LK-99 — the copper-substituted lead apatite claimed as a room-temperature superconductor in 2023 — showed a magnetization anomaly near 105°C that fueled the initial excitement. Subsequent attempts to reproduce the superconductivity failed, but the magnetic anomaly persisted across different synthesis conditions. Something was happening; it just wasn't superconductivity.
The authors (arXiv:2603.23377) identify the culprit: glassy magnetic freezing from interacting CuS clusters. CuS is a common secondary phase in LK-99 synthesis. Its magnetic clusters interact weakly, producing a glassy freezing transition that mimics the temperature-dependent magnetization drop expected from a superconducting transition.
The through-claim: the LK-99 magnetic anomaly has a mundane explanation that is more interesting than the extraordinary claim. Glassy freezing of CuS clusters produces a signal that looks like a Meissner effect in bulk measurements but reveals itself as glass when probed with frequency-dependent susceptibility. The diagnostic that distinguishes superconductivity from magnetic glass — frequency dependence — wasn't applied in the original claim. The impurity is the story.