What’s going on with the reports of a room-temperature superconductor?

In late July, a couple of startling papers appeared on the arXiv, a repository of pre-peer-review manuscripts on topics in physics and astronomy. The papers claim to describe the synthesis of a material that is not only able to superconduct above room temperature, but also above the boiling point of water. And it does so at normal atmospheric pressures.

 

Instead of having to build upon years of work with exotic materials that only work under extreme conditions, the papers seem to describe a material that could be made via some relatively straightforward chemistry and would work if you set it on your desk. It was like finding a shortcut to a material that would revolutionize society.

 

The perfect time to write an article on those results would be when they've been confirmed by multiple labs. But these are not perfect times. Instead, rumors seem to be flying daily about possible confirmation, confusing and contradictory results, and informed discussions of why this material either should or shouldn't work.

 

In this article, we'll explain where things stand and why getting to a place of clarity will be challenging, even if these claims are right.

What’s the original claim?

The more detailed of the two manuscripts describes how to make the material and measurements of its property. The material itself is a variation of a well-known chemical called lead apatite. Apatites are a class of chemicals that form similar crystal structures; this particular version is primarily composed of lead and phosphate groups—all of its constituents are cheap and readily available.

 

The version developed here, which has been termed LK-99, was made by reacting a lead sulfate with a copper-phosphorus compound (the reaction requires high temperatures for over a day under a vacuum). This strips the phosphorus from the copper, oxidizes it, and allows it to displace the sulfur from its compound with the lead. Critically, though, some fraction of the lead itself ends up replaced by copper in the resulting compound.

 

This has a significant impact on the apatite crystal structure because copper is quite a bit smaller than lead. The researchers claim the overall volume of the sample drops by about half of a percentage as a result, and that change is accompanied by shifts in the orientation of various atoms and bonds. That means changes in where the electrons reside within the material.

 

That change appears to be critical to the LK-99's behavior. Superconductivity is associated with a number of very specific properties, and the researchers measure two of them: the expulsion of magnetic field lines (called the Meissner effect) and the existence of a critical temperature at which conductivity changes.

 

It's hard to explain just how strange these experiments are. Under normal circumstances, the superconducting material starts out behaving as a normal chemical and has to be cooled down to the critical point where exceptional behavior emerges. LK-99, by contrast, starts out superconducting and has to be heated beyond the boiling point of water to reach its critical temperature.

 

The only somewhat strange result here comes at temperatures just below the critical temperature. At room temperature and above, the resistance of LK-99 remains at zero as far as the testing equipment is able to measure. But it starts to rise ever so slightly once temperatures reach 60°C and displays a smooth upward slope until the sample hits 90°C, at which point it stays flat until the critical temperature is reached. The researchers did not attempt to explain this.

So we have a simple superconductor, then?

Anything but. The synthesis process does not allow any control over how many lead atoms are swapped out for copper. LK-99's formula is given as Pb10-xCux(PO4)6O. Note the presence of the x, which means just what high school algebra taught you to think it does: unknown. The base unit of the crystal structure has 10 lead atoms in it, and that number is decreased in exact proportion to the number of copper atoms that get added. But what that number is can potentially vary.

 

And it keeps getting more complicated from there. We also don't have control over which of the lead atoms gets swapped out. There are lead atoms located at very specific places in each base unit of the crystal, and not all of these locations are equivalent to each other. There are some indications that it's energetically more favorable to substitute copper in at specific locations, but this synthesis takes place at high temperatures, so energetic favorability may not be a major determinant of what happens; there's energy around to spare.

 

In addition, there's no guarantee that neighboring base units in the same crystal will be identical. So neighboring units in the same crystal could have different numbers of copper atoms located in different places.

 

One bit of relevant information comes from data obtained by shining X-rays through LK-99, which suggests that the majority of the material is in a single conformation. But there are some notable differences between the X-ray data and computer-generated predictions of what that data should look like. These are minor, suggesting that any variation in composition is small. But it's always possible for small differences to radically impact a material's behavior.

 

Consistent with all this messiness, LK-99 is what's called a polycrystalline material. That means it is formed from multiple smaller crystals, each with different orientations, smushed up against each other. It's possible that any superconductivity is the product of a subset of the crystals within the bulk material.

 

One last thing worth mentioning: Theoretical considerations suggest that electrons from specific orbitals of the atoms within the crystal will be doing the superconducting. And those orbitals have an equally specific orientation within the crystal. It's possible that LK-99 only superconducts along certain axes of the crystal. So you could potentially get superconductivity if you put wires on the top and bottom of a crystal, but regular metallic behavior if the wires are on the left and right sides.

Has anyone reproduced this?

Maybe? Ish? While the original draft manuscript contains detailed synthesis instructions, there's a lot of potential for lab-to-lab variation in little things like glassware composition, water pH, and so on that could make reproducing LK-99 difficult. The huge potential for variation in LK-99 described above obviously increases the challenge of making the right version of the chemical. It's also possible for materials to have some partial aspects of a superconductor but not the full suite of expected behaviors.

 

Given all this, it's plausible to expect that we'd see a mix of confirmation of some reported results and failures to replicate, regardless of whether LK-99 was a superconductor or not. And social media has been filled with exactly that.

 

On the more compelling side, we have a video reportedly from a research group that synthesized some LK-99 (it appears to be from the people who posted this report) and showed that it rejects magnetic fields strongly enough to levitate away from them—a hallmark of the Meissner effect. With a strong enough magnet, it's possible to get nearly anything to levitate (including, apparently, mice), but this is done with not especially strong magnets, and clearly at room temperature. And the small chunk of material isn't lifted evenly, consistent with only a small crystal within the sample actually superconducting.

 

On the less compelling side, a different group has apparently synthesized the material but only finds that it superconducts up to about 110 K—nowhere near room temperature. Whatever was made here also doesn't seem to have a critical temperature, instead seeing a gradual increase in resistance above that point, and the Meissner effect tests came up negative. That's pretty inconsistent with the original results and suggests that what they have isn't a typical superconductor at all.

 

Meanwhile, there has been some support on the theory side, as Lawrence Berkeley Lab's Sinéad Griffin ran some density functional theory calculations to probe what the material might be up to. These calculations confirmed that swapping copper into a specific position in the crystal should cause a conformational change in the crystal itself. More significantly, this change causes the appearance of a set of conduction bands that are largely "flat," meaning the energy involved in getting electrons into them hovers around zero.

 

And that's consistent with existing ideas on superconductivity. "If previous assumptions about band flatness driving superconductivity are correct," Griffin writes, "then this result would suggest a much more robust (higher temperature) superconducting phase exists in this system, even compared to well-established high-TC systems." Again, that's a calculation, not experimental evidence. But it at least provides a reason to think the reported results might be valid.

What about those other superconductors?

The news comes at a somewhat awkward time for the field. A similar claim was made about a high-pressure material a few years ago, but that paper ended up being retracted because of problems with some of its data. The same research group came back with a different material that was said to work at room temperature, but that work hasn't been replicated, and the head of the lab has now been accused of scientific misconduct.

 

There is almost no overlap between that work and LK-99. None of the people involved are the same, so there's no reason to suspect problematic research practices. And the chemistry and physics involved are completely different. The earlier work used high pressure to create chemicals with lots of hydrogen and unusual orbital structures. LK-99 uses no hydrogen at all and gets its orbital structures via a conformational change in a crystal lattice that takes place at ambient pressures.

 

Hydrogen was the focus of the earlier work because its low atomic weight influences the behavior of vibrations within the material in a way that promotes the formation of superconducting pairs of electrons. The mechanism behind LK-99 is less clear, but clearly not that.

 

(The LK-99's creators suggest that the conformational change in the crystal creates a sort of standing wave of electrons called a "charge density wave," and superconductivity involves electrons tunneling between wave sites. The modeling paper, by contrast, suggests that giving electrons the opportunity to both superconduct and participate in additional processes like charge density wave formation increases the probability that they'll superconduct. In any case, neither idea involves phonons.)

So when will we actually know anything?

Hopefully soon. The researchers behind the original report are trying to get information out there. In addition to the drafts placed in the arXiv, they have already published a paper on LK-99, albeit in their native Korean. And a group of South Korean scientists working in the field have also announced that they're going to obtain LK-99 samples and try to confirm its reported behavior.

 

There's also lots of activity outside of South Korea. Producing LK-99 is within reach of a lot of labs, and testing it is much easier since it doesn't require low temperatures or high pressures. That will mean a lot of short-term confusion, but it's likely to enable a consensus to emerge sooner.

 

Whether or not this chemical superconducts at ambient temperatures might not be the final question, though. Assuming it does, there will be many questions about how to develop it into a useful material, how much current it can carry, and how to use it most effectively in the huge range of applications it can be put to. But I'm sure we'll all be happy if we end up needing answers to those questions.