Superconductors.ORG reports a milestone discovery of a 20th new high-temperature superconductor since planar weight disparity (PWD) was found to be a robust Tc-enhancement tool in early 2005. This newest superconductor, Sn2Ba2(Y0.5Tm0.5)Cu3O8+, exhibits a critical transition temperature near 96K. And while this Tc improvement is just 10 degrees over the prototype, even greater improvements have been realized in other PWD formulations. (To see a list of all of the new superconductors discovered to date click here.)
Planar weight disparity exists whenever planes within the layered perovskites are alternated light-heavy. PWD was first found to work successfully between alternating CuO2 planes. Then insulating layers were confirmed. And just recently, disparity within the insulating layers was also found to enhance Tc. (Electron/hole doping remains an important consideration.)
Consistently, however, Tc is depressed if the various types of PWD are intermixed within the same unit cell. Only one exception has been found to this rule.
This latest discovery achieves planar weight disparity by positioning a heavy rare earth (Tm) in one CuO2 layer and a light rare earth (Y) in the next CuO2 layer. See below "3212" structure in top row.
An advantage of PWD is it allows the molecule designer to place the heaviest atoms in the CuO2 planes. Previously most high-Tc copper-oxides had Period 6 heavy metals in the insulating layer. By placing heavy rare earths in the CuO2 planes, Period 4 and 5 metals can be used alone or in combination with Period 6 metals in the insulating layer. This allows for an increase in PWD and optimization of overall unit cell weight.
The ten structure types that have responded to PWD are shown below. As can be seen, most are intergrowth structures.
In summary, planar weight disparity has proven invaluable in uncovering new superconductive materials, while at the same time providing insight into the elusive mechanism that facilitates high-temperature superconductivity.
E. Joe Eck
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