FOR years physicists have seemed tantalisingly close to explaining high-temperature superconductivity. But the observation of one of its defining properties in an unexpected material, a magnetic metal, has left them wondering if they understand this exotic phenomenon at all.
Superconductivity usually occurs in metals at temperatures close to absolute zero. Quantum effects at these temperatures cause electrons to stop repelling each other and pair up, reducing the material鈥檚 electrical resistance to zero. Photons fired at the material can break up the electron pairs, but to do this they need to reach a certain energy level. This energy is known as the 鈥済ap鈥�.
The gap also occurs in high-temperature superconductors (HTSs), which are made of copper oxides and can superconduct at temperatures as high as 130 kelvin.
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But the gap in HTSs does not go away when the temperature rises to a point at which the superconductivity disappears, as it does in low-temperature superconductors, prompting scientists to term this the 鈥減seudogap鈥�. Many thought that the pseudogap was closely linked to, and was possibly the precursor to, high-temperature superconductivity.
鈥淔inding these pseudogaps in magnetic metals suggests that they have nothing at all to do with superconductivity鈥�
But now Jan Zaanen of the University of Leiden in the Netherlands and his colleagues have shown that this theory may be flawed. They have observed a pseudogap in manganite, a manganese oxide that is a metallic ferromagnet and which displays very different behaviour to that seen in HTSs (Nature, vol 438, p 474).
鈥淚t is a fascinating result. For years we have debated whether the pseudogap was a foretaste of high-temperature superconductivity, or whether it was instead associated with a different underlying order,鈥� says Andy Schofield, who works on HTSs at the University of Birmingham in the UK. 鈥淭he experiment suggests that what we have seen has nothing to do with superconductivity and is much more common than we had imagined.鈥�
The manganite results link the pseudogap to physical phenomena known as polarons. Normally, polarons are clouds of polarised charges that are induced by electrons moving through manganite鈥檚 crystal lattice. At low temperatures, quantum effects begin to dominate and polarons pair up and start demonstrating a pseudogap.
According to Zaanen, high-temperature superconductors and manganites are related materials and so the similarities are unlikely to be coincidental despite the fact that one family of materials superconducts and the other does not.
Some physicists have suggested that high-temperature superconductivity could be a result of competing quantum effects: a polaron-induced pseudogap, and conventional electron pairs moving without resistance.
鈥淚 think the finding will add fire to the debate on this great mystery of our time,鈥� says Zhi-Xun Shen, a team member at Stanford University in California. 鈥淭here are many important questions now. Are the anomalous HTS properties manifestations of the competing sibling states and if so, what is the underlying physics giving rise to these unruly children?鈥�
