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Copper-doped lead apatite, called LK-99, was initially claimed to be a room temperature superconductor driven by flat electron bands, but was later found to be a wide gap insulator. Despite the lack of room temperature superconductivity, there is growing evidence that LK-99 and related compounds host various strong electron correlation phenomena arising from their flat electron bands. Depending on the copper doping site and crystal structure, LK-99 can exhibit two distinct flat bands crossing the Fermi level in the non-interacting limit: either a single or two entangled flat bands. We explore potential correlated metallic and insulating phases in the flat bands of LK-99 compounds by constructing their correlation phase diagrams, and find both non-Fermi liquid and Mott insulating states. We demonstrate that LK-99 is a charge-transfer Mott insulator driven by strong electron correlations, regardless of the flat band type. We also find that the non-Fermi liquid state in the multi-flat band system exhibits strange metal behaviour, while the corresponding state in the single flat band system exhibits pseudogap behaviour. Our findings align with available experimental observations and provide crucial insights into the correlation phenomenology of LK-99 and related compounds that could arise independently of superconductivity. Overall, our research highlights that LK-99 and related compounds offer a compelling platform for investigating correlation physics in flat band systems.
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This content will become publicly available on July 8, 2026
Investigating Copper Doping in Lead Oxyapatite: Flux Growth of Copper-Containing Pb 10 (PO 4 ) 6 O Single Crystals
A flux growth method for the high-yield synthesis of Pb10(PO4)6O single crystals was developed. This optimized synthesis method was utilized for the systematic doping of copper into the lead apatite structure, where the availability of single crystals allowed for a detailed structural analysis via single crystal X-ray diffraction. Our developed synthesis method resulted in a different type of copper doping in lead apatite where instead of Cu(II) doping onto the lead sites, doping occurred via Cu(I) doping into the disordered channels at the center of the Pb(1)/Pb(2) columns in the form of CuO2 linear units. The presence of Cu(I) as opposed to Cu(II) in the structure was shown via magnetic susceptibility data which also indicated a lack of superconductivity in the modified lead apatite at all achieved doping levels. The knowledge gained from this study enabled a reanalysis of the initial diffraction data reported for LK-99. While this work further supports the lack of superconductivity in the modified lead apatites, it does allow us to suggest a more accurate formula for LK-99: Pb10(PO4)6(CuO)2O1-1.5x.
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- Award ID(s):
- 2401995
- PAR ID:
- 10658986
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- Chemistry of Materials
- Volume:
- 37
- Issue:
- 13
- ISSN:
- 0897-4756
- Page Range / eLocation ID:
- 4894 to 4900
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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