New developments on the crystalline structure of the LK-99 semiconductor that affect theoretical models

2023-08-10 14:18:27

The DFT simulations point to the existence of flat bands in the electronic structure of the LK-99 material. However, on several occasions I have requested caution, as these theoretical studies use the crystal structure of LK-99 proposed by the Koreans (with very slight modifications); if your experimental data is unreliable, why should we trust your theoretical data? The crystal structure of the copper-doped lead apatite Pb10−xCux(PO4)6O used by Koreans is based on that published in 2023 for the oxipiromorphita Pb10(PO4)6O. A new crystal structure for the oxipiromorphita which offers us some surprises (X-ray diffraction of crystals prepared in 1968 is used): a crystalline superstructure is observed compared to the one published in 2003, with the lead ions Pb1 split into Pb1A, not bound to oxygen O’, and Pb1B , linked to them; this implies structural changes that affect the whole crystal and that could hinder copper doping (the replacement of some Pb2 ions by Cu ions). The change might seem small, but it breaks down the degeneracy that might be behind the flat bands; future studies with DFT will have to verify how the new crystalline structure affects the band structure of LK-99. The more the LK-99 material is studied, the more it becomes clear that its superconductivity at room temperature is fictitious.

A new replication of the LK-99 synthesis has also been published, a sample about 15 mm wide and 0.5 mm thick; its color is black after synthesis, but to machine the electrical contacts it has been polished, thus acquiring a brighter color. The diffractogram obtained is very similar to that of the Korean ones (including the presence of Cu2S and Cu2O impurities). The resistivity measurement shows a semiconductor material (note how the resistivity increases as the temperature drops). The most curious thing is that a sharp drop in resistance is observed at a temperature of ~387 K (which could have been misinterpreted by the Koreans as a superconducting transition). The synthesized material is diamagnetic, with a transition at 341 K (ZFC) and 354 K (FC); the data obtained do not allow to decide if this diamagnetic transition has its origin in the impurities or in the LK-99 as such. In any case, what interests you, a new experimental demonstration that LK-99 is not a superconductor at room temperature. In addition, two new theoretical articles have been published, one from the United States with an effective model and the other from Switzerland with DFMT simulations (Dynamical Mean Field Theory), but both fall into the same trap, they assume the flat band structure of the DFT simulations, so I think they are of no interest until they are confirmed with the new crystal structure.

I guess you’re already bored with so much scientific summer soap opera. I’m sorry, but in my opinion, enjoying science live is a real pleasure. So, until September, as long as new articles worth commenting continue to be published, the snake will live on this blog. The paper with the new crystal structure is Sergey V. Krivovichev, “The crystal structure of Pb10(PO4)6O revisited: the evidence of superstructure,” arXiv:2308.04915 [cond-mat.supr-con] (09 Aug 2023), doi: https://doi.org/10.48550/arXiv.2308.04915. La nueva replicación china es Hao Wu, Li Yang, …, Haixin Chang, «Observation of abnormal resistance-temperature behavior along with diamagnetic transition in Pb10−xCux(PO4)6O-based composite,» arXiv:2308.05001 [cond-mat.supr-con] (09 Aug 2023), doi: https://doi.org/10.48550/arXiv.2308.05001. . . . The two theoretical articles are Patrick A. Lee, Zhehao Dai, «Effective model for Pb9Cu(PO4)6O,» arXiv:2308.04480 [cond-mat.supr-con] (08 Aug 2023), doi: https://doi.org/10.48550/arXiv.2308.04480y Changming Yue, Viktor Christiansson, Philipp Werner, «Correlated electronic structure of Pb10−xCux(PO4)6O,» arXiv:2308.04976 [cond-mat.str-el] (09 Aug 2023), doi: https://doi.org/10.48550/arXiv.2308.04976.