First-principles study on the electronic structure of LK-99： JMST, the first heavy release of Institute of Metals, Chinese Academy of Sciences

　　First author: Lai Junwen; Correspondent: Chen Xingqiu

　　Communication unit: Institute of Metals, Chinese Academy of Sciences

　　DOI: https://doi.org/10.1016/j.jmst.2023.08.001

　　01

　　Full text glance

　　LK-99 room temperature superconductivity has aroused widespread concern and controversy. Based on the first-principles calculation, this paper analyzes the mother phase structure of LK-99, and finds that the mother phase structure of LK-99 is an insulator band gap of 2.77 eV, which undergoes an insulator-metal transition through Cu-doped, and two strange flat bands appear near Fermi level. At the same time, we also studied the influence of different elements doping, and it can be determined that the reason why Cu doping leads to the transition of insulator metal is that the doped system has isolated electrons and is doped with elements of the same family as copper, and Au will give ideal results.

　　02

　　Research background

　　Recently, Korean scientists reported a breakthrough discovery, claiming that the first room temperature and atmospheric pressure superconductor was realized in copper-doped apatite lead ore (LK-99). The reported superconducting transition temperature (Tc) is as high as 400 K (at atmospheric pressure), and it is characterized in detail by experiments. In addition, the author provides a detailed sample synthesis method, which will enable other experimental groups to try to reproduce their findings. Despite the strong experimental evidence, there are still several important questions to be answered: what is the electronic structure of LK-99 and its parent compound, what is the role of copper, and can similar effects be achieved by doping other elements?

　　03

　　Research highlights

　　1) The relationship between electronic energy band and superconductivity of LK-99 is analyzed based on theoretical calculation for the first time.

　　2) It is found that LK-99 has a unique flat-band electronic structure near Fermi level, and its energy broadening is smaller than that of most other D-band superconductors. In addition, its main electronic properties can be described by a two-band low-energy efficient model.

　　3) We clarify the electron correlation and the influence of doping with different valence electrons and atomic volume on the flat-band electronic structure.

　　4) Our calculation shows that it is necessary to synthesize samples with doped Cu atoms substituted at Pb1 site to make LK-99 obtain superconductivity, but it is challenging to realize this doping configuration experimentally from the thermodynamic point of view.

　　04

　　Graphic analysis

　　Figure 1 shows the calculated energy band structure and DOS of LK-99. After doping Cu, LK-99 is metallic, and only a semi-filled very flat energy band (dispersion < 0.15 eV) passes through Fermi level. Fermi looks like a football on the surface, consisting of electron and hole pockets. There is another fully occupied flat belt under this semi-filled flat belt. We notice that the two low energy bands of LK-99 are much flatter than other related D-band superconductors. Because there are two flat bands near Fermi level, which are composed of 2p orbitals of O2 atom and the hybridization between Cu-3d state and 2p orbitals of its nearest neighbor O1 atom. It is worth mentioning that the conduction electrons are confined in the layer containing doped copper atoms, while other layers without doped copper atoms seem to be insulated. In addition, the PO4 cell surrounding the cylinder formed by Pb2 atoms also shows insulation characteristics, which leads to the formation of a one-dimensional conduction-like channel mediated by 1/4 O2 atoms along the C axis. More interestingly, we observed four VHS on these two flat belts, which originated from the saddle points of M and L in Brillouin zone. The existence of these unique flat bands may be related to the remarkable superconducting characteristics of LK-99 observed in experiments, which is worthy of further study.

　　Fig. 2 shows that both Ni and Zn doping will lead to energy gap opening. This can be understood from the electron filling. Because nickel has one electron less than copper, the semi-filled flat band at Fermi level becomes completely unfilled, then LK-99 will become a non-conductive semiconductor. When zinc is doped, because zinc has one electron more than copper, the energy band will be completely filled and Fermi level will rise, so LK-99 doped with zinc is also an insulator. Then we studied the doping of Ag and Au. As can be seen from Figures 2(c) and 2 (d), gold and silver are the same family elements of copper. When doped with gold or silver, its electronic structure and chemical environment are roughly the same, and its energy band structure is also roughly the same, that is to say, they are all metallic conductors and both are at Fermi level. However, we find that the doping effect of Au is more similar to that of Cu than that of Ag. This observation can be explained by the closer volume between gold-doped and copper-doped lead apatite. These results show that gold-doped lead apatite may also show superconducting characteristics similar to copper-doped lead apatite.

　　05

　　Conclusion Prospect

　　In this thesis, the first-principles electronic structure of LK-99 and its parent compounds were analyzed comprehensively. The results show that the parent compound of LK-99 without copper doping is an insulator with a large band gap, and the doping of copper leads to the transition of insulator metal and the appearance of strange flat-band electronic structure, which can lay a foundation for studying the role of electronic structure of LK-99 in superconductivity. These results show that LK-99 has a potential superconducting possibility, but more experimental results are urgently needed to verify and discuss this possibility in more detail.

　　06

　　references

　　[1] S. Lee, J.-H. Kim, Y.-W. Kwon, arXiv:2307.12008 (2023).

　　[2] S. Lee, J. Kim, H.-T. Kim, S. Im, S. An, K.H. Auh, arXiv:2307.12037 (2023).

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　　Quote this article

　　Junwen Lai, Jiangxu Li, Peitao Liu*, Yan Sun*, Xing-Qiu Chen, First-principles study on the electronic structure of Pb10?xCux(PO4)6O (x = 0, 1) , J. Mater. Sci. Technol. (2023).

　　https://doi.org/10.1016/j.jmst.2023.08.001.

　　Source: JMST.