Superconductivity is a popular research topic in modern physics and material science, due to its unprecedented and promising implications in technological advancements which could revolutionize daily life. Superconductive substances exhibit zero resistance as they drop below a critical temperature which varies in different materials, enabling efficient transmission of electronic signals and energy (Energy.gov, 2024). As a result, scientists have been trying to find superconductors at room temperature, which would allow multiple applications of superconductivity at everyday temperatures.
LK-99
Recently, a group of South Korean scientists led by Sukbae Lee claimed that they had discovered a room-temperature superconductor, LK-99: a mixture of copper, lead, phosphorus and oxygen. According to them, the substance not only shows zero-resistance at room temperature and pressure, but also exhibits an iconic feature of a superconductor, floating in the air when applied to a magnetic field (Leffer, 2023).
This revolutionizing breakthrough immediately attracted attention and led to excitement from scientists and the general public, incurring numerous attempts to replicate the substance and to verify the discovery. Disappointingly, the fact that LK-99 was a room-temperature superconductor was sadly disproved.
Disproving LK-99’s superconductivity
The phenomena of LK-99 floating in a magnetic field can be easily explained by ferromagnetism, a non superconductive property inherent to iron and rare earth materials. Ferromagnets are intrinsically magnetic, and therefore can be strongly repelled by the applied magnetic field. This explanation is a powerful rebuttal to LK-99’s superconductivity, in addition to scientists’ failure to yield identical results to Lee’s research. Richard Greene, a physicist studying superconductors at the University of Maryland, generally agrees with the primary conclusion, “The experimental papers showing ferromagnetism were pretty convincing, and the new theories are also more carefully done.” (Leffer, 2023)
To further validate this speculation, Prof. Luo Jianlin’s team from the Chinese Academy of Science conducted an experiment that confirmed the invalidity of LK-99’s superconduction claims. The team discovered Cu₂S, or copper (I) sulfide, as an impurity in LK-99, and tested the resistivity of Cu₂S under changing temperature (Fig.1). At around 385 °F, which is close to the “critical temperature” of LK-99, Cu₂S undergoes a significant decline of resistivity by three to four orders of magnitude, along with a structural change that alters its hexagonal shape to a monoclinic structure. When mixed with Cu₂S, LK-99 exhibits a similar result to Lee’s paper, as its resistivity decreases sharply at the predicted temperature. Hence, the substance contributing to the near-zero resistance phenomenon is Cu₂S, not LK-99 (English.cas.cn, 2023).
Some may claim that Cu2S should be recognized as a room-temperature superconductor, but the transition phase of Cu2S is significantly different from real superconductors.
Figure 1: Temperature dependence of resistivity of Cu₂S, and LK-99 mixed with Cu₂S (English.cas.cn, 2023)
Superconductors undergo second-order phase transitions, in which the substance neither releases or absorbs energy. There is no latent heat, no entropy change, and no phase coexistence. Oppositely, first-order phase transitions are accompanied by a change in entropy. The change of phase occurs abruptly during first-order phase transitions, while noticeable fluctuations would be observed before second-order phase transitions. Common examples of the two transitions are the melting process and ferromagnetic change respectively (Dur.ac.uk, 2017). This leads back to our discussion of Cu2S’s qualification as a superconductor, because it goes through first-order phase transition instead of the second, it is not a superconductor (Zhang Nannan, 2023).
Conclusion
LK-99 gained its fame from its similar properties to room-temperature superconductors, but they can be attributed to other non-superconducting phenomena and impurities. Despite its failure, LK-99 is still an important milestone on our journey of discovering superconductors, as it introduced scientific discussions to the general public and contributed to cross-country collaboration.
Works Cited
Dur.ac.uk. (2017). Phase Transitions 3. [online] Available at: https://www.ippp.dur.ac.uk/~krauss/Lectures/NumericalMethods/PhaseTransitions/Lecture/pt3.html. [Accessed 12 May 2024].
Energy.gov. (2024). DOE Explains...Superconductivity. [online] Available at: https://www.energy.gov/science/doe-explainssuperconductivity [Accessed 12 May 2024].
English.cas.cn. (2023). Myth of Room Temperature Superconductivity in Lk-99 Is Shattered----Chinese Academy of Sciences. [online] Available at: https://english.cas.cn/newsroom/research_news/phys/202311/t20231127_645010.shtml [Accessed 12 May 2024].
Leffer, L. (2023). The Superconductor Sensation Has Fizzled, and That’s Fine. [online] Scientific American. Available at: https://www.scientificamerican.com/article/the-superconductor-sensation-has-fizzled-and-thats-fine1/ [Accessed 12 May 2024].
Zhang, N. (2023). Myth of room temperature superconductivity in LK-99 is shattered. [online] Phys.org. Available at: https://phys.org/news/2023-11-myth-room-temperature-superconductivity-lk-.html#google_vignette [Accessed 12 May 2024].
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