Room-temperature superconductivity has long been the holiest of holy grails in condensed-matter physics. Within the past decade, the appearance of new materials that superconduct at relatively balmy temperatures, but only under extreme pressures, has brought a slight yet significant alteration in the quest. To be truly grail-like, a newly synthesized superconductor cannot merely carry electrical current without resistance at room temperature. It must also do it at ambient pressure for it to have practical applications beyond the laboratory – such as levitating trains, efficient power lines or cheaper MRI machines.
So when a not-yet-peer-reviewed paper entitled “The First Room-Temperature Ambient-Pressure Superconductor” appeared on the arXiv preprint server earlier this week, physicists were intrigued – though also sceptical, given recent retractions and allegations of scientific misconduct in the field.
In the paper, Sukbae Lee and Ji-Hoon Kim, both materials scientists at the Quantum Energy Research Centre (Q-Centre) in Seoul, Korea, together with Young-Wan Kwon of Korea University, report that under everyday conditions, a modified form of the mineral lead apatite exhibits tell-tale signs of superconductivity. These signs include the all-important resistance-free flow of current; the expulsion of magnetic field from the material via the Meissner effect; and a critical temperature and critical magnetic field below which the superconducting transition occurs.
Further evidence emerges
To bolster these claims, a further paper appeared shortly afterwards on the arXix, this time written by Lee and Kim in collaboration with their Q-Centre colleagues Sungyeon Im, SooMin An and Keun Ho Auh, plus Hyun-Tak Kim, a physicist at the College of William and Mary in the US. The timing of this paper’s appearance and its longer author list prompted intense online speculation about the team’s motives, with several commenters pointing out that a Nobel Prize (the likely reward for any confirmed discovery of room-temperature, ambient-pressure superconductivity) can only be shared by three people, not six. Speculation aside, the second paper repeats many of the jaw-dropping details of the first, while describing the material’s synthesis in more detail.
As a final piece of evidence, a video posted by Hyun-Tak Kim on arXiv’s ScienceCast platform on 25 July purports to show the material Lee and Ji-Hoon Kim call LK-99 (apparently after their own initials and the year they first synthesized it) levitating atop a magnet. This simple demonstration of the Meissner effect is a staple of undergraduate physics labs – except in this case, the liquid nitrogen required to produce superconductivity in conventional, low-temperature superconductors is nowhere to be seen.
The critics wade in
A few days after the papers appeared on the arXiv – and mere hours after their sensational claims began circulating on social media, crashing Q-Centre’s website in the process – experts in the field urged caution. Richard Greene, a physicist at the University of Maryland, US who has worked on superconducting materials since the 1970s, observed that while the Meissner-effect video “looks impressive” at first glance, superconductivity is not the only phenomenon that can cause objects to levitate. “If you look carefully you see that sample 2 (which was levitated) has a large diamagnetic magnetization in the normal state,” he said. “So it could be levitated just because it’s a diamagnetic material.”
Another physicist, Douglas Natelson of Rice University, US, highlighted apparent inconsistencies in the two papers’ data on magnetic susceptibility, Χ. When Lee, Ji-Hoon Kim and colleagues placed their sample of LK-99 in a magnetic field, the six-authored paper states that the change in the material’s mass susceptibility (that is, Χ divided by density) amounted to 2.5 x 10-4 electromagnetic units per gram. “Assuming a density of about 7 grams per cubic centimetre, that gives Χ = –0.022, about 36 times that of graphite,” Natelson wrote in a Twitter/X thread dedicated to the findings. “That would be exciting, if it’s accurate.”
However, Natelson went on to note that “what appears to be the same data” also appears in Figure 4 of the three-authored paper, but with a completely different scale on the graph’s y-axis. This second set of numbers is, he said, “unphysical”, adding that the “pretty sloppy” discrepancy “does not encourage confidence in the results”.
Wait for reproduction
One bright spot in this confusion is that unlike studies of high-pressure superconductors, the work of Lee, Ji-Hoon Kim and their collaborators required relatively little in the way of specialist equipment. That won’t make attempts at replicating it easy, exactly; as Jennifer Fowlie, a condensed-matter physicist at the SLAC National Laboratory in the US, pointed out on Twitter, the four-day, multi-step, solid-state process the Korean researchers used to synthesize their material is hardly straightforward. (“Some of you haven’t had blisters from overusing your pestle and it shows,” she quipped.)
Tantalum polyhydride joins emerging class of high-pressure superconductors
Still, the absence of highly specialized kit should make replication possible for more than a handful of research groups. And with so much attention devoted to finding it, a solution to the mystery of LK-99 and its possible room-temperature, ambient-pressure superconductivity should not be long in coming. “I think it is best we wait and see if this material, and the results contained within the report, are reproduced by another group in the world,” Nigel Hussey, a superconductivity researcher at the University of Bristol, UK, tells Physics World. “If so, then of course, this would be a sensational breakthrough. For the time being, though, it is simply sensational.”
