Scale-free structural organization of oxygen interstitials in La2CuO4+y

TL;DR

This paper reveals that oxygen interstitials in La2CuO4+y superconductors are organized in a fractal pattern up to 400 microns, which appears to enhance high-temperature superconductivity, indicating a scale-invariant microstructure.

Contribution

It is the first to identify a scale-free, fractal organization of oxygen interstitials in high-Tc cuprates and links this structure to enhanced superconducting properties.

Findings

Oxygen interstitials form fractal distributions up to 400 microns.

Fractal organization correlates with increased superconducting temperature.

High-Tc superconductors exhibit scale-invariant microstructures.

Abstract

It is well known that the microstructures of the transition-metal oxides (refs 1-3), including the high-transition-temperature (high-Tc) copper oxide superconductors (refs 4-7), are complex. This is particularly so when there are oxygen interstitials or vacancies (ref.8), which influence the bulk properties. For example, the oxygen interstitials in the spacer layers separating the superconducting CuO2 planes undergo ordering phenomena in Sr(2)O(1+y)CuO(2) (ref. 9), YBa(2)Cu(3)O(6+y) (ref. 10) and La(2)CuO(4+y) (refs 11-15) that induce enhancements in the transition temperatures with no changes in hole concentrations. It is also known that complex systems often have a scale-invariant structural organization16, but hitherto none had been found in high-Tc materials. Here we report that the ordering of oxygen interstitials in the La(2)O(2+y) spacer layers of La(2)CuO(4+y) high-Tc superconductors is characterized by a fractal distribution up to a maximum limiting size of 400 microns. Intriguingly, these fractal distributions of dopants seem to enhance superconductivity at high temperature.