a) An LK-99 pattern synthesised by Guo, et al. (2023); b) X-ray spectroscopy of the pattern indicated the presence of various components; c) a partly levitating pattern; d) a scanning-electron microscope picture of the pattern.
| Photo Credit: arXiv:2308.03110v2
When we use a room heater on a chilly day and cosy up with a cup of tea, little will we consider the physics that makes the heater work. Most electrical conductors resist the circulate of electrical present, changing a number of the electrical power into warmth. With a heater, we use this impact to generate the heat that we really feel.
While this property of conductors permits us to remain heat within the winter, in most situations, it is undesirable. For instance, a substantial quantity of electrical energy generated is misplaced whereas being transmitted between energy crops and our factories and households as warmth. Tiny wires inside computer systems and cellphones dissipate warmth, draining the batteries within the course of. So it is pure that scientists are in search of supplies that can conduct electrical energy with out resistance, particularly for purposes the place warmth loss is a deal-breaker.
An elusive materials
More than a century in the past, scientists found that many metals turn out to be superconducting – i.e. permit present to circulate with zero resistance – if cooled to beneath -250º C. This gave delivery to a massive physics puzzle: why does a materials turn out to be a superconductor in any respect? The breakthrough got here within the Nineteen Fifties and Sixties, when scientists developed a idea of superconductivity. With this idea, they found that superconductors aren’t simply supplies with zero resistance: they’ve a exceptional new quantum state during which the electrons within the materials work collectively. Several improbable properties of superconductors then got here to gentle, opening the door to new applied sciences – together with superior medical imaging, ‘maglev’ trains, and quantum computer systems.
However, superconductivity additionally remained an extremely-low temperature-phenomenon for a very long time. It was solely within the mid-Nineteen Eighties when scientists found copper-oxide superconductors, whose transition temperature was larger than -200º C. But to today, scientists haven’t made important progress to raise this determine to at or close to ambient circumstances. One of the best transition temperatures has been found in a sulphide compound, nevertheless it must be positioned underneath excessive pressures – like that found on the centre of the earth!
The all-important discovery of an ambient-condition superconductor, which may herald radical new applied sciences, has eluded a number of generations of scientists.
Surprise and scepticism
In July 2023, a group of scientists in South Korea uploaded two preprint papers (right here and right here) claiming that a lead apatite materials was an ambient situation superconductor. Apatites are supplies that have a common association of tetrahedrally formed phosphate ions (i.e. one phosphorus atom and 4 oxygen atoms). When lead ions sit in between these phosphate motifs, it is lead apatite. While apatites have been well-studied, nobody had anticipated that they might be superconductors – not to mention one in ambient circumstances.
The novelty of the South Korean group’s work was to switch 10% of the lead ions in lead apatite with copper, to provide the supposed marvel materials that they’d christened LK-99 (after their very own final names). The group’s two papers elicited a combination of shock and scepticism within the scientific group – shock due to the apatite, and scepticism due to the historical past of superconductivity.
Independent verification
In their papers, the group described subjecting their LK-99 samples to a number of exams. They measured the fabric’s electrical resistance, which appeared to drop beneath a sure temperature. They confirmed that the low resistance state vanished when a sufficiently robust magnetic discipline was utilized. They additionally confirmed that the resistive state was restored if a sufficiently great amount of present was handed by means of the pattern. They even included a picture of the pattern partially levitating over a magnet of their second paper – a well-known check for superconductivity. But whereas all of those knowledge urged superconductivity, the group additionally missed a number of essential exams, together with some to verify the quantum nature of the microscopic state of the system.
Despite their scepticism, analysis teams from world wide labored fervently to breed the South Korean staff’s outcomes. In their second paper, the staff had supplied directions to synthesise LK-99. Researchers in Australia, China, India, the U.S., and a number of European nations adopted them and tried to copy the South Korean staff’s findings – however nobody found conclusive proof of superconductivity of their samples. In reality, the Indian group, from the CSIR-National Physical Laboratory, New Delhi, was one of many first to report that it didn’t discover any indicators of superconductivity in LK-99.
Some teams did discover a drop in resistance, and others found that their samples confirmed partial levitation in a magnetic discipline. Some of the newest work additionally tried to provide LK-99 utilizing different strategies. At least one group was in a position to make a extremely pure crystal – the place all of the ions are recurrently organized in area. It had a brownish-purple hue and was clear, which was uncommon for a superconductor. More remarkably, this single crystal behaved like an insulator, displaying no indicators of superconductivity from low temperatures as much as 800º C. Researchers additionally found that it was ferromagnetic – i.e. it might be magnetised by, say, rubbing a magnet on it. Superconductors can’t have this property.
Science in motion
How can we reconcile these findings with these of the South Korean staff? The key appears to be the way in which the fabric was ready. The South Koreans had made lead sulphate react with copper phosphide to provide polycrystalline LK-99 (i.e. small crystallites randomly organized in area, in contrast to in a single crystal, the place the atoms are organized recurrently over very giant distances) and a few by-products. One of the vital by-products was copper sulphide, which might have turn out to be embedded within the LK-99 matrix. Independent researchers confirmed this through the use of X-rays to ‘look’ contained in the crystal.
Scientists who had been already finding out copper sulphide, for different functions, identified that its association of ions modifications when heated to 100º C, and that the fabric’s resistivity additionally jumps at that temperature for causes fairly unrelated to superconductivity. The South Korean LK-99 samples had proven a bounce in resistance at virtually the identical temperature, that means that the tantalising graphs of their papers had been the handiwork of copper sulphide somewhat than LK-99. Researchers additionally found a extra mundane method to clarify the levitation: that the LK-99 pattern additionally contained impurities (different by-products) that had been diamagnetic, i.e. supplies that might be magnetised however whose magnetic discipline is the wrong way of the utilized discipline. Diamagnetic supplies can even partially levitate above magnets as a consequence.
The present proof suggests that LK-99 is not a superconductor. Even because the replication efforts had been underway, some scientists additionally made fashions of LK-99’s quantum properties. They found that if copper atoms changed a sure set of lead atoms in LK-99, the fabric would have some digital states that are very fascinating in that their kinetic power might tackle very restricted values. These are known as flat-band programs. Electrons in flat-bands can work together strongly with one another and are predicted to kind superconducting phases, however solely at very low temperatures.
The LK-99 story offers a view of science in motion, even because the narrative stays that we’re but to seek out an ambient-condition superconductor.
The creator is a professor on the Centre for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bengaluru.
