Scientists discovered how tiny changes in superhydride structure enable superconductivity at near room temperatures but extreme pressure - offering clues for designing more practical superconductors.
Superconductors allow electricity to flow without resistance, meaning no energy is lost as heat. This property makes them useful for technologies such as MRI scanners, particle accelerators, magnetic-levitation trains and some power-transmission systems. Most superconductors, however, only work at extremely low temperatures - often hundreds of degrees below zero Fahrenheit. Keeping materials that cold requires complex and costly cooling systems, which limits where the superconductors can be used.
Researchers at the U.S. Department of Energy's (DOE) Argonne National Laboratory have helped take a step toward easing that limitation. They have gained new insight into a class of materials called superhydrides that can become superconducting at much higher temperatures - around 10 degrees Fahrenheit.
The research was carried out with collaborators from the University of Illinois Chicago (UIC), the University of Chicago and DOE's Lawrence Livermore National Laboratory. A key tool was the upgraded Advanced Photon Source (APS), a DOE Office of Science user facility at Argonne.
Superhydrides are made mostly of hydrogen, held together in an ordered structure (crystal) by a small number of metal atoms. When subjected to extremely high pressures, these materials can carry electric current with no resistance. In a landmark 2018 study, researchers led by UIC professor Russell Hemley showed that a lanthanum-based superhydride could superconduct near 8 degrees Fahrenheit. The drawback was that it only worked at pressures equivalent to those deep within the Earth (1.88 million atmospheres), making it impractical outside the lab.
In the new study, Hemley and his fellow researchers explored whether changing the material's chemistry could lower the pressure needed for superconductivity. They added a small amount of yttrium to the lanthanum superhydride to make it more stable and reduce the pressure required.
"To reach these extreme pressures, we squeezed a tiny sample between two diamonds," said Maddury Somayazulu, a physicist at the APS. The team's diamond-anvil device can generate pressures as high as five million atmospheres.
After forming the superconducting material at high pressure and temperature, the team used high-energy X-rays from the APS to study its structure (at beamlines 16-ID-B and 13-ID-D). "We focused an intense X-ray beam onto a sample only a few micrometers thick and about ten to twenty micrometers across," said Vitali Prakapenka, a beamline scientist and research professor at the University of Chicago. One micrometer is about 1/70th the width of a human hair.
The recent APS upgrade made these measurements possible. Its brighter, more tightly focused X-ray beam allowed researchers to study extremely small samples while changing the pressure. "That beam allowed us to separate signals coming from the tiny sample itself as opposed to those coming from the surrounding materials and diamond anvils," Prakapenka said.
The team found that small differences in how atoms are arranged in a crystalline lattice can strongly affect superconductivity. They identified two different crystal structures, each becoming superconducting at a slightly different temperature.
"These experiments show what the upgraded APS can do," Somayazulu said. "We can now study atomic-level structures with unprecedented detail in materials under extreme pressure."
Although the pressures used in the experiments are still very high - about 1.4 million times atmospheric pressure - the researchers see this as part of a longer path forward. They are adding more elements to lower the pressure further with the goal of making these materials practical.
Diamonds provide a useful comparison, Somayazulu explained. Natural diamonds form deep inside the Earth under extreme pressure and temperature. Scientists later learned how to synthesize them in the lab, and eventually how to produce them without such intense conditions. Researchers believe superhydrides could follow a similar path.
"If we understand the physics well enough, we may be able to stabilize these structures at much lower pressures but still attain superconductivity close to room temperature," Prakapenka said.
Experimental data from the APS will help guide theoretical models and AI tools in that search for new materials. Instead of testing only a few combinations at quite-challenging-to-reach extreme conditions, scientists can use AI to explore many possible multi-element compositions. They can then focus experiments on the most promising ones.
"The calculations are very demanding," Prakapenka said. "Theorists rely on high-quality experimental data to make their predictions more accurate."
Finding a material that superconducts at near room temperature and normal pressure could reshape the nation's electrical infrastructure.
The research was supported by the DOE Office of Basic Energy Sciences, DOE National Nuclear Security Administration and the National Science Foundation. Contributors include Maddury Somayazulu, Russell Hemley, Vitali Prakapenka, Abdul Haseeb Manayil-Marathamkottil, Kui Wang, Nilesh Salke, Muhtar Ahart, Alexander Mark, Rostislav Hrubiak, Stella Chariton, Dean Smith and Nenad Velisavljevic.