Superconductors conduct electricity without resistance and could therefore revolutionize electronics and power grids. So far, however, the loss-free flow of electrons only worked with deep-frozen materials. Now, for the first time, researchers have succeeded in making a material superconducting at room temperature. The combination of carbon, sulfur and hydrogen becomes a superconductor at 15 degrees Celsius – but only under the extreme pressure of 267 gigapascals. This is more than double the pressure at the deepest point of the Mariana Trench. The scientists hope, however, to be able to optimize their material in such a way that it becomes a superconductor with less pressure.
Superconductivity, discovered in 1911, gives materials two decisive properties: On the one hand, their electrical resistance disappears, so that in this state they conduct electrons without loss. On the other hand, they repel magnetic fields, which deflects magnetic field lines around the material. This allows superconductors to float on a magnetic field – and this in turn enables applications such as magnetic levitation trains. But superconductors are also used in particle accelerators, in magnetic resonance tomography or in quantum computers. So far, however, these superconductors only lose their electrical resistance at ultra-cold temperatures, so they have to be cooled in a complex manner – for example with liquid nitrogen or helium. “The cost of keeping these materials so cold is so high that it has not yet been possible to realize their full potential,” explains senior author Ranga Dias of the University of Rochester in New York.
Hydrogen as a key player
That is why scientists have been looking for decades for materials that turn into superconductors even with little cooling or even at room temperature. They achieved initial successes with high-temperature superconductors such as cuprates, copper compounds that become superconducting at temperatures as low as minus 100 degrees. In 2015, a research group achieved an important breakthrough with a completely new type of superconductor: under high pressure, they made hydrogen sulfide (H2S) pass into the loss-free power line at minus 70 degrees. The scientists attribute this primarily to the properties of hydrogen: “To create a high-temperature superconductor, you need strong bonds and light elements – hydrogen is the lightest atom and the hydrogen bond is one of the strongest,” explains Dias.
Based on this, other teams have since made compounds of metals and hydrogen into superconductivity and gradually raised the transition temperature. The previous record was set in 2019 by a team led by Mikhael Eremets from the Max Planck Institute for Chemistry in Mainz with a lanthanum hydride. Under high pressure, this material became superconducting at minus 23 degrees.
Superconductivity at just under 15 degrees
But only now has the decisive breakthrough to superconductors at room temperature been achieved. For this, Dias, first author Elliot Snider and her colleagues assumed hydrogen sulfide as the starting material and added hydrogen and methane (CH4) to it. Under an initially comparatively moderate pressure of around four gigapascals, these components react with one another and a crystalline compound is formed, in the lattice of which methane replaces some of the hydrogen sulfide molecules. In addition, inclusions filled with hydrogen arise in the crystal. If this carbon-containing sulfur-hydrogen compound is put under further pressure, it changes to the superconducting state at around 267 gigapascals and a temperature of 14.55 degrees. “This transition to superconductivity was demonstrated by a sharp drop in electrical resistance, which fell to zero within less than one degree,” report Elliot and his colleagues.
This is the first time that a material has been made superconductive at a temperature above zero degrees – an important breakthrough, as the researchers emphasize: “Because of the need for cooling, superconducting materials have so far not been able to transform the world as much as it can many have introduced themselves, ”says Dias. “But our discovery is now tearing down these barriers and opening the door to many new potential applications.” The specialist magazine “Nature” reports on its breakthrough as the cover story. But even if superconductivity has now become a reality at room temperature, there is still a lot to be done before it can be used in practice, as Dias and his team admit. Because you still need a diamond die cell and enormous pressure to produce a tiny amount of this superconductor. But the scientists are confident that by further optimizing the proportions of the three elements, among other things, they can significantly reduce the pressure required for the transition to superconductivity. “A chemical tuning of our three-part system could preserve the properties of room temperature superconductivity at lower pressure,” said Dias and his team.
Source: Elliot Snider (University of Rochester, New York) et al., Nature, doi: 10.1038 / s41586-020-2801-z