Scientists at the University of Rochester recently made a breakthrough: They’ve created a superconducting material that exhibits superconductivity at 69 degrees Fahrenheit and 10 kilobars (145,000 pounds per square inch, or psi) of pressure. This superconducting material, a nitrogen-doped lutetium hydride (NDLH), is made at both a temperature and pressure low enough for practical applications.
Sea level pressure is about 15 psi, so 145,000 psi may still seem like a very high pressure; however, stress engineering techniques are commonly used in chip fabrication, for example, and feature materials that are held together by significantly higher internal chemical pressures.
Scientists previously announced the creation of two materials, yttrium superhydride and carbonaceous sulfur hydride, that are superconducting at 58 degrees Fahrenheit/39 million pounds per square inch and 12 degrees Fahrenheit/26 million pounds per square inch, respectively.
For this new study, scientists collected the data outside the lab. It really was a collective effort.
Hydrides are made by combining rare earth metals with hydrogen and then adding nitrogen or carbon. In recent years, they have provided scientists with a tantalizing ‘working recipe’ for making superconducting materials.
Technically speaking, rare-earth hydrides take the form of cage-like structures called clathrates, where the rare-earth ions serve as carrier donors and provide enough electrons to promote the dissociation of the H2 molecules. Carbon and nitrogen aid in material stabilization. The bottom line is that superconductivity can occur at lower pressures.
Scientists have used other rare earth metals in addition to yttrium. Yet the resulting compounds become superconducting at pressures or temperatures that are still impractical for applications.
That’s why scientists looked elsewhere along the periodic table.
Ranga Dias, an assistant professor of mechanical engineering and physics, said: “Lutetium seemed” a good candidate to try. It has highly localized, fully filled 14 electrons in its f-orbital configuration that suppress phonon softening and enhance the electron-phonon coupling necessary for superconductivity to occur at ambient temperatures.
“The key question was: how are we going to stabilize this in order to lower the necessary pressure? And that is where nitrogen came into the picture.”
“Like carbon, nitrogen has a rigid atomic structure that can create a more stable, cage-like lattice in a material and hardens the low-frequency optical phonons. This structure provides stability so that superconductivity occurs at lower pressures.”
A pure sample of lutetium was placed in a reaction chamber with a gas combination of 99 percent hydrogen and 1 percent nitrogen. The mixture was then allowed to react at 392 degrees Fahrenheit for two to three days.
Scientists noted, “The resulting lutetium-nitrogen-hydrogen compound initially had a ‘shiny bluish color’. When the compound was then compressed in a diamond anvil cell, a “surprising visual transformation” took place: from blue to pink at the onset of superconductivity, then to a bright red non-superconducting metallic state.
Dias says, “It was a very bright red. I was shocked to see colors of this intensity. We humorously suggested a code name for the material in this state – ‘red matter’ – after a material Spock made in the popular Star Trek movie from 2009. The code name stuck.”
“The 145,000 psi pressure required to induce superconductivity is nearly two orders of magnitude lower than the previous low pressure.”
With this study, scientists have now answered whether superconducting material can exist at ambient temperatures and pressures low enough for practical applications.
Dias says, “A path to superconducting consumer electronics, energy transfer lines, transportation and significant improvements in magnetic confinement for fusion is now a reality. We believe we are now in the modern superconducting era.”
“The nitrogen-doped lutetium hydride will significantly accelerate progress in the development of tokamak machines to achieve fusion. Instead of using powerful, converging laser beams to implode a fuel pellet, tokamaks rely on strong magnetic fields emitted from a doughnut-shaped casing to trap, trap and ignite superheated plasmas. NDLH, which produces a huge magnetic field at room temperature, will be a game-changer for the emerging technology.”
- Dasenbrock-Gammon, N., Snider, E., McBride, R. et al. Evidence of near-environmental superconductivity in an N-doped lutetium hydride. Nature 615, 244-250 (2023). DOI: 10.1038/s41586-023-05742-0