2023-08-15 12:40:03
Unidentified superconducting objects (OSNIs), like UFOs, are shrouded in fascination and mystery. New OSNIs are published every year, but very few receive media attention. LK-99 is the new star of the usology. Today four articles have been published on arXiv on this material, one from India that synthesizes it and does not observe superconductivity, and another three theorists (German, Swedish and American) who conclude that it cannot be a superconducting material. The only one worth noting is the German one, who tries to answer the question: does the doping of LK-99 with holes or electrons allow superconductivity at room temperature? Using a theoretical model for two bands with the FLEX method (fluctuation exchange) the conclusion is that orbital and spin fluctuations do not allow it; as the figure illustrates, according to the FLEX method, the largest superconducting eigenvalue λSC < 0.1 for realistic values of the parameter U, according to the authors (as you surely expect, I have many doubts about the appropriate value of this parameter, which changes too much from one article to another, with everyone saying that they have chosen the most suitable one). But what is relevant is that, like most theoretical articles, they are based on the flat band structure observed by DFT (which should be questioned) and, furthermore, the FLEX method is a valid approximation outside the strong correlation regime. . Therefore, in my opinion, the new theoretical results that point to an insulating or even Mott-type state add very little to what we already know.
Regarding the Indian article, it compares the doping of LK-99 with nickel and with copper, in order to avoid copper impurities, such as Cu2S. Experimental results show that nickel doping gives rise to a paramagnetic insulator, rather than a diamagnetic semiconductor for copper. Beyond curiosity, the article does not contribute anything either. The other two articles are pure theoretical entertainment. The Swedish article even states it in its title; the author compares various materials related to LK-99 with SP-DFT (spin polarized density functional theory), using CASTEP software. Comparison of the results for Pb10(PO4)6O, Pb20(O2)(PO4)12, and Pb19Cu(PO4)6(O)2 does not provide information on the superconductivity of LK-99, but suggests that apatites doped with copper are interesting as per. And the American article, which calls itself a tutorial, compares with DFT+U (with a value of U = 4 eV) the copper doping of Pb10(PO4)6O, with the vanadium doping of SrTiO3, to conclude that the structure of flat bands, much vaunted by other theorists, cannot be observed in LK-99 when it is synthesized without atomic doping control. If the Cu ions dope the Pb(2) ions, the flat bands are observed, but if they dope the Pb(1) they are not observed, the latter being more favorable from an energetic point of view; without doping control, more Pb(1) will be doped than Pb(2) and the flat band structure will not be observed. Furthermore, the resulting material should be transparent at room temperature. All this was already known.
The new articles are: Indian article is H. Singh, A. Gautam, …, A. Ganguli, “On the Experimental Evidence for Possible Superconductivity in LK99,” arXiv:2308.06589 [cond-mat.supr-con] (12 Aug 2023), doi: https://doi.org/10.48550/arXiv.2308.06589; el artículo sueco es Itai Panas, «Entertaining the Possibility of RT Superconductivity in LK-99,» arXiv:2308.06684 [cond-mat.supr-con] (13 Aug 2023), doi: https://doi.org/10.48550/arXiv.2308.06684; the German article Niklas Witt, Liang Si, …, Tim Wehling, “No superconductivity in Pb9Cu(PO4)6O found in orbital and spin fluctuation exchange calculations,” arXiv:2 [cond-mat.supr-con] (14 Aug 2023), doi: https://doi.org/10.48550/arXiv.2308.07261; y el artículo estadounidense es Alexandru B. Georgescu,» Cu-doped Pb10(PO4)6O, and V doped SrTiO3 —a tutorial on electron-crystal lattice coupling in insulating materials with transition metal dopants,» arXiv:2308.07295 [cond-mat.str-el] (14 Aug 2023), doi: https://doi.org/10.48550/arXiv.2308.07295.
By the way, there is a fifth article that I have not echoed, because it seems to me nonsense, excuse me. A new statistical law of superconducting materials is proposed. According to this law, the critical temperature depends on the distance between the electronic bands associated with the different ions that make up the material. They support the idea with a figure made with the eye of a good cubero to support their idea. And they claim that based on DFT calculations for lead apatite Pb10(PO4)6O, their new empirical law predicts that it should be a superconductor at room temperature (no copper doping, which is untrue). And then they say that LK-99 will be too. Bullshit no matter how you look at it. But if you’re interested in reading it firsthand, it’s Ning Chen, Yang Liu, Yang Li, “Statistical Material Law Support for Room Temperature Superconductivity in the Lead Apatite System,” arXiv:2308.06349 [cond-mat.supr-con] (11 Aug 2023), doi: https://doi.org/10.48550/arXiv.2308.06349.
#articles #LK99 #add