Figure 1: Image of LK-99 (Source)
The news of a supposed superconductor at room temperature and ambient pressure is shaking the world. Well known as LK-99, this material is a modified-lead apatite crystal structure.
Before discussing whether LK-99 is a superconductor, it is important to define what a superconductor is. A superconductor is a material that can conduct direct current (DC) electricity without energy loss, meaning it has no resistance. Also, a superconductor does not allow magnetic fields to penetrate. When there is a small imperfection in the superconductor, it allows a magnetic field to penetrate, causing the superconductor to levitate when placed on top of a magnet. This phenomenon is known as the Meissner effect. Researchers believed that LK-99 could be a superconductor because it levitated over a magnet. However, according to researchers at Peking University, the levitation effect displayed by LK-99 is believed to be caused by ferromagnetism, the same concept behind how magnets attract iron. Therefore, LK-99 was not displaying the Meissner effect.
South Korean researchers also claimed that LK-99 showed a significant drop in resistivity. They specifically noted that as the sample cooled, at 104.8℃, it showed a tenfold drop in resistivity. However, Prashant Jain, a professor at the University of Illinois at Urbana-Champaign, noted that this temperature is very close to the temperature at which Copper Sulfide (Cu2S) undergoes a phase transition. This causes the resistivity of air-exposed Cu2S to drop dramatically.
This made sense because the reaction that synthesizes LK-99 uses an unbalanced recipe. Producing LK-99 also results in 17 parts copper and 5 parts sulfur. This created impurities, especially Cu2S. Consequently, it made sense that the South Korean researchers had observed a drop in resistivity. It was not because of LK-99 but the Cu2S impurities. Researchers at the Beijing National Laboratory for Condensed Matter Physics and Institute of Physics reinforced this claim when they tested two samples of LK-99, one with 5% Cu2S content and another with 70% Cu2S content. They observed no significant drops in resistivity in the 5% content but a significant drop in resistivity in the 70% content.
Given that both claims related to the Meissner effect and drops in resistivity have been debunked, and the Korean Society of Superconductivity and Cryogenics has announced that there is no evidence supporting the superconductivity of LK-99, it can be concluded that LK-99 is not a superconductor.
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