Superconductors.ORG reports an analysis of 14-year-old data(1) on Sr substitution in YBCO has revealed that the critical transition temperature (Tc) of superconducting copper-oxides can be improved by increasing weight disparity within the insulating layers. Previously Tc enhancement through planar weight disparity was found to occur only by alternating heavy insulating layers with light insulating layers; or by alternating heavy CuO2 planes with light CuO2 planes. (Click HERE for background on planar weight disparity).
In the above plot strontium is substituted into the barium atomic site in increments of 0.2 atom. Since Sr is lighter than the Ba that is being replaced, the overall unit cell weight decreases with increasing substitution. And Tc decreases as well, as overall molecular weight drops below ideal. However, there are 3 different points where Tc briefly spikes above the curved line. Those points represent the mix ratios where planar weight disparity comes into play. The two outermost spikes were expected, as they represent planar weight disparity between alternating insulating layers in the following way:
Sr.66 = (BaSr)-Y-(BaBa)-Y-(SrBa) = light-heavy-light.
Sr1.33 = (SrBa)-Y-(SrSr)-Y-(BaSr) = heavy-light-heavy.
However, the center spike in the plot can only be accounted for through the weight differences of Sr and Ba WITHIN the insulating layer. This is because all of the insulating layers have the same gross weight and electron/hole doping is not a factor. Specifically:
Sr1.0 = (SrBa)-Y-(SrBa)-Y-(SrBa) = same-same-same.
Although the 5-degree improvement in Tc at Sr = 1.0 is not sufficient to overcome the gross decline in Tc due to overall molecular weight loss, it does confirm that creating weight disparity within the insulating layers of copper-oxide superconductors is a viable tool for promoting Tc in theoretical molecules of the future. To date some 20 new superconductors have been discovered by incorporating planar weight disparity into molecular design.
In the quest to understand the mechanisms of high-temperature superconductivity, the contribution of planar weight disparity may prove to be the most significant of all.