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

a) An LK-99 sample synthesised by Guo, et al. (2023); b) X-ray spectroscopy of the sample indicated the presence of different elements; c) a partly levitating sample; d) a scanning-electron microscope image of the sample.
| Photo Credit: arXiv:2308.03110v2

When we use a room heater on a cold day and cosy up with a cup of tea, little do we think of the physics that makes the heater work. Most electrical conductors resist the flow of electric current, converting some of the electrical energy into heat. With a heater, we use this effect to generate the warmth that we feel.

While this property of conductors allows us to stay warm in the winter, in most instances, it is undesirable. For example, a substantial amount of electricity generated is lost while being transmitted between power plants and our factories and households as heat. Tiny wires inside computers and cellphones 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: why does a material become a superconductor at all? 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 when 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 (here and here) 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. While apatites have been well-studied, no one had anticipated that they could be superconductors – let alone one in ambient conditions.

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). The group’s two papers elicited a mixture of surprise and scepticism in the scientific community – surprise because of the apatite, and scepticism because of the history of superconductivity.

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. They also showed that the resistive state was restored if a sufficiently large amount of current was passed through the sample. They even included an image of the sample partially levitating over a magnet in their second paper – a famous test for superconductivity. But while all of these data suggested superconductivity, the group also missed several crucial tests, including some to confirm the quantum nature of the microscopic state of the system.

Despite their scepticism, research groups from around the world worked fervently to reproduce the South Korean team’s results. 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 groups did find a drop in resistance, and others found that their samples showed partial levitation in a magnetic field. 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

How can we reconcile these findings with those of the South Korean team? The key seems to be the way the material was prepared. 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.

Scientists who were already studying copper sulphide, for other purposes, pointed out that its arrangement of ions changes  when heated to 100º C, and that the material’s resistivity also jumps at that temperature for reasons quite unrelated to superconductivity. The South Korean LK-99 samples had shown a jump in resistance at almost the same temperature, meaning that the tantalising graphs in their papers were the handiwork of copper sulphide rather than LK-99. 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.

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.

The LK-99 story provides a view of science in action, even as the narrative remains that we are yet to find an ambient-condition superconductor.

The author is a professor at the Centre for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bengaluru.