University of Rochester scientists have created a superconducting material at temperatures and pressures low enough for practical applications. “With this material, the dawn of ambient superconductivity and applied technologies has come,” says the team led by Ranga Dias, assistant professor of mechanical engineering and physics. In a paper in Nature, the researchers describe nitrogen-doped lutetium hydride (NDLH) that exhibits superconductivity at 69 degrees Fahrenheit and a pressure of 10 kilobars (1,45,000 pounds per square inch or psi).
Although 1,45,000 psi may still seem extraordinarily high (the pressure at sea level is about 15 psi), strain engineering techniques commonly used in chip manufacturing, for example, involve materials held together by internal chemical pressures that are even higher.
Scientists have been following this breakthrough in condensed matter physics for more than a century. Superconducting materials have two key properties: electrical resistance disappears and magnetic fields that are repelled pass through the superconducting material. Such materials could enable:
Power grids that transmit electricity without the loss of up to 200 million megawatt hours (MWh) of energy that now occurs due to resistance in the wires.
•More affordable medical imaging and scanning techniques such as MRI and magnetocardiography.
• Faster and more efficient electronics for digital logic and memory device technology.
•Tokamak machines use magnetic fields to confine plasma to achieve fusion as a source of unlimited energy
Previously, in papers in Nature and Physical Review Letters, Dias’ team reported the creation of two materials carbonic sulfur hydride and yttrium superhydride that are superconducting at 58 degrees Fahrenheit/39 million psi and 12 degrees Fahrenheit/26 million psi, respectively.
Given the importance of the new discovery, Dias and his team went to unusual lengths to document their research and deflect criticism that developed in the wake of the previous Nature paper, which led to a retraction by the journal’s editors.
The previous paper has been resubmitted to Nature with new data that confirms the earlier work, Dias says. The new data was collected outside the lab, at Argonne and Brookhaven National Laboratories in front of an audience of scientists who saw the superconducting transition live. A similar approach was used for the new document.
Hydrides, created by combining rare-earth metals with hydrogen and then adding nitrogen or carbon, have provided researchers with an enticing “working recipe” for creating superconducting materials in recent years. Technically speaking, rare earth metal hydrides form cage structures similar to clathrates, where the rare earth metal ions act as carriers, providing enough electrons to increase the dissociation of H2 molecules. Nitrogen and carbon help stabilize materials. Bottom line: less pressure is needed for superconductivity to occur.
In addition to yttrium, the researchers used other rare earth metals. However, the resulting compounds become superconducting at temperatures or pressures that are still not practical for applications.
So this time, Dias looked elsewhere in the periodic table. Lutetium seemed like “a good candidate to try,” says Dias. It has highly localized fully filled 14 electrons in its orbital configuration that suppress phonon softening and provide the enhancement of electron-phonon coupling required for superconductivity that occurs at ambient temperatures. “The key question was how do we stabilize it to lower the required pressure? And that’s where nitrogen came into the picture.”
Nitrogen, like carbon, has a rigid atomic structure that can be used to create a more stable cage-like lattice in the material and, according to Dias, stiffens low-frequency optical phonons. This structure provides stability for superconductivity to occur at lower pressure.
Dias’ team created a gas mixture of 99 percent hydrogen and one percent nitrogen, placed it in a reaction chamber with a pure lutetium sample, and let the components react for two to three days at 392 degrees Fahrenheit.
The resulting lutetium-nitrogen-hydrogen compound initially had a “shiny bluish color,” the paper says. When the compound was then compressed in a diamond anvil cell, a “surprising visual transformation” occurred: from blue to pink at the onset of superconductivity and then to a bright red non-superconducting metallic state.
“It was a very bright red,” says Dias. “I was shocked to see colors of this intensity. We jokingly suggested a code name for the material in this state – ‘red matter’ – after the material Spock created in the popular 2009 Star Trek film.” Code name stuck. The 145,000 psi pressure required to induce superconductivity is nearly two orders of magnitude lower than the previous low pressure created in Dias’ lab.