Superconductivity offers the promise of hyper-efficient electric motors, ultra powerful magnets, and the transmission of electricity without losses. The reality, however, has fallen considerably short of that promise, as superconducting materials are difficult and expensive to manufacture, requiring a constant bath of liquid nitrogen to keep them cold enough to operate.
And progress at identifying new high-temperature superconductors went through an extended stall, with no new contenders for decades.
But behind that stall, researchers were getting a better understanding of the physics involved with superconductivity, and that understanding seems to be paying off. A few years back, researchers found that a high-pressure form of hydrogen sulfide would superconduct at 203K (-70°C), roughly 65K higher than any previous material. Now, following up on suggestions from computer modeling, researchers have discovered that a metal-hydrogen compound (LaH10) can superconduct all the way up to 250K. That’s roughly -25°C, a temperature that can be reached by a good freezer.
Unfortunately, its superconductivity is dependent upon pressure and required compressing the sample between two diamonds. But the results do tell us that our understanding is on the right track, and there are undoubtedly additional chemicals worth examining.
While superconductivity isn’t fully understood, we have developed a solid theoretical understanding of some of the physics underpinning it. One of the key factors we’ve identified involves high-frequency vibrations of the crystal lattice formed by the material. These lattice vibrations depend in part on the structure of the crystal and partly on the atoms within that structure. Lighter atoms can vibrate more readily and so would make superconductivity easier to achieve.
That’s what inspired the exploration of hydrogen sulfide—the presence of hydrogen in the material would allow the high-frequency lattice vibrations that are needed for superconductivity. In fact, indications are that the high pressures needed for it to superconduct forced some of the sulfur out of its crystal structure, creating a chemical that was even richer in hydrogen.
Similar calculations (which relied on density functional theory) suggested that high pressure could create hydrogen-rich compounds centered on metals like yttrium and lanthanum. These would have the metal surrounded by a cage of hydrogen atoms. The calculations suggested that these chemicals could superconduct at much higher temperatures.
Earlier studies had examined the behavior of metals complexed with an average of six hydrogen atoms (calcium and yttrium). But there were indications that it was possible to push the hydrogen content even higher. And, last year, there was a report of the synthesis of a lanthanum compound with a very large hydrogen content: LaH10. “These superhydrides can be considered as a close realization of metallic hydrogen,” the authors suggest. The people who made it did see some changes in electrical resistance at different temperatures but didn’t fully characterize those. So, an international team of scientists decided to do exactly that.
Making the chemical itself was as simple as placing some lanthanum metal in a hydrogen atmosphere and then crushing it between diamond anvils. At 270 Gigapascals—roughly two million atmospheres of pressure—the mixture would form a small patch of LaH10 between the anvils. To test superconductivity, the researchers layered a tiny wire on each of the diamond anvils, allowing the lanthanum hydride to conduct current between them. The whole setup could be dropped to very cold temperatures.
At certain combinations of pressure and temperature, the researchers found that “the electrical resistance decreased sharply to zero.” In other words, the compound started superconducting. The highest temperature at which they consistently saw this happen was 250K, which is -23°C. That’s the sort of temperature your freezer could potentially reach if cranked up to its highest setting, and it’s a temperature that occurs regularly in the polar regions on Earth. It’s much, much warmer than dry ice, much less than liquid nitrogen.
This peak in superconductivity occurred at 170 Gigapascals, and increasing the pressure further tended to cause the critical temperature to drop. This suggests that the best performance is very sensitive to the structure of the LaH10.
One of the challenges of measuring superconductivity at these temperatures is that the wire feeding the current into the sample doesn’t superconduct at these temperatures, so you’re not directly measuring the lack of resistance to the current. So, to provide a sense of whether this really was superconduction, the researchers checked factors that should influence it. Superconductors are very sensitive to the presence of magnetic fields, and LaH10 saw its critical temperature drop when magnetic fields were applied to it.
In addition, the whole reason to look at the compound in the first place was the presence of lots of lightweight hydrogen atoms. The researchers made them heavier by swapping in deuterium, a hydrogen isotope with an extra neutron. This also lowered the critical temperature. For these reasons, the researchers concluded that LaH10 really was superconducting under these conditions.
Obviously, it’s not especially practical to set up diamond anvils everywhere we might want to have access to a superconductor. But the results are potentially important for two reasons. One, they suggest that we understand at least some aspects of superconductivity well enough to predict chemicals that will display this behavior. While the calculations are compute-intensive enough that we probably can’t just start doing a blind screen of chemicals, they should let us identify promising classes of chemicals.
The second good thing here is that the authors were able to figure out the structure of the LaH10 in its superconducting form. And that may allow us to identify some of the critical features that enable its behavior. While this particular chemical will probably never be a practical solution, the hope is that it will help us identify something that could.