LK-99 is a newly discovered material with potential room temperature superconductivity. It has a gray-black appearance. Its structure is slightly modified from lead-apatite by introducing small amounts of copper.
LK-99 was first synthesized by a team of researchers including Professor Sukbae Lee from Korea University in 2023. The team claims it functions as a superconductor at ambient pressure and below 400K (-173°C; -280°F).
The chemical composition of LK-99 is approximately Pb9Cu(PO4)6O. In the structure, approximately one quarter of Pb(II) ions are replaced by Cu(II) ions.
The material is claimed to be a room temperature superconductor. A video shows a sample of LK-99 partially levitating over a large magnet, indicating potential diamagnetism. However, its superconducting properties need further confirmation.
The discovery of LK-99 was announced in a preprint paper on arXiv before peer review. Materials scientists are skeptical about the claims and replication attempts are underway by research groups worldwide.
LK-99 has a hexagonal structure slightly modified from lead-apatite, by introducing small amounts of copper. The material was first discovered and manufactured by a team of researchers including Sukbae Lee (이석배), and Ji-Hoon Kim (김지훈) from Korea University (KU). The team claims it functions as a superconductor at ambient pressure and below 400 K (127 °C; 260 °F). As of 31 July 2023, the material has not been confirmed to be superconducting at any temperature. The synthesis of LK-99 and observation of its superconductivity has not been peer reviewed or independently replicated.
The chemical composition of LK-99 is approximately Pb9Cu(PO4)6O such that—compared to pure lead-apatite (Pb10(PO4)6O)—approximately one quarter of Pb(II) ions in position 2 of the apatite structure are replaced by Cu(II) ions. Partial replacement of Pb2+ ions (measuring 133 picometres) with Cu2+ ions (measuring 87 picometres) is said to cause a 0.48% reduction in volume, creating internal stress inside the material. The internal stress is claimed to cause a heterojunction quantum well between the Pb(1) and oxygen within the phosphate ([PO4]3−) generating a superconducting quantum well (SQW).
Lee et al. claim to show LK-99 exhibits a response to a magnetic field (potentially due to the Meissner effect) when chemical vapor deposition is used to apply LK-99 to a non-magnetic copper sample. Pure lead-apatite is an insulator, but Lee et al. claim copper-doped lead-apatite forming LK-99 is a superconductor, or at higher temperatures, a metal. They do not claim to have observed any change in behavior across a transition temperature.
The paper explain their mechanism based on a 2021 paper by Hyun-Tak Kim describing a BR-BCS theory of superconductivity where the BR term comes from a 1970 classical work by W.F Brinkam and T.M. Rice and BCS term comes from the standard Bardeen–Cooper–Schrieffer theory of superconductivity, nevertheless the paper is far for mainstream physics currently having less than 10 citations and being published in Scientific reports a journal with a lax peer review and a history of controversial papers. They also use ideas from the theory of hole superconductivity by J.E.Hirsch another work of controversial nature.
On August 1, Sinead Griffin of Lawrence Berkeley National Laboratory published a preprint analyzing the reported structure of LK-99 with density functional theory. This analysis suggested a potential mechanism for copper-substituted lead apatite to develop correlated isolated flat bands, a common signature of high-transition-temperature superconductors.
The name LK-99 is from the initials of discoverers Lee and JH Kim, and the year of discovery (1999). The pair had originally been working with Professor Tong-Shik Choi (최동식) at Korea University in the 1990s. In 2008, researchers from Korea University founded the Quantum Energy Research Centre [Q-Centre]. Lee would later become CEO of Q-Centre, and Kim would become director of research and development (R&D) at Q-Centre.
When Tong-Shik Choi died in 2017, he requested in his will that LK-99 research be continued. Q-Centre got new funding in the same year and interest in LK-99 research was renewed in 2018. An initial paper was submitted to Nature in 2020, but rejected. Similarly-presented research on room-temperature superconductors by Ranga P. Dias had been published in Nature earlier that year, and received with skepticism—Dias's paper would subsequently be retracted in 2022 after its data was found to have been falsified.
Lee and JH Kim filed a patent application in 2021 which was published on 3 March 2023. A Korean trademark application for "LK-99" was filed on 4 April 2023 by the Q-Centre.
A series of academic publications summarizing initial findings came out in 2023, with a total of seven authors across four publications. The first publication appeared on arXiv on 22 July, listing Young-Wan Kwon, former Q-Centre CTO, as third author. A second preprint listed as third author Hyun-Tak Kim, former principal researcher at the Electronics & Telecommunications Research Institute and professor at the College of William & Mary.
The findings were submitted to APL Materials on 23 July 2023 for peer review. On 28 July 2023 Kwon presented the findings of the group at a symposium held at Korea University. That same day, Yonhap News Agency published an article quoting an official from Korea University as saying that Kwon was no longer in contact with the University. The article also quoted Lee saying that Kwon had left the Q-Centre Research Institute four months previously; that the academic papers on LK-99 were not finished; and that the papers had been uploaded to arXiv without the other authors' permission.
Materials scientists and superconductor researchers responded with skepticism. The highest-temperature superconductors known at the time of publication had a critical temperature of 250 K (−20 °C; −10 °F) at pressures of over 170 gigapascals (1,700,000 atm; 25,000,000 psi). The highest-temperature superconductors at atmospheric pressure (1 atm) had a critical temperature of at most 150 K.
As of 31 July 2023, the measured properties do not prove that LK-99 is a superconductor as the published material does not fully explain how the LK-99's magnetisation can change, demonstrate its specific heat capacity, or demonstrate it crossing its transition temperature. An alternative explanation for LK-99's stated partial magnetic levitation could be solely from non-superconductive diamagnetism.
As of July 2023, the experiment has not been successfully replicated, despite the initial experiments being completed in 2020. After the July 2023 publications release, independent groups reported that they had begun attempting to reproduce the synthesis. The results of those independent tests are expected within weeks.
The chemical composition of LK-99 approximates to Pb9Cu(PO4)6O. This implies that, in comparison with pure lead-apatite (Pb10(PO4)6O), around one quarter of Pb(II) ions situated at position 2 of the apatite structure are substituted by Cu(II) ions.
Lee et al. devised a method for the chemical synthesis of the LK-99 material by generating lanarkite from a 1:1 molar mixture of lead(II) oxide (PbO) and lead(II) sulfate (Pb(SO4)) powders, then heating at 725 °C (1,000 K; 1,340 °F) for 24 hours:
PbO + Pb(SO4) → Pb2(SO4)O
In addition, copper(I) phosphide (Cu3P) is produced by mixing copper (Cu) and phosphorus (P) powders in a 3:1 molar ratio in a sealed tube under vacuum and heated to 550 °C (820 K; 1,000 °F) for 48 hours:
Cu + P → Cu3P
The lanarkite and copper phosphide crystals are ground into a powder, placed in a sealed tube under vacuum, and heated to 925 °C (1,200 K; 1,700 °F) for between 5‒20 hours, leading to the formation of LK-99:
Pb2(SO4)O + Cu3P + O2 (g) → Pb10-xCux(PO4)6O + S (g), where (0.9 < x < 1.1)