2023-08-03 11:18:49
We already know that the hot material, LK-99, a copper-doped lead apatite, is not a superconductor at room temperature and pressure; all the evidence points to its being a semiconductor at room temperature. However, just as lead is a type I superconductor with a critical temperature of 7.2 K, LK-99 apatite could be a superconductor. The first synthesis of ultrapure LK-99 showing zero resistance below a critical temperature of 110 K (−163 °C), well above the temperature of liquid nitrogen (77 K), is published on arXiv. The record for cuprates is around 150 K (−123 °C), but LK-99 could be very useful if its high-temperature superconductivity is confirmed. But, be careful, of the six samples studied, only one has zero resistance (the others are semiconductors). Furthermore, the Meissner effect has not been observed in any sample. It also doesn’t seem to show a critical magnetic field (up to 9 tesla). I have serious doubts that it is a superconductor at 100 K. We will have to wait for future resistivity measurements, but it looks like a fiasco.
The problem of resistivity measurement is the small size of the samples and their great fragility (perhaps for this reason other groups that have synthesized this material have not been able to measure it). The machining of the contacts is very delicate and can incur in accidental short circuits induced by the low temperature that produce a zero resistance that is fictitious. In my opinion it is a bad sign that it was only observed in one of the six synthesized samples of Pb10-xCux(PO4)6O, with 0.9
LK-99 apatite promises to be an interesting material for both experimental and theoretical physicists (seven theoretical papers have already been published, most observing flat bands with DFT, which does not imply superconductivity in any case). Whether or not LK-99 is a high-temperature superconductor, it opens the door to the study of apatite superconductivity. It may end up being a new family of superconductors, like the cuprates, iron pnictures, and nickelates. The new article is Qiang Hou, Wei Wei, …, Zhixiang Shi, “Observation of zero resistance above 100∘ K in Pb10-xCux(PO4)6O,” arXiv:2308.01192 [cond-mat.supr-con] (02 Aug 2023), doi: https://doi.org/10.48550/arXiv.2308.01192.
Note that observing flat bands around the Fermi level means that there are strongly correlated electronic states, nothing more. There are superconductors that have this type of bands (such as MATBG with Tc ~ 1.7 K), but there is no proven cause-and-effect relationship between flat bands and superconductivity. New theoretical papers observing such bands in LK-99 using DFT with VASP are J. Cabezas-Escares, NF Barrera, …, F. Munoz, “Theoretical insight on the LK-99 material,” arXiv:2308.01135 [cond-mat.supr-con] (02 Aug 2023), doi: https://doi.org/10.48550/arXiv.2308.01135; Junwen Lai, Jiangxu Li, …, Xing-Qiu Chen, «First-principles study on the electronic structure of Pb10-xCux(PO4)6O (x=0, 1),» arXiv:2307.16040 [cond-mat.mtrl-sci] (29 Jul 2023), two: https://doi.org/10.48550/arXiv.2307.16040; Liang Si, Karsten Held, «Electronic structure of the putative room-temperature superconductor Pb9Cu1(PO4)6O,» arXiv:2308.00676 [cond-mat.supr-con] (01 Aug 2023), doi: https://doi.org/10.48550/arXiv.2308.00676; Rafal Kurleto, Stephan Lany, …, Daniel S. Dessau, «Pb-apatite framework as a generator of novel flat-band CuO based physics, including possible room temperature superconductivity,» arXiv:2308.00698 [cond-mat.supr-con] (01 Aug 2023), doi: https://doi.org/10.48550/arXiv.2308.00698; in addition to Griffin’s quoted in LCMF, 01 Aug 2023.
In addition, there are theoretical articles presenting very simplified models of LK-99 to facilitate the understanding of its potential properties (in my opinion of very little interest due to its simplicity), such as Omid Tavakol, Thomas Scaffidi, «Minimal model for the flat bands in copper -substituted lead phosphate apatite,” arXiv:2308.01315 [cond-mat.supr-con] (02 Aug 2023), doi: https://doi.org/10.48550/arXiv.2308.01315; G. Baskaran, «Broad Band Mott Localization is all you need for Hot Superconductivity: Atom Mott Insulator Theory for Cu-Pb Apatite,» arXiv:2308.01307 [cond-mat.supr-con] (02 Aug 2023), doi: https://doi.org/10.48550/arXiv.2308.01307.
In the new article an attempt has been made to synthesize LK-99 by four different procedures (in the figure they are S1, S2, S3 and S4). Only S1 and S2 have been successful, being a sample of S1 the one that has allowed us to observe zero resistance (without Meissner effect). The comparison between the X-ray diffraction spectrum of the S1 sample and the one that appears in the article by the Koreans indicates that a synthesis of higher purity has been achieved. I will not go into the (minor) details that have made it possible to achieve this (those interested can consult the article). Of course, zero resistance (a drop in resistivity of four orders of magnitude) is not enough to speak of a superconducting state. I would have liked to see measurements of magnetic susceptibility, thermal capacity, etc. Proclaiming that a material is a superconductor requires extensive study of many physical properties, which together support the observed superconductivity. A single property tells us nothing. Be careful not to extrapolate what is said on social networks and less rigorous media.
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