures closer to room temperature than liquid nitrogen temperatures? Nobody knows. The one thing we know is that whatever the ultimate theory of high-temperature superconductivity ends up being, it will be quantum to the core.

If it turns out to be possible to make superconductors that will work at higher temperatures, that discovery would be nothing short of revolutionary. Lossless electric wires, magnetically levitated trains, higher-efficiency motors and other devices-- if any of these things are to become reality, they will take humans and dogs who know quantum physics to do it.

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So are you going to make a full series out of all of these dog posts into a book? Could make you some cash!! You are such an amazing, intelligent man.

Superconductors also come up in the movie Avatar -- Unobtanium, the material being mined in Pandora, is a high temperature superconductor. It's in such high demand presumably because of the energy crisis on Earth.

Love your blog, and can't wait until your book arrives from Amazon! I'm a materials engineering student at nearby RPI -- it's great to read your references to the local area.

The symmetry of Cooper pair condensation excludes a chiral lattice. Are there empirical exceptions?

William A. Little, Phys. Rev. 134 A1416-A1424 (1964)
Exciton-based ambient temperature superconductors: polyacetylenes substituted with polarizable chromophores, [-C(Ar)=(Ar)C-]n. Now easy to synthesize though the benzil, then the dimethylene, then ADMET polymerization.

http://www.mazepath.com/uncleal/pave1.png
Stereogram. Pi-bonds and hydrogens omitted for simplicity.

Somebody should look.