LK-99: the 'world-first' superconductor that was copper sulfide and iron

On 22 July 2023 two preprints appeared on arXiv from the Quantum Energy Research Centre, a small Seoul start-up run by Lee Sukbae and Kim Ji-Hoon. Their material, LK-99 — a copper-doped lead apatite named for Lee, Kim and the year 1999, when they began studying it at Korea University — was, they said, 'for the first time in the world', a superconductor at room temperature and everyday pressure, up to 127 °C.

The claim went global within days. Prediction markets briefly gave replication a 60% chance; a Huazhong University video of a sample 'partially levitating' went viral on Chinese social media; the team sent a second video to the New York Times; Korean and Chinese 'superconductor stocks' surged so hard the Korean exchange warned against speculative bets. Scientists asked to test a sample. The team said it would share one only after its own journal review was complete — as physicist Michael Fuhrer put it, 'there are many ways to get fooled'.

Replication did not wait for permission. A dozen labs failed to reproduce the effect within three weeks; Southeast University's reported resistance drop below 110 K was rejected by experts; Peking University found no Meissner effect and no zero resistance. The Korean Society of Superconductivity set up a verification committee, and the explanation emerged by mid-August: the dramatic resistivity drop came from a copper sulfide impurity with a phase transition near 104–127 °C, and the 'levitation' from ferromagnetic iron flecks. On 16 August Nature declared LK-99 not a superconductor.

The preprints were never formally retracted. In November the Chinese Academy of Sciences published its impurity analysis in the journal Matter — 'another nail in the coffin' — concluding the superconductivity-like behaviour was caused by the structural phase transition of copper sulfide. LK-99 went from Nobel talk to cautionary tale in under a month: the price of announcing a 24-year result with no sample anyone else could hold.

The team guarded its sample, so speculation filled the vacuum: viral videos and a stock frenzy ran until copper sulfide impurities and iron flecks explained the effect.

Announced before verified: two unreviewed preprints, a 'world first', and no sample anyone could test — the claim outran the evidence before the world could answer.

The team was split before the world even replied: a co-author said the first paper contained 'many defects' and was uploaded without his permission.

The show outran the science: the levitation and the resistance drop were consistent with mundane physics — a copper sulfide phase transition and iron impurities — that controls would have caught.

Refusing to share samples let speculation fill the vacuum: prediction markets, viral videos and a stock frenzy ran for weeks while the only people who held the material guarded it.

Extraordinary claims need a sample, not a preprint. The world held nothing, so it filled the gap with viral videos and a stock frenzy — until copper sulfide explained everything.

The preprints were never formally retracted and the journal submission stayed in limbo; the team blamed each other instead. In November 2023 the Chinese Academy of Sciences published its impurity analysis in Matter, concluding the superconductivity-like behaviour came from the structural phase transition of copper sulfide — 'another nail in the coffin'. The Korean verification committee's verdict was that no evidence supported the claim. LK-99 became shorthand for hype, and the material itself — a real purple lead apatite — conducts like a rock.