LK-99 is a material that a team of South Korean researchers claimed in July 2023 to be the first room-temperature, ambient-pressure superconductor. The claim sparked enormous excitement online and in the scientific community because, if true, it would have been one of the most important discoveries in modern physics. Within weeks, however, independent labs around the world failed to reproduce the results, and the scientific consensus quickly settled: LK-99 is not a superconductor.
What the Original Claim Said
Researchers Sukbae Lee and Ji-Hoon Kim, along with colleagues at a small Korean startup called the Quantum Energy Research Centre, posted two papers to arXiv (a preprint server where scientists share work before peer review). They claimed to have synthesized a material with a critical temperature at or above 400 Kelvin (127°C), meaning it would superconduct well above room temperature and at normal atmospheric pressure. Both of those conditions, if real, would have shattered existing records. The best confirmed superconductors require either extreme cold (often colder than -100°C) or crushing pressures to function.
The material itself is a modified form of lead-apatite, a mineral with the base formula Pb₁₀(PO₄)₆O. The researchers replaced some of the lead atoms with copper, creating what they called LK-99. They argued that the copper substitution caused a tiny structural distortion, shrinking the crystal’s volume by about 0.48%, and that this distortion alone was responsible for superconductivity at room temperature.
Why It Went Viral
A room-temperature superconductor working at normal pressure would be transformative. Superconductors carry electricity with zero resistance, meaning no energy lost as heat. Today’s power grids lose significant energy during transmission. Rewiring them with a room-temperature superconductor would eliminate those losses entirely. Computers built with superconducting components wouldn’t overheat and would waste far less energy. Maglev trains, which currently require expensive cooling systems to keep their superconducting magnets working, could become practical for everyday transit. Superconducting energy storage, MRI machines, and quantum computers would all become cheaper and more accessible.
The researchers also posted a video showing a small dark pellet partially hovering above a magnet. This “half-levitation” looked like it could be the Meissner effect, a hallmark of superconductivity where a material expels magnetic fields and floats. The combination of the papers and the video turned LK-99 into a global sensation overnight, with labs racing to replicate the results and social media tracking every development in real time.
What the Levitation Actually Was
The partial levitation turned out to be far less remarkable than it appeared. Researchers who investigated the behavior carefully found that the “half-levitation” seen in the video could be produced by both ferromagnetic and diamagnetic materials. It wasn’t unique to superconductors at all.
A ferromagnetic substance (one attracted to magnets, like iron) and a diamagnetic substance (one weakly repelled by magnets) can both balance on the edge of a magnet if their weight, shape, and magnetic force happen to align. The partial levitation is simply a matter of geometry and force balance. The LK-99 samples that showed this behavior were identified as ferromagnetic, not superconducting. The inhomogeneity of the samples, meaning they were a messy mix of different phases and impurities, contributed to the misleading visual effect.
Why the Resistance Dropped
One of the key pieces of evidence the original team presented was a sharp drop in electrical resistance at a specific temperature, which they interpreted as the material transitioning into a superconducting state. Independent analysis identified the real culprit: copper sulfide (Cu₂S), a known impurity that forms during the synthesis process.
Cu₂S undergoes a structural phase transition at around 385 K (about 112°C), shifting from a hexagonal crystal structure at high temperatures to a monoclinic structure at lower temperatures. This first-order phase transition causes a sudden drop in resistivity that looks superficially like a superconducting transition but has nothing to do with superconductivity. The temperature at which this drop occurs lines up closely with the “critical temperature” the Korean team reported, strongly suggesting the resistance data was simply showing a well-known property of a contaminant, not a new state of matter.
Replication Attempts Failed
Labs around the world synthesized LK-99 samples and tested them. The results were consistent and discouraging for the original claim. Four-probe resistivity measurements, the standard way to check for superconductivity, showed that LK-99 at room temperature behaves like an insulator. It has extremely high resistance, the opposite of what a superconductor should show.
Magnetic susceptibility measurements found that the material does exhibit some diamagnetic behavior, but not the kind associated with superconductivity. A true superconductor shows perfect diamagnetism, completely expelling magnetic fields from its interior. LK-99’s weak diamagnetism fell far short of that threshold. One group reported a partial success in achieving superconductivity in copper-doped lead apatite, but only below 110 K (about -163°C), which is far below the claimed 400 K and within the range of already-known superconducting materials. It offered no support for the room-temperature claim.
A peer-reviewed study published in ACS Omega stated plainly: the experimental results reveal no superconductivity in LK-99. The synthesized phase is highly resistive and behaves like an insulator at room temperature.
What LK-99 Actually Is
Computational studies confirmed that the base compounds of lead-apatite are insulators with large band gaps, meaning electrons can’t flow freely through them. When copper is introduced, it distorts the crystal structure, shifting it to a lower-symmetry form and creating what physicists call a “flat band” in its electronic structure. Flat bands had generated theoretical interest as potential ingredients for exotic electronic behavior, which is part of why the claim attracted serious scientific attention initially. But in practice, the copper doping makes LK-99 behave as a semiconductor, not a superconductor. The electronic properties are interesting from a materials science perspective but don’t produce zero-resistance conductivity.
Adding sulfur impurities (another byproduct of the synthesis) further alters the band structure but still doesn’t produce superconductivity. The material is a messy, multi-phase mixture where the most dramatic-looking behaviors, the resistance drop and the partial levitation, are explained entirely by known properties of copper sulfide impurities and simple magnetic force balancing.
Where Things Stand
LK-99 is not a superconductor. No peer-reviewed study has replicated the original claims, and the specific observations that made it look promising have been explained by conventional physics. The resistance drop comes from a copper sulfide impurity. The levitation comes from ferromagnetism and geometry. The material itself is an insulator at room temperature.
The episode did highlight how hungry the world is for a genuine room-temperature superconductor and how quickly the scientific community can mobilize to test a claim. It also served as a reminder of why peer review exists: the original papers were preprints that hadn’t gone through formal vetting, and the extraordinary claims they made didn’t survive contact with independent replication.