Faultory

Case file 80 · Science & Discovery

The flake that hung halfway

LK-99 claim, 2023: did a copper-doped lead compound superconduct at room temperature?

Subject
LK-99 superconductor claim (2023)
Reading
12 min
Evidence
5 exhibits
Reasoning
3 steps
Level
Intermediate
A small blue-grey piece floating above a silver ring magnet against a warm, blurred background.
6 March 2015 A high-temperature superconductor levitating above a ring magnet, shown for comparison. It is not a photograph of LK-99. Photo: Julian Litzel · CC BY-SA 3.0 · Wikimedia Commons.
Illustrative signature. An illustrative strip chart of electrical resistance falling in a steep step as the sample warms, not plotted data. Drawn to explain the case, not plotted from source data.

01 · The briefing

Begin with the question.

On 22 July 2023, Sukbae Lee and colleagues posted two preprints to arXiv claiming that a material called LK-99 was a superconductor at room temperature and ordinary atmospheric pressure. The compound is a copper-doped lead apatite, written Pb10-xCux(PO4)6O, and it was made by a solid-state method. A superconductor carries current with no electrical resistance and, in its strongest form, pushes magnetic fields out of its interior, which is the Meissner effect. A confirmed material of that kind would be a major discovery, so the claim spread fast through social media, with live replication attempts and images of small flakes that seemed to float.

Two readings competed. Supporters pointed to the levitation and to a drop in electrical resistance. Sceptics suggested that a magnet can hold a magnetic flake at an angle, and that an impurity can make a material look like a superconductor without being one. This investigation separates three things: what the preprints claimed, what the replication studies measured, and which explanation those measurements support. The question is not only whether LK-99 is a superconductor, but why it looked like one, and which test would have settled the matter.

The timeline

  1. Two arXiv preprints by Lee and colleagues claim room-temperature, ambient-pressure superconductivity for LK-99.

  2. Guo, Li and Jia report that small flakes of LK-99-like samples half-levitate over a Nd2Fe14B magnet, without a Meissner effect or zero resistance.

  3. Zhu and colleagues report a sharp drop in resistance in samples containing copper sulfide, linked to a structural transition, with no zero resistivity.

  4. Jain reports a copper(I) sulfide phase transition at 104 °C and argues that a valid test requires LK-99 made without copper sulfide.

  5. Nature news reports that replication efforts had pieced together why LK-99 looked superconducting.

A researched reconstruction for learning. These reasoning steps simplify a complex record; no score can capture the full history.

02 · The evidence

What does the record show?

A dark, irregular pellet sitting on the top face of a silver cylindrical magnet.
On the evidence table25 July 2023 A dark LK-99 pellet on a silver magnet, in an image dated July 2023. Photo: Hyun-Tak Kim · CC BY 4.0 · Wikimedia Commons.

Read each exhibit before settling on an explanation. The source links below every exhibit lead to the record behind it.

Exhibit 01

The claim as posted

The preprint abstract describes a modified lead apatite, Pb10-xCux(PO4)6O with x between 0.9 and 1.1, made by solid-state synthesis. The authors describe the material as an ohmic metal above its claimed critical temperature and interpret levitation as the Meissner effect at room temperature and atmospheric pressure. This establishes what was claimed and how the authors read their own observations. It does not establish superconductivity, because it reports the authors' interpretation and was posted before peer review.

Source record s1arxiv.org

Exhibit 02

A magnetic explanation for the flake

Guo, Li and Jia made LK-99-like ceramic samples by solid-state sintering. Powder X-ray diffraction showed the main phases were the lead apatite and copper(I) sulfide. In some small flakes they observed half levitation above an Nd2Fe14B magnet. They attribute it to soft ferromagnetism combined with the flake's shape, and state that their measurements show no Meissner effect and no zero resistance. This supports a magnetic explanation for the floating flake that does not need superconductivity. It does not rule out other behaviour in samples that were not tested.

Source record s2arxiv.org

Exhibit 03

A resistance drop tied to copper sulfide

Zhu, Wu, Li and Luo measured the transport and magnetic properties of pure Cu2S and of LK-99 samples containing it. They report a sharp, superconducting-like transition with thermal hysteresis, but no zero resistivity. They argue that the drop is most likely a reduction in resistivity caused by a first-order structural phase transition of Cu2S at around 385 K. Measuring pure Cu2S strengthens the link. The authors present this as their argument, and the exact mechanism in every sample is not settled by this paper alone.

Source record s3arxiv.org

Exhibit 04

A test that removes the impurity

Jain's preprint states that copper(I) sulfide has a known phase transition at 104 °C, from an ordered low-temperature phase to a high-temperature superionic phase. It argues that the polycrystalline LK-99 samples contain a significant fraction of copper sulfide, so the apparent superconductivity may be linked to that transition. It concludes that LK-99 must be synthesised without any copper sulfide before its superconducting properties can be validated. This gives a clear test design. Its 104 °C figure differs from the 385 K given by Zhu and colleagues, and the sources do not resolve that gap.

Source record s4arxiv.org

Exhibit 05

The news verdict and what could be read

Nature's news report of 16 August 2023 describes replication efforts as having pieced together why LK-99 displayed superconducting-like behaviour. Its reference list cites the preprints discussed here. This establishes that a major science news outlet treated the replication results as settling the question. This case uses only the article's standfirst and reference list, not its detailed argument.

Source record s5nature.com

03 · Your reasoning

Follow the strongest explanation.

Choose an interpretation, then compare it with the evidence. You can revise your choice. The goal is understanding, not a perfect score.

Reasoning step 01 of 03

Why did one LK-99 flake hang at a tilt over a magnet?

Evidence-based explanation

Guo, Li and Jia report half levitation in small flakes over a Nd2Fe14B magnet and attribute it to soft ferromagnetism combined with shape anisotropy. That explains the tilt without a superconducting current. The caveat is that their samples were LK-99-like materials made by their own group, so the study shows one plausible explanation rather than proving what the original flake was doing.

Weigh it against

Exhibit 02A magnetic explanation for the flakeExhibit 01The claim as posted

Reasoning step 02 of 03

What best explains the sharp drop in resistance near 385 K reported in copper sulfide samples?

Evidence-based explanation

Zhu and colleagues report a sharp resistivity drop, with thermal hysteresis but no zero resistivity, in LK-99 samples that contain Cu2S. They argue the drop comes from a first-order structural transition of Cu2S near 385 K, and they also measured pure Cu2S. Jain's preprint gives 104 °C for a copper sulfide transition, so the exact temperature is not settled by these sources.

Weigh it against

Exhibit 03A resistance drop tied to copper sulfideExhibit 04A test that removes the impurity

Reasoning step 03 of 03

Which test would best settle whether LK-99 is a superconductor?

Evidence-based explanation

Jain's preprint states that LK-99 must be synthesised without copper(I) sulfide to allow unambiguous validation of its superconducting properties. A sample free of the impurity tests the confound that Zhu and colleagues argue is most likely behind the resistance drop. The test is decisive only if it also shows zero resistance and Meissner expulsion, not just a change in the readings.

Weigh it against

Exhibit 04A test that removes the impurityExhibit 05The news verdict and what could be read

04 · The findings

The chain behind the outcome.

The best-supported conclusion is that LK-99 was not shown to be a superconductor. Replication studies within weeks offered two effects that could account for the headline observations without superconductivity. The first is magnetic. Guo, Li and Jia observed half levitation in small flakes over a magnet, attributed it to soft ferromagnetism combined with shape, and reported no Meissner effect and no zero resistance. The second is a copper sulfide transition. Zhu and colleagues argue that the sharp drop in resistance is most likely due to Cu2S, which has a known structural change, and Jain makes the same link. The two sources give different temperatures, about 385 K in one and 104 °C in the other, and these were not reconciled in the cited sources. Nature's news report of 16 August 2023 described replication efforts as having pieced together why the material looked superconducting. The lesson is that a claim can be partly right about a measurement and wrong about its cause, and that the decisive test removes the suspected impurity.

Two log-scale plots of electrical resistance against temperature, with a small photograph of a sample on a green board inset.
How it ended2 August 2023 A still from an August 2023 livestream: a Southeast University group's resistance plot for an LK-99 flake, marking zero resistance near 110 K, far below room temperature. Photo: Sun Yueyue et al. · CC BY-SA 4.0 · Wikimedia Commons.

Contributing causes and conditions

  1. A solid-state synthesis that left copper(I) sulfide as a significant impurity
  2. A structural phase transition in Cu2S that lowers electrical resistivity near 104 °C or 385 K
  3. Thermal hysteresis that makes the drop look like a sharp superconducting transition
  4. Small flakes whose shape and soft ferromagnetism let a magnet hold them partly up
  5. Preprints circulated before peer review, so early measurements spread faster than controlled tests
  6. Replications that found no zero resistance and no Meissner expulsion in the samples they tested

OutcomeLK-99 superconductor claim (2023)

Take the method with you

  1. Separate what was observed from what was interpreted, and check which of the two a headline result actually establishes.
  2. Look for the test that removes the suspected confound, not a repeat of the same measurement under the same conditions.
  3. Before treating agreement or disagreement between replications as decisive, check whether they used the same material and measurements.

You can read findings at any time. Exploring every reasoning step marks this case as complete in your notebook.

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The open record

Sources & further reading

Read the original record and its scope before relying on a conclusion. Source references support the exhibit summaries; Faultory’s reasoning prompts are editorial reconstructions.

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  3. s3
  4. s4
  5. s5

Written and curated for Faultory by Yunus Emre Vurgun, with AI-assisted research and drafting. Sources and interpretations can need correction. Read our editorial standards or report an issue.

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