How scientists found that LK-99 is probably not a superconductor

General Studies Paper 3

Introduction

• Tiny wires inside computers and cell phones dissipate heat, draining the batteries in the process. So it is natural that scientists are looking for materials that can conduct electricity without resistance, especially for applications where heat loss is a deal-breaker.

An elusive material

• More than a century ago, scientists discovered that many metals become superconducting – i.e. allow current to flow with zero resistance – if cooled to below -250º C.
• This gave birth to a big physics puzzle about superconductor. The breakthrough came in the 1950s and 1960s, when scientists developed a theory of superconductivity.
• With this theory, they found that superconductors aren’t just materials with zero resistance: they have a remarkable new quantum state in which the electrons in the material work together.
• Several fantastic properties of superconductors then came to light, opening the door to new technologies – including advanced medical imaging, ‘maglev’ trains, and quantum computers.
• However, superconductivity also remained an extremely-low temperature-phenomenon for a long time.
• It was only in the mid-1980s that scientists discovered copper-oxide superconductors, whose transition temperature was higher than -200º C.
• But to this day, scientists haven’t made significant progress to elevate this figure to at or near ambient conditions.
• One of the highest transition temperatures has been found in a sulphide compound, but it needs to be placed under extreme pressures – like that found at the centre of the earth!
• The all-important discovery of an ambient-condition superconductor, which can herald radical new technologies, has eluded several generations of scientists.

Surprise and scepticism

• In July 2023, a group of scientists in South Korea uploaded two preprint papers claiming that a lead apatite material was an ambient condition superconductor.
• Apatites are materials that have a regular arrangement of tetrahedrally shaped phosphate ions (i.e. one phosphorus atom and four oxygen atoms).
• When lead ions sit in between these phosphate motifs, it is lead apatite.
• The novelty of the South Korean group’s work was to replace 10% of the lead ions in lead apatite with copper, to produce the supposed wonder material that they had christened LK-99 (after their own last names).

Independent verification

• In their papers, the group described subjecting their LK-99 samples to a variety of tests. They measured the material’s electric resistance, which seemed to drop below a certain temperature.
• They showed that the low resistance state vanished when a sufficiently strong magnetic field was applied.
• In their second paper, the team had provided instructions to synthesise LK-99.
• Researchers in Australia, China, India, the U.S., and several European countries followed them and tried to replicate the South Korean team’s findings – but no one found conclusive evidence of superconductivity in their samples.
• In fact, the Indian group, from the CSIR-National Physical Laboratory, New Delhi, was one of the first to report that it didn’t find any signs of superconductivity in LK-99.
• Some of the most recent work also tried to produce LK-99 using alternative methods. At least one group was able to make a highly pure crystal – where all the ions are regularly arranged in space.
• It had a brownish-purple hue and was transparent, which was unusual for a superconductor.
• More remarkably, this single crystal behaved like an insulator, showing no signs of superconductivity from low temperatures up to 800º C.
• Researchers also found that it was ferromagnetic – i.e. it could be magnetised by, say, rubbing a magnet on it. Superconductors cannot have this property.

Science in action

• The South Koreans had made lead sulphate react with copper phosphide to produce polycrystalline LK-99 (i.e. small crystallites randomly arranged in space, unlike in a single crystal, where the atoms are arranged regularly over very large distances) and some by-products.
• One of the important by-products was copper sulphide, which could have become embedded in the LK-99 matrix.
• Independent researchers confirmed this by using X-rays to ‘look’ inside the crystal.
• Researchers also found a more mundane way to explain the levitation: that the LK-99 sample also contained impurities (other by-products) that were diamagnetic, i.e. materials that could be magnetised but whose magnetic field is the opposite direction of the applied field. Diamagnetic materials can also partially levitate above magnets as a result.

Conclusion

• The current evidence suggests that LK-99 is not a superconductor. Even as the replication efforts were underway, some scientists also made models of LK-99’s quantum properties. They found that if copper atoms replaced a certain set of lead atoms in LK-99, the material would have some electronic states that are very interesting in that their kinetic energy could take on very restricted values. These are called flat-band systems. Electrons in flat-bands can interact strongly with each other and are predicted to form superconducting phases, but only at very low temperatures.