Technology attributes
LK-99 is a material with a modified lead-apatite structure created by a team of South Korean researchers. In two papers published in July 2023, the researchers claimed LK-99 demonstrates superconducting properties at room temperature and ambient pressure, due to abrupt drops in resistivity and the material levitating above a magnet. After numerous attempts to replicate the results reported in the original papers, it was shown that LK-99 is not a room-temperature, ambient pressure superconductor. Research shows impurities in the material (in particular copper sulfide) caused sharp drops in electrical resistivity and partial levitation over a magnet.
The material is named after the principal authors (LK-99, Lee-Kim 1999). LK-99 is a compound of lanarkite (Pb₂SO₅) and copper phosphide (Cu₃P) baked in a four-day, multi-step, small batch, solid-state synthesis process. LK-99 has a modified-lead apatite crystal structure with the composition:
All previously confirmed superconductors function only at extreme temperatures and pressures. The development of a room-temperature superconductor has the potential to transform a number of technologies, radically increasing the efficiency of electrical systems, and leading to new use cases.
In two papers published on July 22, 2023, the researchers (led by Sukbae Lee and Ji-Hoon Kim) from Korea University, Seoul, describe initial experiments with LK-99 dating back to 2020. The researchers claimed LK-99 is a room-temperature superconductor with a critical temperature above 400K (Kelvin) or 127oC. Above this critical temperature, the material exhibits Ohmic metal characteristics. Below it, LK-99 demonstrates superconducting properties at ambient pressure. The papers were accompanied by a video showing the material levitating above a magnet, a phenomenon caused by the Meissner effect. The claims made by the papers were met with skepticism by the scientific community, with teams around the world attempting to recreate their results.
The papers state the material's superconductivity originates from two factors:
- Volume contraction that results from an insulator-metal transition caused by substituting lead with copper
- On-site repulsive Coulomb interaction that is enhanced by the structural deformation in the one-dimensional chain (Pb2-O1/2-Pb2 along the c-axis) structure due to superconducting condensation at the critical temperature
The structural distortion by slight volume contraction (0.48%) is not caused by external factors, such as temperature or pressure. The substitution of Pb2+ ions with Cu2+ ions in the insulating network of Pb(2)-phosphate generates stress, concurrently transferring Pb(1) of the cylindrical column resulting in distortion of the column interface, generating superconducting quantum wells.
Following the publication of the LK-99 papers, Sinéad Griffin from Lawrence Berkeley National Laboratory simulated the material, showing that the claims were theoretically possible in a July 31 paper.
On August 1, 2023, a team from the Huazhong University of Science and Technology in Wuhan, China, claimed to have replicated LK-99 floating above a magnet. However, they did not show the low resistance required to verify the claims of room-temperature superconductivity. On August 3, 2023, scientists at Southeast University in Nanjing, China, reported preliminary results showing zero electrical resistance in a sample of LK-99 that they produced themselves. However, they only achieved zero resistance at -163oC (110K), not room temperature. Zero resistance was measured using a four-point probe method showing zero resistance at -163oC and normal air pressure. They also verified that LK-99 transitioned in and out of zero resistance depending on whether it was subject to a strong electric field.
Separate teams at Peking University and the Chinese Academy of Sciences (CAS) found mundane explanations for the phenomena described in the initial papers. Research by US and European scientists combined theoretical and experimental evidence to show how the LK-99 structure could not be a superconductor. Another team synthesized and studied pure samples of LK-99, removing doubts the scientific community had about its structure and confirming it is an insulator, not a superconductor.
