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LK-99 Article: Studies by Beihang University and CSIR-National Physical Laboratory

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The recent claim of ambient-pressure room-temperature superconductivity in LK-99 has generated significant excitement and attention within the scientific community. In this blog post, we will explore the findings of two research teams from Beihang University in China and CSIR-National Physical Laboratory in India, who conducted cross-verifications to ascertain the superconducting nature of LK-99. Before delving into those research results, let's first understand the basic concept of superconductors. 


What are the requirements to be a superconductor? 


Superconductors exhibit two crucial properties: zero electrical resistance below a certain transition temperature and the Meissner effect, which excludes external magnetic fields from the superconductor. 


1) Zero Electrical Resistance: 


In regular conductors like copper wires, we encounter electrical resistance, which leads to energy loss and scattering of electrons as they move through the material. However, in superconductors, electrons form pairs called Cooper pairs through a quantum mechanical phenomenon. At temperatures below the critical temperature (Tc), these Cooper pairs condense into a coherent state, allowing them to flow through the material without any scattering or energy loss. 


When a current is applied to a superconductor, these Cooper pairs move effortlessly through the lattice structure, creating a persistent current that circulates indefinitely without any dissipation of energy. This property of zero electrical resistance distinguishes superconductors from regular conductors and holds immense promise for revolutionizing various technologies. Imagine a world where electrical power can be transmitted over long distances without any loss, or where electronic devices can operate without overheating due to resistive losses. Superconductors have the potential to make these visions a reality, transforming energy distribution and electronic technology. 



2) The Meissner Effect: 


 The Meissner effect is another fascinating phenomenon observed in superconductors. When a superconductor transitions to its superconducting state below Tc, it expels almost all magnetic flux lines from its interior. As a result, the magnetic field is pushed out of the superconducting material, making it behave like a perfect diamagnet. This expulsion of magnetic fields occurs due to the formation of Cooper pairs, which disrupt the alignment of magnetic moments in the lattice structure, effectively canceling out any external magnetic fields. 


The Meissner effect is responsible for the strong repulsive force that a superconductor exhibits against magnetic fields, making it appear as if the material is actively pushing the magnet away. One impressive application of the Meissner effect is magnetic levitation, where a superconductor is cooled below its critical temperature and placed in proximity to a magnet. The repulsive force between the superconductor and the magnet allows the superconductor to levitate above the magnet, defying gravity. This levitation phenomenon has garnered significant attention and forms the basis for potential transportation technologies like Maglev trains. 


Types of Superconductors: 


 Superconductors can be broadly classified into two types based on their response to external magnetic fields: Type 1 and Type 2. 



Type 1 Superconductors: 


Type 1 superconductors are often referred to as "soft" superconductors. They have a single critical magnetic field value and exhibit a sharp transition from the normal to the superconducting state. Below their critical temperature, they expel all magnetic fields from their interior and exhibit perfect diamagnetism. However, Type 1 superconductors are limited in their ability to sustain strong magnetic fields. As the external magnetic field approaches their critical value, they undergo a phase transition to the normal state. This behavior restricts their practical applications in high-field environments. 



Type 2 Superconductors: 


Type 2 superconductors, also known as "hard" superconductors, are more versatile than Type 1. They exhibit a gradual transition from the normal to the superconducting state and can sustain higher critical magnetic fields. Below their critical temperature, Type 2 superconductors allow magnetic flux to partially penetrate their interior in the form of quantized vortices or magnetic flux tubes. These vortices can move through the material in response to changes in the external magnetic field, but they do not lead to a complete loss of superconductivity. Type 2 superconductors find extensive use in various applications, especially those requiring strong magnetic fields. They are crucial for applications such as Maglev trains, magnetic resonance imaging (MRI) machines, and high-energy physics experiments that rely on powerful superconducting magnets. 



The CSIR-National Physical Laboratory Group, "LK-99 needs more careful re-examination and investigation": 


 Kumar et al. from CSIR-National Physical Laboratory also conducted their cross-verification of LK-99, as reported in arXiv: 2307.16402.  The original paper argued that room-temperature superconductivity is achieved by doping Copper in a known compound Lead Apatite ${\rm Pb}_{10}({\rm PO}_4)_6{\rm O}$, which is referred to as LK-99. To cross-verify the properties of LK-99, they synthesized LK-99 by following the previously reported method. Specifically, they synthesized Pb2SO5 using the ingredients ${\rm PbSO}_4$ and PbO. Subsequently, using X-ray diffraction (XRD) measurements, they analyzed the crushed powder samples and confirmed that they resemble sample pieces of LK-99. However, according to their study, LK-99 lacks significant properties of superconductors, particularly the Meissner Effect. If the synthesized LK-99 were a superconductor at room temperature, it should repel a permanent magnet due to the Meissner effect, displaying antimagnetic properties. Contrarily, their results show that LK-99 exhibits properties of a paramagnetic body, meaning it is attractive to the magnet. In conclusion, Liu et al. assert that LK-99 is not a superconductor at room temperature [1]. 



The Beihang University Group, "LK-99 is not a superconductor": 


Liu et al. from Beihang University undertook a comprehensive study to verify the superconducting properties of LK-99, the modified lead-apatite compound, as reported in arXiv: 2307.16802. They followed the proposed method to synthesize LK-99 and synthesized ${\rm Pb_2SO_5}$, ${\rm Cu_3P}$, and finally the modified lead-apatite ${\rm Pb_{10-x}Cu_x(PO_4)_6O}$ to verify the claim of LK-99. Using the obtained samples, they checked the electrical transport and magnetic properties of LK-99. However, at room temperature, they confirmed that LK-99 is a semiconductor with a resistivity on the order of $10^4$ Ω·cm, rather than a superconductor. Furthermore, regarding magnetism, they also confirmed that LK-99 exhibits no repulsion for the magnet. In conclusion, they could not find properties of superconductivity in LK-99 at the claimed room temperature, and they insist that more careful re-examination and investigation are needed [2]. 



Conclusion: 


 In conclusion, the recent claims of ambient-pressure room-temperature superconductivity in modified lead-apatite (LK-99) have sparked immense excitement and scientific interest. Although the cross-verification studies conducted by Beihang University and CSIR-National Physical Laboratory have not yet passed peer review and require further verification, the independent results from both groups imply that LK-99 has no possibility of being a room temperature superconductor. Perhaps, additional cross-verifications for LK-99 will be undertaken by other research teams. Even if LK-99 is not confirmed as a room temperature superconductor, the process of verification has given rise to new ideas, fostering hope for further progress in the field of superconductivity. 


References: 


[1] K. Kumar, N.K. Karn, and V.P.S. Awana, Synthesis of possible room temperature superconductor LK-99: Pb9Cu(PO4)6O, arXiv:2307.16402. 

[2] L. Liu et al., Semiconducting transport in Pb10-xCux(PO4)6O sintered from Pb2SO5 and Cu3P, arXiv:2307.16802.

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