SUPERCONDUCTORS could be made to work at higher temperatures than ever before, thanks to the discovery that one such material has an internal structure that behaves like a fractal. Its structure is also similar to the way the internet and some social networks are connected up.
With no electrical resistance, superconductors can conduct a large current with no energy lost as heat. This makes them extremely useful for a host of applications, from maglev trains to particle accelerators. However, most superconducting materials only work at temperatures close to absolute zero, though certain compounds containing copper and oxygen work at just over 100 kelvin.
To better understand these materials, of the Sapienza University of Rome in Italy and colleagues studied different crystalline forms of lanthanum copper oxide. It superconducts at between 16 and 40 kelvin if it is “doped” with extra oxygen atoms, known as interstitials.
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When Bianconi’s team used X-ray crystallography to deduce the way the interstitials are arranged in their crystals, a pattern emerged (see picture). The pattern was the same whether the crystal structure was examined at millimetre or micrometre scales – it was behaving like a fractal (Nature, ). “It’s a completely unexpected result,” says Bianconi.
Their analysis also revealed that the superconductor is a “scale-free” network, meaning its structure obeys the same mathematics as can be used to describe connections within the internet and some social networks. “I find it plainly mysterious,” says condensed matter physicist of Leiden University in the Netherlands. “It is telling us something very deep.”
The researchers also found that the greater the length scales at which the pattern persisted – or the more complete its “fractality” – the higher the maximum temperature at which the crystal could superconduct. Bianconi speculates that the scale-free distribution of the interstitial oxygen helps preserve the “quantum coherence” of electrons in the crystal. Superconductivity is thought to depend on this property, which breaks down as temperature rises.
“The scale-free distribution of the oxygen in the material helps to preserve superconductivity”
Engineering superconductors to increase their fractality could yield materials that work at yet higher temperatures, he adds, making it much easier to harness them for practical applications.
