A room-temperature superconductor, if one could be found, would be amazing. Several research groups have been looking for one by following a 2004 proposal by Neil Ashcroft: In a pressurized hydrogen-rich material, vibrations of the lightweight atoms could nudge the electrons into Cooper pairs at high temperatures. Sulfur hydride, for example, was found to superconduct at 203 K (see Physics Today, July 2016, page 21), and other hydrides show signs of superconductivity at even warmer temperatures (see “Pressurized superconductors approach room-temperature realm,” Physics Today online, 23 August 2018). But the field has been cutthroat and controversial, with reproducibility challenges and some retracted high-profile papers.
The experiments are genuinely difficult. Most candidate hydrides don’t even exist unless squeezed to hundreds of gigapascals, or millions of atmospheres, and the diamond anvil cells that achieve those extreme conditions constrain the measurements that can be made. Two key properties identify a material as a superconductor—its electrical resistance drops to zero, and it expels magnetic fields from its bulk—and measuring both on the same compressed sample has been nearly impossible. For the former, the diamond anvil cell needs to be large enough to contain electrical leads; for the latter, it needs to be small enough to fit inside the loop of a superconducting quantum interference device, or SQUID.
Now researchers led by Harvard University’s Norman Yao and Boston University’s Christopher Laumann have developed a solution that’s almost deceptive in its simplicity. Instead of a SQUID to measure the hydride’s magnetic properties, they use the diamond itself. Specifically, they use defects called nitrogen–vacancy (NV) centers, whose spectroscopic properties are known to be sensitive to magnetic fields. As shown in the figure, the researchers embed a diamond anvil with NV centers and fit the cell with electrical transport leads. Then they measure the compressed hydride’s electrical and magnetic properties with a single apparatus.
The idea of combining NV centers and diamond anvil cells is not new. But because NV centers lose their spectroscopic contrast when compressed to 50 GPa, they weren’t expected to be applicable to the superconducting hydrides at higher pressures. With the help of University of Chicago theorist Giulia Galli, Yao and colleagues pinpointed the problem: In a typical diamond anvil cell, an NV center’s axis—the line between the nitrogen atom and the vacant site—is oblique to the surface that presses against the sample. The pressure at that angle distorts the defect’s symmetry and destroys its signal.
By cutting their diamonds differently, Yao and colleagues positioned their NV centers perpendicular to the sample surface, which preserved the defects’ utility at extreme pressures. For their proof-of-principle experiment, they looked at cerium hydride. Not only could they confirm its superconducting status electrically and magnetically, but they could directly image the sample’s inhomogeneous structure. Although cerium hydride’s superconducting temperature of 91 K is modest by hydride standards, extending the method to other materials could provide much-needed clarity to the quest for a room-temperature superconductor. (P. Bhattacharyya et al., Nature 627, 73, 2024.)
