https://arxiv.org/abs/2308.01723
>In fact, I find that Cu on this Pb(2) is 1.08 eV more energetically favorable than Cu on the Pb(1) site, suggesting possible difficulties in robustly obtaining Cu substituted on the Pb(1) site.
https://arxiv.org/abs/2307.16892
The paper from Dr. Griffin at LBNL suggests copper atoms have to be placed in a specific (but less likely) position in the molecule to result in the desired flat band characteristic. Also, the original authors and the labs who were able to replicate LK-99 are reporting they had to make multiple batches to even find a tiny piece that shows levitation. This suggests that you just have to be very lucky to produce a sample with high enough concentration of LK-99 to observe levitation.
If we can somehow confirm that LK-99 is truly a room temperature superconductor, billions of dollars of R&D fund will pour in to improve the fabrication process. When the first transistor was invented, people probably weren't imagining that we'll be mass producing them in nanometer scale in the future. Or maybe LK-99 will be stuck in a lab like graphene. Who knows?
I think the more likely explanation is that the particles do touch each other but the interface is not superconductive. In other words, it is a polycrystalline material, and most of it is LK-99, but the grain boundaries are not a very good conductor. In conventional superconductors grain boundaries don't disrupt superconductivity because they are 3D superconductors, but in this allegedly 1D superconductor the superconducting channels in most cases don't meet at the grain boundaries, so the current has to overcome the resistance of some material that is almost an insulator.
If that is the case it will be difficult to produce a material that is macroscopically superconducting. But I hope researchers will be able to make single crystals that are large enough for resistance measurements so that finally it can be determined if this material is a superconductor or not. For practical uses the best result that can be achieved with this material may be a metal-LK-99 composite where the LK-99 particles lower the resistivity of the metal by 50-90%.
The Korean team appears to have been stuck for several years by the lack of reproducibility of this synthesis method. While it was a great discovery that has shown that this material must have some very interesting properties, perhaps even superconductivity at ambient temperature and pressure, in order to be able to measure its properties and be able to evaluate the possible practical applications, a much more precise method for enforcing the desired crystal structure is required, than mixing powders and baking them into a ceramic.
Perhaps such a method for producing samples with deterministic properties would be to develop first a method to grow monocrystals of the special kind of lead phosphate that forms the base crystal structure, maybe by drawing the crystals from melt.
Once monocrystals of this kind of lead phosphate are available, they could be doped with copper, e.g. by ion implantation. By controlling and varying the parameters of the process, e.g. the angle of incidence and the velocity of the ions and the thermal profile used for annealing, it is likely that reproducible samples can be produced, where the copper ions substitute lead in the useful places and not in the others.
By this method it would be possible to produce only thin layers of LK-99, but that should be enough to enable the characterization of the material.
Moreover, because LK-99 is very fragile, it is unlikely that it could be used to make cables or coils. Practical uses where LK-99 would be deposited as thin films are much more likely.
As an alternative to ion implantation, which might be able to produce thicker layers, perhaps once monocrystals of the base lead phosphate are available it may be possible to develop some method of chemical vapor deposition, to grow epitaxially a layer of LK-99 over the base crystal, but with such a method it is less obvious if there is any way to control which lead atoms are substituted, though this may depend on the orientation of the base crystal.
