Superconductivity is well known among so-called "traditional" superconductors. However, unconventional superconductors have recently emerged, but it is not clear how they work.
A team at HZDR, along with colleagues from the CEA, Tohoku University in Japan, and the Max Planck Institute for Solid State Chemical Physics, now explains why a new material continues to superconduct even in extremely high magnetic fields – a property that is missing from super-strong magnetic fields. Conventional superconductors. This discovery has the potential to enable previously unimaginable applications of the technology. The study was published in Nature Communications.
Uranium ditelluride, or UTE 2 for short, is a leader in superconducting materials," says Dr. Toni Helm of the HZDR Dresden High Magnetic Laboratory (HLD). "As discovered in 2019, this compound can conduct electricity without loss, but in a different way than conventional superconductors. ”
Since then, research groups around the world have become interested in this material. Among them is Helm's team, who are one step closer to understanding this compound.
To fully understand the hype surrounding this material, we need to take a closer look at superconductivity," the physicist explained. "This phenomenon is caused by the movement of electrons in the material. Whenever they collide with atoms, they lose energy in the form of heat. This manifests itself as resistance. Electrons can avoid this by arranging themselves in the form of pairs, the so-called Cooper pairs".
The Cooper pair describes two electrons combining at low temperatures, passing through a solid without friction. They use the vibrations of the surrounding atoms as a kind of wave that can surf without losing energy. These atomic vibrations explain the traditional superconductivity.
However, for many years, it has also been known that Cooper pairs in superconductors are formed by effects that are not yet fully understood," said the physicist. One possible form of unconventional superconductivity is spin triplet superconductivity, which is thought to take advantage of magnetic fluctuations.
There are also some metals where the conduction electrons come together," Helm explains. "Together, they shield the magnetism of the material, which is represented by the extremely high mass of individual particles (for electrons). ”
This superconducting material is known as a heavy fermion superconductor. Thus, as the current experiments have shown, ute 2 may be both a spin triplet and a heavy fermion superconductor. In addition to this, it is also a heavyweight world champion – to date, no other known heavy fermion superconducting material has been able to achieve it under similar or higher magnetic fields. This is also confirmed by this study.
Superconductivity depends on two factors: the critical transition temperature and the critical magnetic field. If the temperature is below the critical transition temperature, the resistance drops to zero and the material becomes superconductive. The external magnetic field also affects superconductivity. If these exceed the threshold, the effect collapses.
Physicists have a rule of thumb for this," Helm said. "In many conventional superconductors, the transition temperature value in Kelvin is about one to two times the value of the critical magnetic field strength in Tesla. In spin triplet superconductors, this ratio is usually much higher. ”
Through the study of the heavyweight ute 2, the researchers are now able to raise the bar even higher: in 16 Kelvin ( 271.)55°C), the critical magnetic field strength reaches 73 Tesla, and the ratio is set to 45 – a record.
So far, heavy fermion superconductors have not aroused interest in technical applications," the physicist explained. "They have very low transition temperatures, and the effort required to cool them is relatively high. ”
However, their insensitivity to external magnetic fields can compensate for this shortcoming. This is because lossless current transfer is currently mainly used in superconducting magnets, such as magnetic resonance imaging (MRI) scanners. However, the magnetic field also affects the superconductor itself.
A material that can withstand very high magnetic fields and still be able to conduct electricity without loss would represent a big step forward.
Of course, UTE 2 cannot be used to make leads for superconducting electromagnets," Helm said. "Firstly, the properties of the material make it unsuitable for this work, and secondly, it is radioactive. But it's well suited to explore the physics behind spin triplet superconductivity. ”
Based on their experiments, the researchers developed a model that can explain superconductivity, with extremely high stability to magnetic fields. To do this, they looked at samples that were a few microns thick – a fraction of the thickness of a human hair (about 70 microns). As a result, the radioactive radiation emitted by the sample is still much lower than the natural background.
To obtain and shape such tiny samples, Helm uses a high-precision ion beam with a diameter of only a few nanometers as a cutting tool. UTE 2 is an air-sensitive material. Therefore, the Helm prepares the sample in a vacuum and then seals it with epoxy adhesive.
In order to conclusively prove that our material is a spin triplet superconductor, we must spectroscopy it when exposed to a strong magnetic field. However, current spectroscopic methods still struggle in magnetic fields above 40 Tesla. Together with other teams, we are "also working on the development of new technologies. Ultimately, this will allow us to provide clear evidence," Helm said.
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