Claimed superconductor LK-99 is an online sensation — but replication efforts fall short

[Hartmann352]

nature

04 August 2023

Social media is abuzz with chatter about the material, but some scientists are pushing back on the hype.

By Dan Garisto


Levitation is one hallmark of superconductivity.Credit: Patrick Gaillardin/LookAt Sciences/SPL

A South Korean team’s claim to have discovered a superconductor that works at room temperature and ambient pressure has become a viral sensation — and prompted a slew of replication efforts by scientists and amateurs alike. But initial efforts to experimentally and theoretically reproduce the buzzworthy result have come up short, and researchers remain deeply sceptical.

The team, led by Sukbae Lee and Ji-Hoon Kim at the start-up firm Quantum Energy Research Centre in Seoul, reported in preprints published on the arXiv server 25 July1,2 that a compound of copper, lead, phosphorus and oxygen, dubbed LK-99, is a superconductor at ambient pressure and temperatures up to at least 127 °C (400 kelvin). The team says that samples show two key hallmarks of superconductivity: zero electrical resistance and the Meissner effect, in which the material expels magnetic fields, leading samples to levitate above a magnet. Previous efforts have achieved superconductivity only in materials at incredibly low temperatures or extremely high pressures. No material has ever been confirmed to be a superconductor under ambient conditions.

LK-99’s purported superconductivity drew immediate scrutiny from scientists. “My first impression was ‘no’,” says Inna Vishik, a condensed-matter experimentalist at the University of California, Davis. “These ‘Unidentified Superconducting Objects’, as they’re sometimes called, reliably show up on the arXiv. There’s a new one every year or so.” Advances in superconductivity are often touted for their potential impact on technologies such as computer chips and maglev trains, but Vishik points out that such excitement might be misplaced. Historically, progress in superconductivity has had tremendous benefits for basic science, but little in the way of everyday applications. There’s no guarantee that a room-temperature superconductor would be of practical use, Vishik says.

The first attempts to replicate the LK-99 results have not improved the material’s prospects. None of the studies provides direct evidence for any superconductivity. (The Korean team did not respond to Nature’s request for comment.)

Two separate experimental efforts — by teams at the National Physical Laboratory of India in New Delhi3 and Beihang University in Beijing4 — reported synthesizing LK-99, but did not observe signs of superconductivity. A third experiment by researchers at Southeast University in Nanjing, China,5 found no Meissner effect, but measured near-zero resistance in LK-99 at −163 °C (110 K) — which is far below room temperature, but high for superconductors.

Theorists have also entered the fray. Several studies6–9 used a computational technique called density functional theory (DFT) to calculate LK-99’s electronic structure. The DFT calculations suggest interesting electronic features that, in other materials, have been associated with behaviour such as ferromagnetism and superconductivity. But none of the studies found evidence that LK-99 could be a superconductor at ambient conditions.

Early efforts​

Replicators first attempted to synthesize LK-99, following the process described by the Korean team, which involved mixing powdered components and two stages of heating, with 925 °C reached during the second stage. (The high temperatures and use of lead have prompted concerns about amateur replication attempts, which researchers say are dangerous.)

To confirm the resulting materials’ structure and identity, replicators used X-ray diffraction, an atomic imaging technique. The Beihang team concluded that its sample’s structure was “highly consistent” with that of LK-99.
A member of the National Physical Laboratory of India team, physicist Veerpal Singh Awana, acknowledged small differences between their sample and that of the Korean team. “Our LK-99 is very similar to that as the reported superconducting LK-99,” he says.

But Robert Palgrave, a chemist at University College London, says that both X-ray diffraction patterns obtained by these replication attempts are significantly different from the Korean team’s patterns and from each other. (Members of the Beihang team did not respond to Nature’s request for comment.)

The Southeastern University team obtained X-ray diffraction data that are more consistent with the Korean team’s sample, according to Palgrave. But several researchers have questioned the claim of achieving zero resistance at −163 °C. Evan Zalys-Geller, a condensed-matter physicist at the Massachusetts Institute of Technology in Cambridge, says that the resistance measurement wasn’t sensitive enough to distinguish between a superconductor and a low-resistance metal such as copper. (Members of the Southeastern University team did not respond to a request for comment.)

Theory troubles​

Uncertainty about the structure of LK-99 limits the conclusions that researchers can draw from theoretical calculations, which assume a given structure.

On 31 July, a theoretical analysis posted on Twitter prompted excitement among online enthusiasts. Sinéad Griffin, who studies quantum materials at Lawrence Berkeley National Laboratory in Berkeley, California, shared her paper6, accompanied by a GIF of former US president Barack Obama performing a ‘mic drop’. The optimism was prompted by Griffin’s use of DFT to find that LK-99 has ‘flat bands’, indicating that electrons in the material are strongly correlated with each other. “Flat-band systems tend to show interesting physics,” Vishik says. “So when a material is predicted to have a flat band, people get kind of excited.”

Griffin later rebuffed the optimism, tweeting: “My paper did *not* prove nor give evidence of superconductivity”.

Other theoretical analyses also suggested the presence of flat bands, but all of them rely on the same assumption about the structure, says Leslie Schoop, a solid-state chemist at Princeton University in New Jersey. “In a nutshell, I don’t believe any of the DFT before I know the correct crystal structure,” she says.
Griffin agrees that knowing the structure is essential. But she says that the structure found by the Korean team is similar to that of other lead phosphate minerals. “So it’s not too bizarre to think it possible.”
Even if future experiments confirm flat bands, the feature does not mean the material would display room-temperature superconductivity, Schoop says. The association between flat bands and superconductivity comes from other materials, such as ‘twisted’ layers of graphene — slightly offset sheets of atomically thin carbon — which displayed superconductivity at −271 °C (1.7 K) and featured flat bands10. But this does not provide evidence for superconductivity at the researchers’ claimed temperatures in the lead-based LK-99, Schoop says.

Viral videos​

The limited success of the replication attempts has not quelled speculation online. Unverified videos of samples, supposedly levitating because of superconductivity, have circulated as viral evidence, despite the fact that many materials and objects — such as graphite, frogs and pliers — can exhibit similar magnetic behaviour.
Previous room-temperature superconductivity claims, including one made in March by the controversial physicist Ranga Dias, have made headlines. But the attention associated with LK-99 has surpassed that for many of its predecessors.

Frustrated by the atmosphere of hype, some scientists have taken to mimicking the levitation videos with everyday materials suspended by string and other props. “I opened Twitter up one day and noticed a bunch of sketchy videos with little floating pebbles,” says Eric Aspling, a physicist at Binghamton University in New York. In response, he uploaded a video featuring a “sample of LK-99 shaped as a fork” suspended by tape. “I thought, ‘How can anybody be convinced by this?’,” he says.

doi: https://doi.org/10.1038/d41586-023-02481-0

UPDATES & CORRECTIONS​

• Correction 07 August 2023: The original version of this story stated that the material LK-99 is claimed to be a superconductor at temperatures above 127 °C. In fact, the researchers claim LK-99 has this property up to at least 127 °C. The picture caption has also been edited for accuracy.

References​

1. Lee, S. et al. Preprint at https://arxiv.org/abs/2307.12037 (2023).
2. Lee, S., Kim, J.-H. & Kwon, Y.-K. Preprint at https://arxiv.org/abs/2307.12008 (2023).
3. Kumar, K., Karn, N. K. & Awana, V. P. S. Preprint at https://arxiv.org/abs/2307.16402 (2023).
4. Liu, L. et al. Preprint at https://arxiv.org/abs/2307.16802 (2023).
5. Hou, Q., Wei, W., Zhou, X., Sun, Y. & Shi, Z. Preprint at https://arxiv.org/abs/2308.01192(2023).
6. Griffin, S. M. Preprint at https://arxiv.org/abs/2307.16892 (2023).
7. Lai, J., Li, J., Liu, P., Sun, Y. & Chen, X.-Q. Preprint at https://arxiv.org/abs/2307.16040(2023)
8. Kurleto, R. et al. Preprint at https://arxiv.org/abs/2308.00698 (2023).
9. Si, L. & Held, K. Preprint at https://arxiv.org/abs/2308.00676 (2023).
10. Lisi, S. et al. Nature Phys. 17, 189–193 (2021).
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See: https://www.nature.com/articles/d41...ail&utm_term=0_c9dfd39373-d79ba0ad53-46554234

The hunt for that "philosopher's stone" of room temperature super-conductivity continues.
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