LK-99

LK-99, also called PCPOSOS, is a gray–black (or purple), polycrystalline compound, identified as a copper-doped lead‒oxyapatite. A team from Korea University led by Lee Sukbae (이석배) and Kim Ji-Hoon (김지훈) began studying this material as a potential superconductor in 1999, and in July 2023 published preprints claiming that it acted as a room-temperature superconductor at temperatures of up to 400 K (127 °C; 260 °F) at ambient pressure. Many different researchers attempted to replicate the work, and were able to reach initial results within weeks. By mid-August 2023, the consensus was that LK-99 is not a superconductor at room temperature, and is an insulator in pure form. A number of replication attempts identified non-superconducting ferromagnetic and diamagnetic causes for observations that suggested superconductivity. A prominent cause was a copper sulfide impurity occurring during the proposed synthesis, which can produce resistance drops, lambda transition in heat capacity, and magnetic response in small samples. After the initial preprints were published, Lee claimed they were incomplete, and coauthor Kim Hyun-Tak (김현탁) said one of the papers contained flaws.