A Puzzling Question

In our previous work, QevosAgent autonomously completed DFT calculations on LK-99, obtaining a 3.2 eV bandgap and confirming it as an insulator. But at that time, I raised a question:

"I have a doubt: since DFT calculations can directly yield a 3.2 eV bandgap, indicating an insulator, the LK-99 authors should easily have thought of this calculation. Why did they still believe it was a superconductor? Were there some special considerations that made them think certain mechanisms might still give it superconducting possibilities?"

This question touches on the deep tension between theory and experiment in scientific exploration. Today, QevosAgent will systematically梳理 the complete scientific story of LK-99 from controversy to truth based on in-depth research.

The Birth of LK-99 and the Core Contradiction

In July 2023, Korean scientists Lee Sukbae and Kim Ji-Hoon's team published two preprint papers on arXiv, claiming to have discovered a material named LK-99 (Pb${9}$Cu(PO${4}$)$_{6}$O) that exhibited superconductivity at room temperature (below approximately 132°C) and ambient pressure. This claim immediately caused a huge sensation in the global scientific community and the public, as room-temperature ambient-pressure superconductivity has long been the holy grail of condensed matter physics.

However, almost simultaneously, multiple research teams through Density Functional Theory (DFT) calculations found that LK-99's electronic structure showed a bandgap of approximately 3.2 eV, making it an insulator rather than a superconductor. This created the core contradiction: Since DFT calculations can directly predict LK-99 as an insulator, why did the original authors still insist it was a superconductor?

LK-99 Authors' Experimental Evidence

Diamagnetism (Magnetic Levitation)

The authors claimed to observe extremely strong diamagnetism in LK-99 samples, with diamagnetic strength reportedly exceeding graphite by 5000 times. They showed a video displaying LK-99 samples partially levitating above magnets. The authors believed such strong diamagnetism could only be explained by the Meissner effect of superconductors.

Resistance Drop

At approximately 132°C (405K), the authors observed a sudden drop in resistance of LK-99 samples. Although the resistance did not drop to zero, the authors attributed this to insufficient sample purity, believing high-quality samples should achieve zero resistance.

The Authors' Logical Chain

The authors' reasoning was:

1. Strong diamagnetism + resistance drop = superconductivity
2. Imperfect behavior (resistance not reaching zero, incomplete levitation) = caused by impurities or defects
3. Therefore, LK-99 is a superconductor, just with insufficient sample quality

Why Might the Authors Ignore DFT's "Insulator" Verdict?

1. Experimental Primacy

The authors took experimental phenomena as primary evidence, believing DFT calculations might not fully capture the complexity of actual samples. Their logic was: experiments showed diamagnetism and resistance drop, which is evidence of superconductivity.

2. Legitimate Doubts About DFT Limitations

Standard DFT (LDA/GGA) indeed has known limitations when dealing with strongly correlated electron systems:

• Strong correlation failures: DFT often fails in predicting Mott insulators, high-temperature superconductor parent materials, etc.
• Structural differences: DFT is based on ideal crystal structures, while actual samples are polycrystalline, defective, and strained
• Electron correlation effects: If LK-99 involves strongly correlated Cu d-electrons, standard DFT might incorrectly predict the ground state

3. The Temptation of Flat Band Theory

The theoretical community found isolated flat bands in LK-99's band structure. Flat bands mean electrons have extremely large effective mass and minimal kinetic energy, enhancing Coulomb interactions, which theoretically could support unconventional superconductivity. This provided the authors with some theoretical psychological support.

Possible Special Mechanisms: If It Were a Superconductor, What Would Be the Mechanism?

Although ultimately disproven, these theoretical explorations revealed LK-99's complex electronic structure:

Correlated Flat Band Superconductivity

Cu doping in the apatite framework creates localized Cu-d orbital states, forming isolated flat bands. If half-filled flat bands exist, under appropriate doping, superconducting pairing might be mediated by antiferromagnetic exchange, similar to cuprate superconductors.

Spin-Orbit Coupling (SOC) and Topological Effects

Pb is a heavy element with extremely strong SOC effects. SOC might open or close bandgaps, or even induce topologically non-trivial band structures. Theoretically, topological surface states could support superconductivity, but room-temperature topological superconductivity is extremely difficult to achieve.

Interface/Defect and Non-Equilibrium States

Grain boundaries, defects, or strain might create localized superconducting regions. Metastable phases during synthesis might have electronic properties different from the ground state, while DFT calculates the ground state.

Scientific Community's Reproduction and Truth

The Real Culprit of Resistance Drop: Cu₂S Impurity Phase Transition

Multiple studies ultimately revealed that LK-99 samples contained copper(I) sulfide (Cu₂S) impurities. Cu₂S undergoes a first-order structural phase transition at approximately 104°C (from diamagnetic β-phase to metallic α-phase), causing the resistance drop. This perfectly explains the "resistance drop" phenomenon, but it's just an impurity phase transition, not a superconducting transition.

The Real Culprit of Diamagnetism: Ferrimagnetism

The observed "partial levitation" was not the complete diamagnetic Meissner effect, but rather the result of ferrimagnetism of impurity phases interacting with the external magnetic field. True superconducting levitation completely expels magnetic field lines, while LK-99's levitation was unstable and partial.

Multiple Reproduction Results

Teams from the Chinese Academy of Sciences, UC Davis, Stanford, MIT, Harvard, and others all failed to reproduce superconductivity, confirming the samples as insulators. Nature magazine officially published confirming LK-99 is not a superconductor.

Latest Research Progress (2024-2026)

Although the superconductivity dream was shattered, LK-99 family materials have shown rich intrinsic quantum behaviors, becoming a new platform for studying complex quantum phenomena.

Glassy Magnetic Freezing

The latest research (arXiv:2603.23377, 2026) systematically confirmed that the magnetization anomalies observed in LK-99 family materials (such as ZFC/FC bifurcation, magnetic memory effects) originate from glassy magnetic freezing of interacting clusters in CuS (covellite) impurity phases. This clarified magnetic behaviors previously misinterpreted as "vortex glass" or superconducting signals, emphasizing the complexity of impurity phases in multiphase materials.

Structural Reinterpretation

New research (arXiv:2410.03722, 2024) proposed that Cu does not substitute Pb but is located in apatite channels, forming [O-Cu-O] molecular ions. The localized nature of flat bands originates from the localization of these molecular ions. Fully reduced LK-99 is a wide-bandgap insulator (with intermediate bands), while partial reduction allows hole hopping. This provides a new structural basis for understanding flat bands.

Dynamical Stability and Anharmonic Effects

Research (npj Computational Materials, 2024) proved that copper-doped lead apatite is dynamically stable at room temperature. Anharmonic phonon-phonon interactions play a key role in stabilizing certain copper-doped phases, reconciling early theoretical controversies about its instability.

Strong Correlation and Mott Insulator

Research (Materials Horizons, 2024) confirmed LK-99 is a charge-transfer Mott insulator. The correlated metallic state exhibits non-Fermi liquid characteristics, and strong correlation effects in flat bands may lead to complex electronic phase diagrams.

New Material Screening and Future Potential

The latest research (arXiv:2509.20260, 2025) developed DFT screening programs, identifying Pb₉Cu(VO₄)₆Br₂ as the most promising candidate material, with thermodynamic stability and symmetry robustness of electronic structures near the Fermi level. The apatite platform can serve as an ideal system for studying flat-band-related phenomena (such as flat-band superconductivity, topological properties).

Conclusion and Outlook

The LK-99 event is a paradigm of scientific self-correction. It demonstrates:

1. Risks of preprint dissemination: Unreviewed research may cause over-hype
2. The scientific community's powerful self-correction ability: Rapid reproduction and cross-validation (experiment + theory) ultimately reveal the truth
3. DFT is not omnipotent: Especially in strongly correlated systems, multi-angle experimental evidence is needed
4. Value of unexpected discoveries: Although LK-99 is not a room-temperature superconductor, it opened a new chapter in researching complex quantum behaviors such as flat bands, strong correlations, and glassy magnetism in copper-doped apatite systems

QevosAgent demonstrated the powerful capabilities of AI Agents in materials science research by autonomously completing DFT calculations, literature research, and cross-validation. From calculation to verification, from controversy to truth, the entire process reflects the rigor of scientific exploration and the huge potential of AI-assisted research.

This article is based on in-depth research work autonomously completed by QevosAgent, comprehensively analyzing multiple key research papers on LK-99 and related systems from 2023-2026.