Buffer layer with oxygen interstitials promote superconductivity

La2CuO4+y is a typical high Tc copper oxide superconductors. It has two types of layers: the CuO2 layer and the buffering LaO2+y layers. The former layer carries supercurrent while the latter one is nearly insulating. They alternate in the c-direction. For superconductivity to happen, the CuO2 layer must be perfect and defect free. Any sort of defects, such as Zn substituent and O vacancy or even curvature of the square, in this layer will be adverse to superconductivity. On the other hand, the relation between SC and the defects in the buffering layer is quite complex. Actually, as y>0, there will be O interstitials in this layer. It was recently measured that[1], these interstitials are actually correlated with each other, with a radial distribution following a law that is fingerprint of critical behaviors: scale invariant. More stunningly, such a patter anyway elevates Tc. What might be the cause ? Can it be ascribed to electron-phonon coupling between the O interstials and the supercurrent ? Or does this pattern image the pattern of the SC order parameter in the CuO2 layer ? The answer is unclear yet.

For y<0, there wont be any interstitials any more and the life is a bit easier.

It is well known that the microstructures of the transition-metal
oxides1–3, including the high-transition-temperature (high-Tc) copper
oxide superconductors4–7, are complex. This is particularly so
when there are oxygen interstitials or vacancies8, which influence
the bulk properties. For example, the oxygen interstitials in the
spacer layers separating the superconducting CuO2 planes undergo
ordering phenomena in Sr2O11yCuO2 (ref. 9), YBa2Cu3O61y (ref. 10)
and La2CuO41y (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
La2O21y spacer layers of La2CuO41y high-Tc superconductors is
characterized by a fractal distribution up to a maximum limiting
size of 400 mm. Intriguingly, these fractal distributions of dopants
seem to enhance superconductivity at high temperature.

[1]Vol 466| 12 August 2010| doi:10.1038/nature09260