The room-temperature superconductor that wasn’t

The summer of room-temperature superconductivity was short-lived. It started with some manuscripts placed on the arXiv toward the end of July, which purportedly described how to synthesize a compound called LK-99, which would act as a superconductor at temperatures above the boiling point of water. High enough that, if its synthesis and material properties worked out, it could allow us to replace metals with superconductors in a huge range of applications.

Confusion quickly followed, as the nature of the chemical involved made it difficult to know when you were looking at the behavior of LK-99 and when you were looking at related chemicals or even impurities.

But the materials science community responded remarkably quickly. By the end of August, pure samples had been prepared, the role of impurities explored, and a strong consensus had developed: LK-99 was not a superconductor. Best yet, the work nicely provided explanations for why it had behaved a bit like one in a number of situations.

A complex chemical

As we discussed in detail in our story linked above, LK-99 is a complicated chemical. It's largely a lead-phosphate crystal, but some of the lead atoms are displaced by copper. How many copper atoms are present may not only vary from preparation to preparation; they can potentially vary between different areas of the same crystal. Its chemical formula is technically Pb10-xCux(PO4)6O, with the x representing the unknown number of replacements.

It's also what's called a polycrystalline material, meaning that a single chunk of it may be a composite of multiple crystals with different orientations. So, for any properties that depend on the orientation of the crystal, it's easy to end up examining a composite of behaviors from multiple individual crystals.

Finally, the preparation of LK-99 described in the initial report produced it via a chemical reaction, leaving open the possibility that there were contaminants or byproducts affecting the measurements of its properties.

This last possibility turned out to be a key to explaining one of the reported properties of LK-99: a sharp transition in its ability to conduct current that occurred just above the boiling point of water.

Prashant Jain is a professor in the Chemistry Department at the University of Illinois, and he's had funding to work on copper sulfide since 2011. He noticed two critical things about the initial reports of LK-99. One is that the reactions used to produce it could potentially produce copper sulfide as an additional product. The second is that the temperature where LK-99 supposedly started superconducting (104° C) is also the temperature where copper sulfide undergoes a phase transition.

Changing phase and conductivity

In a manuscript deposited on the arXiv, Jain describes this complicated phase transition in detail. Above 104° C, copper sulfide remains a solid but becomes more disordered, which allows its constituent ions to move more readily, increasing its conductivity somewhat. But Jain notes that things get more complicated when the material is exposed to air, as oxygen can react with some of the copper and pull it out of the copper sulfide structure.

This creates charged "holes" in the structure of the copper sulfide, which can also conduct electricity. And they do so far more effectively in the ordered stage of the crystal that forms below 104° C. While these two effects can offset each other, the holes are far more effective at conducting current, so this effect dominates, leading to much higher conductivity below 104° C—an effect similar to that ascribed to the onset of superconductivity.

Jain quite reasonably concludes that "LK-99 must be synthesized without any copper sulfide to allow unambiguous validation of the superconducting properties." And, fortunately, a group at the Max Planck Institute for Solid State Research has done just that.

Their method of producing LK-99 is significantly different from the original report, and its starting materials contain no sulfur whatsoever. It contains a number of additional cycles of grinding and heating to thoroughly mix its materials and then goes through a final step that promotes the growth of single, uniform crystals. As a result, the group was able to characterize LK-99 without the complications from a polycrystalline form, and potentially with far less (and different) contamination from reaction byproducts.

It turns out it's an insulator. In fact, instead of having freely conducting electrons that can absorb light at a variety of wavelengths, their LK-99 crystals were semi-transparent. Which suggests that any changes in conductivity are coming from something else, like the contaminant highlighted by Jain.

About the levitation

But changes in conductivity aren't the only aspects of superconductivity that were originally reported for LK-99. Superconductors don't allow magnetic field lines to pass through them; if you place a magnet near superconducting material, it will levitate to keep this from happening. A number of early reports on LK-99 were accompanied by images of crystals partly levitating, as if a small part of the complex crystal were expelling the magnetic field.

Jain provided a potential explanation for some of this, noting that the copper sulfide contaminant is diamagnetic, meaning it shows magnetic properties when placed in a magnetic field. But the group in Germany, using its single crystal, shows that LK-99 itself is diamagnetic and, so, would be expected to react to the presence of a magnetic field.

Complicating matters further, they showed that the substitution of copper isn't even across even a single crystal. Some areas of the crystal will still contain lead that has not been substituted at all; other areas will see multiple lead atoms in a single crystal unit swapped out for copper. Overall, they found that the x of the Pb10-xCux(PO4)6O formula averaged 1.2 in their preps but varied considerably depending on where in the crystal you looked.

When the local copper concentration was high enough, the material behaved like a standard magnet. This provides an additional opportunity for some portions of the material to be repelled by having a magnet placed near it. A separate study, done by a Chinese collaboration, showed that both levitating and non-levitating LK-99 samples had localized patches that displayed some combination of standard magnetism and diamagnetism, essentially replicating this result.

With both the magnetic and conductive behaviors seemingly explained without superconductivity, LK-99 appears to be a fairly run-of-the-mill insulator when prepared in a pure form. Considering all the complexities involved, this consensus was arrived at remarkably quickly. The only remaining drama is likely to be whether the people who originally described it will continue to argue in favor of their initial reports in the face of all this evidence.