The benefit of fractal dirt

Abstract

Measurements of X-ray diffraction on small patches of a copper oxide superconductor reveal that oxygen crystal defects form fractal structures that seem to promote high-temperature superconductivity. The oxygen interstitials in the spacer layers that separate the superconducting CuO2 planes undergo ordering phenomena in Sr2O1+y CuO2, YBa2Cu3O6+y and La2CuO4+y that induce enhancements in the transition temperatures (Tc) with no changes in hole concentrations. It is also known that complex systems often have a scale-invariant structural organization, but hitherto none had been found in high-Tc copper oxides. Fratini et al. report that the ordering of oxygen interstitials in the La2O2+y spacer layers of La2CuO4+y high-Tc superconductors is characterized by a fractal distribution up to a maximum limiting size of 0.5 millimetres. Intriguingly, these fractal distributions of dopants appear to enhance superconductivity at high temperature.