LK-99 is the name of the material that is being hotly debated around the world these days: A Korean research group published results at the end of July 2023 suggesting that it could be a superconductor that remains superconducting even at room temperature and normal atmospheric pressure, i.e. conducts electricity completely without electrical resistance. Superconductors known to date retain their properties only when either cooled to very low temperatures or subjected to extremely high pressure.
If this assumption is confirmed, it would be a huge breakthrough: Such a high-temperature superconductor has repeatedly been referred to as the -Holy Grail- of materials science. Such a material would revolutionize the way we generate, transport and store electricity and use electric motors. However, there are still justified doubts. At the Vienna University of Technology, the material has now been analyzed with computer simulations and some interesting discoveries have been made: The calculated electron states are actually quite favorable for superconductivity. Of course, this is not yet proof - but it is another reason to pay serious attention to the new material.
The first step is the band structure
Prof. Liang Si of Northwestern University Xi’an and Karsten of the Institute of Solid State Physics at the Vienna University of Technology started computer simulations to analyze the new material LK-99 immediately after the discovery became known. -The decisive factor is the so-called band structure of the material," explains Karsten Held. -It tells us what combinations of speed and energy are possible for the electrons in this material. If you know this band structure, you can tell a lot about the electrical properties of the material.Using density functional theory, Liang Si and Karsten Held were able to calculate this band structure. They found that the electrical repulsion between the electrons means that the material in its pure form should actually be a so-called Mott insulator - a material that does not conduct any current at all, in a sense the opposite of a superconductor. In the experiments, therefore, a doped version of the material was probably used unintentionally - i.e. one in which certain additional atoms were incorporated. And if additional electrons are added to the material by this kind of doping, then the result looks completely different.
-We see relatively flat lines in the band structure, and we know that there are various mechanisms that can lead to superconductivity in such a band structure-, says Karsten Held. So it does seem to be within the realm of possibility that LK-99 (with suitable doping) is a superconductor. -Confirmed by another research group in Beijing, who concluded in initial experiments that LK-99 is a paramagnetic insulator. You have to dope the material to get the band structure that potentially enables superconductivity,- Held said.
Three other research groups also performed density functional theory calculations at the same time, with similar results. -This is not yet proof of high-temperature superconductivity; it is still possible that it is not a superconductor. But our results at least nourish a bit of hope that it might indeed be a long-sought high-temperature superconductor-, says Karsten Held.
Superconductivity or diamagnetism?
When a superconductor is placed on a magnet, electric current begins to flow on the surface of the superconductor, which in turn generates a magnetic field. The superconductor is repelled by the magnet and can thus float above the magnet. Therefore, one of the central arguments that LK-99 is a superconductor was a video showing LK-99 floating above a magnet. These experiments have since been confirmed by other experimental groups.However, it was criticized that in this case it could also be a different effect - there are different forms of magnetism: The best known is ferromagnetism, which is shown, for example, by pieces of iron that can be attracted with a magnet. Paramagnetic materials can also be attracted with a magnet, but unlike iron, they cannot be permanently magnetized themselves. The opposite of this is diamagnetism: diamagnetic materials are repelled by a magnet.
-Thus it would be conceivable that LK-99, if it hovers over a magnet, could also be an ordinary diamagnet. This has been suspected again and again in recent days," says Karsten Held. However, according to his own theoretical calculations, this is now less likely: -The electronic properties that we have calculated do not lead us to expect that LK-99 is a diamond magnet. On the contrary, given the distribution of electrons, one would rather suspect that LK-99 should be paramagnetic.-The experiments from Beijing show the same. This would mean that levitation of the LK-99 samples would indeed indicate a transition to the superconducting state.
So has the Holy Grail of materials science now been found? Many more steps are needed to verify this. -There are still very good reasons to be skeptical-, says Karsten Held. -I wouldn’t bet my money at the moment that this is indeed a high-temperature superconductor-at least not at 1:1 odds-but the results at least show that LK-99 is indeed a very interesting material that deserves closer attention. It remains exciting.-
Original publication
Not peer-reviewed, open access on arxiv:https://arxiv.org/abs/2308.00676
