The name is 99. LK-99!
This is the name that a group of South Korean scientists named Sukbae Lee, Ji-Hoon Kim, and Young-Wan Kwon have conferred to a material that is – they recently reported – a superconductor at room temperature and pressure (in preprint papers available here and here). The material is a copper-doped lead apatite, a type of phosphate mineral.
While the labels ‘L’ and K can commonsensically be traced to the initials of the three scientists, the number 99 continues to be a bit of a puzzle (although some have associated it with the year of its discovery). Nonetheless, the significance of christening this claimed novel apparently-superconducting material after themselves cannot be lost. If indeed independent scientists are able to confirm that LK-99 is an ambient-condition superconductor, the scientists will have etched their names in history in more than just the material’s moniker.
Why the quest for a room-temperature superconductor?
The scientists’ claim has unsurprisingly caught the community of physicists by storm. We are taught as early as middle school that an electric current carried by a metal wire suffers losses owing to the wire’s electrical resistance. Indeed, a significant amount of electricity generated in power plants is lost in transmission for this reason. What if we could make materials that would offer no resistance to current flow?
Scientists discovered such materials more than a century ago. They found that elemental mercury, a liquid metal at ambient conditions, becomes a superconductor at an unimaginably cold temperature of -268 degrees Celsius. Years of painstaking research revealed that superconductivity is a rather common phenomenon in metals if they can be cooled down to similar temperatures.
In fact, in the late 1970s, scientists believed that we can’t have a superconductor at more than -240 degrees Celsius, which is well below the liquefaction temperature of nitrogen, -195 degrees Celsius. At the same time, it became clear that superconductors aren’t just perfect conductors of electricity – they also have many other exotic properties, as a result of their unique quantum nature. Physicists are currently using these exotic properties to help build, among other things, quantum computers and other sophisticated devices that could change the course of human evolution.
Against this background, the paramount importance of discovering a material that is a superconductor in ambient conditions should be evident.
What did tests of LK-99 reveal?
The South Korean group’s new work occurred in a rather unexpected material called an apatite. Apatites are minerals that have a phosphate scaffold with a tetrahedral, or pyramidal, motif: one phosphorus atom is surrounded by four oxygen atoms. Other atoms can sit in between these pyramids; different apatites have different properties based on which atoms these are. A mineral called hydroxyapatite contributes to the strength of tooth enamel and the bones of living organisms.
The novelty of the Korean group’s work is to start with lead apatite, obtained by filling the space between the phosphate pyramids with lead and oxygen ions. Then, some of the lead atoms are replaced with those of copper. This process is called a substitution.
The group reported that at 10% copper substitution, the wonder material LK-99 arises: copper-substituted lead appetite. The group subjected this material to a variety of tests and claimed that it has essentially zero resistance to the flow of an electric current. When the scientists increased the amount of current beyond a threshold value, called the critical current, a resistance to current flow suddenly appears – which is just as expected in a superconductor.
We also know that an external magnetic field is detrimental to superconductivity. The investigators found that in the presence of a magnetic field, the material continues to be a superconductor until the field strength crosses a critical threshold – another positive sign.
Crucially, the dependence of the critical current and the critical magnetic field was found to be qualitatively consistent with the known behaviour of superconductors. The group also reported heat capacity data – i.e. the amount of energy required to raise the temperature of the material by 1 centigrade per gram – but this was less convincing.
Why are copper oxides of interest?
In 1986, superconductivity physics witnessed a revolution when scientists found that some copper oxide materials became superconducting at above -240 degrees Celsius. Then again, despite the best efforts of a generation of scientists, the maximum temperature achieved in this system wouldn’t exceed -100 degrees Celsius, that too under an immense pressure.
More recently, scientists have synthesised sulphide and hydride materials that become superconductors at near room temperatures but under extreme pressure, such as that found at the centre of earth, which is achievable only in laboratory conditions.
Because of the lucre of a room-temperature superconductor, the field hasn’t been without controversy either. Some recent claims of superconductivity in a hydride material didn’t withstand scrutiny. The holy grail of an ambient condition superconductor has thus remained one of the most elusive and coveted prizes of the field. The South Korean group’s claim, if proved true, will therefore be groundbreaking.
Indeed, the wider scientific community has responded in a subdued and cautious way, owing in part to these and other controversies.
Are there problems in the new work?
Some of the data related to LK-99’s magnetic properties in the two papers appear to have some technical errors. Independent scientists have also called the data in some places “a bit sloppy” and “a bit fishy”. Then again, these views haven’t dampened interest in the material.
In the second paper by the same group (including more authors who contributed to the work), the researchers have provided instructions on preparing the material. Many research groups around the world will attempt to reproduce these results; there are already some rumours that independent scientists have done so. We will have to wait for the results of their studies.
At least one group in India, at the CSIR-National Physical Laboratory, New Delhi, attempted to replicate the findings and failed; but its findings were posted as an image on the lead investigator’s personal Facebook page, not in a journal.
There are signs that all is not well among the group members that have claimed such a momentous discovery. For example, the third author of the first paper is not among the authors of the second paper. The first paper has also come under criticism because it appears as if it was written in a hurry. Some have speculated that the number of its authors was limited to three because that’s how many people can receive a Nobel Prize at a time.
All this said, Drs. Lee, Kim, and Kwon themselves seem to have no doubts about their work, if we go by their paper’s words: “We believe that our new development will be a brand-new historical event that opens a new era for humankind”. Will copper-substituted lead apatite be a hit, or will it come a cropper? Time will tell.
The author is a professor at the Centre for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bengaluru.
