Out of cold storage

Work is progressing on several fronts to take superconductors out of the laboratory fridge into the real-world.

Superconductivity could open up a new generation of low-loss power cables, super-powerful magnets and a whole new class of high-speed electronic circuits. The promise has been there since Dutch physicist Heike Kamerlingh Onnes first described the phenomenon in 1911. But progress is not quick, particularly in materials that can superconduct at temperatures above even those found in the deep Antarctic winter.

The first materials discovered to exhibit superconductivity were very pure metals, or alloys of pure metals, cooled close to absolute zero with liquid helium. These low-temperature superconductors (LTS) find practical application in things such as MRI machines, but the cooling requirements are onerous. So-called high-temperature superconductors (HTS), which do not need the embrace of liquid helium are usually ceramic combinations of metals such as yttrium, barium and strontium with copper oxides. The first candidates were discovered at IBM's Zurich research lab in 1986, and superconduct at temperatures from around 90K up to almost 140K, making them easier to cool with liquid gases or mechanical refrigerators. Their mechanical properties make them difficult, but not impossible, to turn into wires or superconducting magnets.

The search for new materials and material combinations that will superconduct led, in 2000, to the discovery of superconductivity at 39K in magnesium diboride, followed by its rapid uptake as a practical material for use in fault current limiters and magnetic resonance imaging (MRI) machines. New materials families, such as pnictides, which are layerings of iron and arsenic atoms, have also recently been found to have superconducting properties. Some carbon-based organic materials also display superconductivity. Such is the desire to discover a room-temperature superconductor that it seems as if most elements on the periodic table, alone or with others, have been cooled, squeezed or otherwise manipulated into some form of superconduction.

Theory lags behind. In the better understood area of low-temperature superconductivity, as an electron passes between the positively charged ions of the atomic lattice through which it is flowing it attracts them, distorting the lattice and creating a concentration of positive charge that draws a second electron into its wake. Think of children running across an infinitely large pocket-sprung mattress, with one naturally lining up in the dip created by the other. The two electrons form what is known as a Cooper pair, with which the lattice does not have the energy to interact and whose lower energy state enables the electrons to overcome their mutual repulsion.

Given that conduction involves more than two electrons at a time, the model has to be broadened out at this point. Theory suggests that the Cooper pairs overlap, and their wave functions begin to cohere to form an extended electronic state that passes unimpeded through the lattice. If you replace the two youngsters on the mattress with the Midwich Cuckoos, you get the idea.

High-temperature superconductivity is not yet fully understood, although the phenomenon seems to be intimately bound up with the material structure. Some theories say it is about the layering of atoms in the ceramic materials, others that it is to do with the hole sites in the oxygen atoms.

Surface states

Research at Brookhaven National Laboratory in the US suggests that the roughness of a substrate on which a superconducting film is deposited can increase its current-carrying capacity, and even that superconductivity can be induced at the interfaces between two non-superconductors - both of which suggest that the effect is bound up in the atomic structures of these materials.

Another research team at Brookhaven says it has found Cooper pairs in an HTS material above its critical temperature, but without the coherence of states that isolates electrons from the lattice's influence in LTS materials.

The effort to increase the temperature at which materials superconduct will continue, but as Professor Tim Coombs, of the electrical engineering department at Cambridge University puts it, this may not immediately yield practically useful results.

"There are many, many superconducting materials, but only a few have the properties you need. Using some of them would be like trying to make a bridge out of butter - butter that you had left out in the sun."

But work presses on as seen in the projects surveyed on this and the last page - each working towards a warmer future for the successors to today's super-chilled superconductors.

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