Superconductor theory falls under IBM testsIBM researchers have finally measured Josephson vortices and found them 20 times wider than the high temperature superconductor theory predicts; The IBM team used scanning SQUID microscope. Melanie Reynolds. Serious doubt has now been cast on the front-running high temperature superconductor (HTS) theory by a team from IBM’s Thomas J. Watson Research Centre. HTS materials have a structure like a pack of playing cards, with flat atomic layers stacked one upon another. The theory, known as interlayer tunnelling (ILT), is that the ability of a material to carry electricity with no resistance results from increased coupling between adjacent atomic layers when copper compounds enter the superconducting state. Testing this theory has proved difficult. It involves the measurement of the strength of the coupling between layers. Only two HTS materials behave simply enough to be studied, one thallium-cuprate and a mercury-cuprate. Supercurrent tornado… An end-on view of a Josephson vortex in a thallium-based superconductor taken with a scanning SQUID microscope. The material lattice squashes the vortex giving it an oval cross-section. Existing theory predicts it should be 1?m across at its widest point. Measurements of this image show that it is 20 times wider. Infrared optical measuring techniques on the thallium-based cuprate have so far found no evidence for the interlayer coupling and only indirect measurements of magnetic susceptibilities have been possible on the mercury based cuprate. The Watson team have used IBM’s high resolution scanning SQUID microscope which, at 4?m resolution, is thought to be the world’s most precise, to make a more direct measurement of the vortices. The microscope maps out magnetic fields and was used to image the tubes of magnetic flux between the atomic layers, known as Josephson vortices. The vortices were measured as 20?m across which is 20 times wider than the ILT theory predicts. John Kirtley is the Watson scientist that made the doubt-casting measurements. “There is always a factor of two floating around,” said Kirtley, commenting on the certainty with which theoretical physicists predict reality, “but 20 is several factors of two.” Perhaps surprisingly, he still has some faith in the ILT: “It is the only theory that takes into account what goes on between the layers as well as what happens inside them. I think that physicists will have to look around for a way to modify ILT in view of the measurements.” And does Kirtley have a pet theory to explain HTS behaviour? “No,” he said emphatically. The History of Superconductivity The first instance of superconductivity was observed in 1911 when mercury was cooled by liquid helium to below the critical temperature of 4.2K. It was not until the 1960’s that a practical low-temperature superconducting metal wire, of niobium and titanium alloy, was made. It worked at temperatures below 20K. The move towards high temperature superconductors was made in 1986 when two scientists at IBM detected superconductivity in ceramic compounds at a temperature up to 35K, a discovery for which they were subsequently awarded the Nobel Prize. Further advances saw the critical temperature rise with superconductivity observed at 94K. This meant that superconductors could be cooled using cheaper and more readily available liquid nitrogen. Commercial applications for superconductivity are now appearing in fields including electric power, transportation, electronics and medicine. Current applications include, magnetic resonance imaging, wireless communication base stations and ultra-fast logic circuits.