Physicists Set a New Record for High-Temperature Superconductivity

And what that actually means.

Aug 23 2015, 11:30am

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203 degrees Kelvin isn't exactly balmy. It winds up being about -70 degrees Celsius actually, but that's still warm enough—hot enough by superconductance standards—to constitute a huge advance in warm-temperature superconducting technology. This temperate leap, which comes courtesy of physicists at the Max Planck Institute, brings superconductivity into the range of natural Earth surface temperatures, offering a glimpse of an electronics engineering holy grail: a room-temperature superconductor.

The group's work is published in the current issue of Nature.

The big deal behind superconductivity isn't hard to see (even literally). By definition, a superconductor offers exactly zero electrical resistance—no energy is lost as heat. The classic illustration of the concept is the infinite electric loop, in which some current supplied to a ring of superconducting material will persist indefinitely (billions of years, according to calculations) with no additional power source.

So, imagine a smart-phone or laptop that requires no accommodations for cooling; it'd be the end of one of the central challenges of designing and building electronics. No more heat sinks, no more inefficiency. Superconductivity doesn't come easy, however.

To see how the Max Planck group's high-temperature superconductor works, we need to know a bit about how superconductors work, generally. You can think of electrical resistance in the most literal, macroscopic sense. Electrons cruise along through some conductor and are impeded by other particles, and the result is like an electronic Plinko game, only gravity is replaced by voltage. Particles collide, energy is lost, and heat is released.

In a superconductor, all of the pegs are removed from the Plinko board and the disc is free to fall unimpeded. This happens as those pegs, which are the free electrons hanging around in a conductor, suddenly start pairing up into what are known as Cooper pairs. Electrons shouldn't do this, of course, because as particles with like charges they naturally repel. But given the right conditions they do the opposite and attract, albeit very weakly.

It goes like this. The electrons couple to the conductor's underlying lattice structure via quasiparticles called phonons, which are essentially units of vibrational energy. In a material with an underlying lattice structure, like a crystal or metal, the particles don't get to vibrate on their own and so the whole arrangement has to vibrate together, which is manifested as phonons. The rough sketch of it is that as an electron cruises around a rigid lattice, it exerts a force on it, and the lattice bends a bit.

Other electrons are attracted to this slight cavity (which is really a bit of positive charge) and we wind up with pairs of electrons hanging out together.

Together, the particles are able to travel through the conductor impervious to the usual scattering mechanisms of electrical resistance. The catch is that the bonds between the particles of Cooper pairs are all but nothing in terms of energy (10-3 eV) while at room temperature those same particles encounter energies up to 1/40th of an eV, which is a whole lot more by several orders of magnitude.

So, superconductivity usually requires extreme cold because this is where Cooper pairings can exist in the absence of overriding thermal energy. The normal temperature threshold for this is about 10 degrees Kelvin, or -263 degrees Celsius. I say usually because there are ways to create superconductivity "unconventionally" at pretty high temperatures using elaborate composite materials—and mechanisms outside of the lattice-Cooper pair relationship above—rather than the more fundamental-seeming superconductivity seen in cold-temperature Cooper pair-based arrangements. Cheating, in other words.

The Max Planck team offers conventional superconductivity at unconventional temperatures, basically. This possibility has been long theorized, and the notion that there is a fundamental temperature limit to superconductivity was debunked as far back as 1970.

"In the late 1960s, [Neil Ashcroft] and [Vitaly Ginzburg] proposed that, if hydrogen could become metallic, the energy of its ionic vibrations would be so high that even a moderately strong electron–ion coupling could result in a rather high [superconductivity transition temperature]," writes physicist Igor I. Mazin in a separate Nature commentary. "Unfortunately, metallization of hydrogen has proved to be extremely difficult. It was then pointed out that hydrogen-rich compounds might be better targets, but it is only now that this idea has been realized."

The key composite behind the current research, what made the new record possible, is sulfur hydride—yes, farts. This compound is what allowed for the 203 K threshold, along with 90 gigapascals of pressure, the creation of which has only recently become possible. So, we're still not talking about everyday conditions, but, overall, we are now much, much closer.