Superconductivity at Ordinary Temperatures

Superconductivity was discovered more than 100 years ago. Electricity can be carried on a wire with exactly zero resistance. Think of hundreds of thousands of miles of wires in this country alone, with huge transformers that ramp up the voltage to hundreds of thousands of volts in order to minimize losses in the wire (which are inversely proportional to voltage) — and still transmission losses are over 20% between the power plant and your home.

With superconducting wires, the losses would be zero, and (maybe) the losses could be eliminated.

There are other applications as well. MRI scans at hospitals are expensive mostly because they require huge electromagnets. Superconducting magnets would make MRI technology much more accessible. And superconducting magnets would allow trains to levitate above the tracks with no wheels and no rolling resistance.

The catch has always been that superconductivity only happens at very low temperatures — so low that the metal must be bathed in liquid helium, just a few degrees above absolute zero. Not so practical.

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Then, in 1986, an exotic material was manufactured that conducted at temperatures that were low but not ultra-low. We’re talking in the neighborhood of 100 degrees above absolute zero, which is still deep in negative territory on our usual Centigrade or Fahrenheit scales. Liquid nitrogen could be used in place of liquid helium. Liquid nitrogen is a much more common and cheaper material, easier to make and much easier to store; and our atmosphere has a limitless supply of nitrogen, whereas helium is rare and expensive.

Liquid nitrogen is cheap enough, but the alloys that conduct at liquid nitrogen temperature are difficult to manufacture and to shape into wires, and they contain rare earths. So there have been no practical applications. The biggest magnets in the world, using huge currents that require superconductivity, are buried under the Alps at the European Center for Nuclear Research (CERN). These still use conventional liquid helium superconductors, though there is perpetual talk about switching them over to “high-temperature” superconductors, where “high temperature” means liquid nitrogen.

The news

Just this Tuesday, a physicist from University of Rochester spoke at the annual meeting of the American Physical Society, where he announced he had a material that superconducts at room temperature — no refrigeration needed. Wednesday, their paper appeared in Nature.

There are lots of caveats.

  • The superconducting material is made from lutetium. If you haven’t heard of lutetium, I can’t blame you. It’s hard to find, and costs three times as much as gold.
  • Its superconductivity depends on an enormous pressure, available routinely in physics labs but not commercially practical.
  • About half the physicists who look at the data are not convinced that the Rochester lab is really seeing superconductivity.
  • …because, sadly, the Rochester group has had a controversial history with accusations of fraudulent scientific claims.

Still, it’s an exciting time. If the discovery pans out, it’s a whole new class of materials in a whole new temperature range. This announcement is sure to lead to new lines of research and a spate of new discoveries. Stay tuned.

Article in Quanta Magazine

Theory of superconductivity

Superconductivity was discovered in 1911, when quantum mechanics was a gleam in the eye of Max Planck, and there was not yet any basis for understanding how it comes about. Today, superconductivity is understood in a general way, but there is yet no theory detailed enough that we might predict what we have to do in order to make better superconductors. 

I wrote a few weeks ago about bosons and fermions as two classes of particles in quantum mechanics. Fermions are contrarians. “Whatever he’s doing, I’m going to do something else.” Bosons are hypersocial. “Let’s do this all together as one.”

Electrons are fermions, and they can be convinced to move in the same direction by continually applying an electrical force (a voltage), but they have no inclination to cooperate with one another. But pairs of electrons are bosons. If some force links electrons together in pairs, then they will tend to move in a coordinated way. Once they start moving in a circuit together, they would rather stay in that collective state, and no voltage is necessary to keep the current flowing. 

How are the electrons joined in pairs? The electrons’ need to be contrarian can be satisfied merely by spinning in the opposite direction, then two electrons have no objection to being in (otherwise) the same quantum state. This pairing of electrons has been a well-entrenched part of the quantum theory of chemistry from the outset, in the 1930s. Normally, electrons are paired as they are tightly bound within a single atom. In metals, there are free electrons, “conduction electrons” that are not connected to any particular atom. And in superconductors, some small fraction of these conduction electrons pair up (with opposite spins) so that they can behave as boson pairs rather than individual fermions. 

This is the BCS theory (1957), which won a Nobel for John Bardeen, Leon Cooper, and John Schrieffer. It is understood in a general, qualitative way. But searching for materials and conditions that support superconductivity remains a trial-and-error experimental science.

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