What is Superconductivity? (Part 2)

Continued from Part 1.

The incredible phenomenon of superconductivity was discovered by Onnes in 1911. I have taken advantage of a fictional centenary celebration at Leyden, in the Netherlands, to interview some of the protagonists in this marvellous adventure and have them guide us through a reconstruction of superconductivity’s history up to its open-ended finale.

The big revolution started in the 1980s. Klaus Bechgaard in Copenhagen synthesised the first organic superconductor at high pressure and at temperatures lower than 1.2 K, but the “next big thing” happened in 1986, am I right Dr. Bednorz?

Bednorz: Yes, Muller and I, while working at IBM research labs in Switzerland and studying perovskites, a particular class of ceramic insulating materials, discovered that they are superconductors at temperatures higher than 30 K, which was thought to be the limit by Bardeen, Cooper and Schrieffer.

Muller: Lanthanum-based perovskite showed superconductivity at 35 K, but the interesting thing came later. When substituting lanthanum with yttrium we found superconductivity arising at 92 K! I remember the scientific community being baffled.

Why is that?

Muller: Not only did we go far beyond the theoretical limit of 30 K, but we also went above the temperature of liquid nitrogen – 77 K. That meant opening the door to technological applications for superconductors. In fact, liquid nitrogen is far cheaper than liquid helium. And the problem is that ceramic materials are not good for making into wires or thin films as they are not yielding and tensile like metals.

They have been called unconventional superconductors, am I right? Why is it so?

Anderson: Because BSC theory, which works very well with low temperature metallic superconductors, cannot predict some properties of this class of superconductors. One of the most striking features of these perovskites are the cuprate copper oxide planes, which develop in a sort of 2D chain. This is a key structural unit along which the superconductivity occurs.

The parent compounds of these materials are antiferromagnetic Mott insulators. This means that the spin of the electrons are aligned anti-parallel and that electrons cannot conduct because of a strong columbic repulsion, which can be decreased by doping the material. Doping consists of inserting other elements which add charge in the material. This effect cannot be explained by BSC theory.

What is the highest temperature so far reached by a superconductor?

Chao: It is a cuprate-based structure with mercury, barium and calcium. The critical temperature is 138 K, but under high pressure it can reache 150 K.

And that’s not all, is it? Superconductivity still holds many surprises.

Hosono: Yes, it does. In 2001 Nagamatsu and other Japanese researchers discovered magnesium diboride superconductor, operating at temperatures of 39 K, above the limits predicted by BCS theory, but very different to the unconventional perovskites superconductors from Bednorz-Muller.

Magnesium diboride is one of the cheapest superconducting materials and is far more efficient than niobium alloys for superconductive magnet applications in particle accelerators and medicine.

Then, in 2008, we discovered a new class of superconductors called oxypnictides. They are based on iron, arsenic, oxygen and rare elements such as lanthanum and do not contain cuprates, copper oxide planes. When doped with fluorine they become superconductors. The compound with lanthanum had a critical temperature of 26 K, but Prof. Zhao found that substituting lanthanum with samarium could raise the temperature to 55 K.

Zhao: They are still unconventional superconductors but the physics behind seems different from the class of perovskites, discovered by Bednorz and Muller.

Superconductivity never ceases to wonder us, with new materials showing this property, from carbon nanotubes to tungsten oxide doped with sodium. But how do all these new superconductors work? What is the physics and the explanation of superconductivity behind these unconventional approaches?

Grant: This is what I call ‘The Great Quantum Conundrum’: there are many ideas under the sun, but nobody knows exactly which are right. Cooper electron pairs also seem to be present in unconventional superconductors and they show an isotope effect that was observed by Maxwell and Reynolds in conventional superconductors. Hence, phonons seem to play a role again in creating the electron pairs. Nevertheless, there is also circumstantial evidence that spin fluctuations are also important. The spins or intrinsic magnetic moments of electrons are responsible for a coupling that makes them act as pairs. Still, what exactly a spin fluctuation is and the way this allows the electrons not to ‘see’ the crystal lattice, nobody has clearly explained.

So it seems to me that a big controversy is still present here. It’s incredible that over the last 100 years, when accurate models of the structure of matter and the universe have been created, and ingenious experiments have been designed to test them, we still do not have an exhaustive explanation of superconductivity!

Prof. Onnes, what do you think? Do phonon interactions play a role or are spin fluctuations alone responsible for your greatest discovery of the Twentieth Century?

Onnes: Phonon what? Spin fluctuation? What are you talking about?

Hmm …Yes, let’s leave it to 2012. Let me offer you another cocktail. Here’s to 100 years of superconductivity!

Image: flickr | agaudin

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