The reaction that synthesizes LK-99 uses an unbalanced recipe: for every 1 part copper-doped lead phosphate crystal — pure LK-99 — it makes, it produces 17 parts copper and 5 parts sulfur. These leftovers lead to numerous impurities — especially copper sulfide, which the Korean team reported in its sample.
Jain, a copper-sulfide expert, remembered 104ºC as the temperature at which Cu2S undergoes a phase transition if exposed to air. Below that temperature, Cu2S’s resistivity drops dramatically — a signal almost identical to LK-99’s purported superconducting phase transition. “I was almost in disbelief that they missed it.” Jain published a preprint on the important confounding effect on 7 August.
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“That was the moment where I said, ‘Well, obviously, that’s what made them think this was a superconductor,’” says Fuhrer. “The nail in the coffin was this copper sulfide thing.”
Science is hard. Kudos to everyone involved for trying to replicate it and figuring this puzzle out.