First room-temperature superconductor reported – PAPER RETRACTED

EDITOR'S NOTE: The paper this report was based on contained questionable data and has been retracted over the objections of the paper's authors.

In the period after the discovery of high-temperature superconductors, there wasn't a good conceptual understanding of why those compounds worked. While there was a burst of progress toward higher temperatures, it quickly ground to a halt, largely because it was fueled by trial and error. Recent years brought a better understanding of the mechanisms that enable superconductivity, and we're seeing a second burst of rapidly rising temperatures.

The key to the progress has been a new focus on hydrogen-rich compounds, built on the knowledge that hydrogen's vibrations within a solid help encourage the formation of superconducting electron pairs. By using ultra-high pressures, researchers have been able to force hydrogen into solids that turned out to superconduct at temperatures that could be reached without resorting to liquid nitrogen.

Now, researchers have cleared a major psychological barrier by demonstrating the first chemical that superconducts at room temperature. There are just two catches: we're not entirely sure what the chemical is, and it only works at 2.5 million atmospheres of pressure.

Vibrating hydrogen

In a normal material, electrons travel as individuals, bumping into atoms and defects and creating electrical resistance in the process. To superconduct in these materials, the electrons have to find a partner to form what's called a Cooper pair. Once paired up, these electrons can travel through the material as a quantum fluid, essentially taking any path available to them. This allows them to avoid all the imperfections that might otherwise slow them down. You've got superconductivity.

To encourage superconductivity, you have to encourage the formation of Cooper pairs, which involves overcoming electrons' natural repulsion. The physics that allows this repulsion to be overcome involves interactions between electrons and the vibrations—called phonons—in the material they reside within.

High-frequency phonons are the most effective at encouraging Cooper pair formation. And that brings us to hydrogen. The low mass of a hydrogen atom makes high frequency vibrations easier to achieve, which means hydrogen-rich materials are good candidates for high-temperature superconductors.

To review, hydrogen atoms yield high-frequency phonons, which encourage electron pairs, which enable superconductivity. So we just need to figure out how to make materials with a high hydrogen content.

So far, the two most prominent examples of this have both used ultra-high pressures to create these materials. In one, the pressures were applied to hydrogen sulfide, causing it to break down and squeeze out a mixture of pure sulfur and some chemical with an unusual and unclear ratio of hydrogen to sulfur. In another, a mix of hydrogen and lanthanum were squeezed together to form a lanthanum hydride, which contained lots of hydrogen within its crystal structure. Both reached record temperatures for the time.

Under pressure

While lots of hydrogen-rich materials are predicted to exist, most of them have been disappointments when tested for superconductivity. The authors of the new paper tried a somewhat different approach: take two elements that naturally form hydrogen-rich chemicals, mix them together with some hydrogen, and mash the whole mix under ultra-high pressures. The chemicals used for this were sulfur (which forms H₂S) and carbon (which forms methane, CH₄). These were placed in a diamond anvil along with some platinum electrodes to test their conductivity.

Once compressed to 4 GigaPascals (one GigaPascal is roughly equal to 10,000 atmospheres of pressure), the researchers exposed it to a green laser for several hours. The laser's wavelength was chosen to break sulfur-sulfur bonds, allowing the formation of sulfur-hydrogen compounds. Whatever formed isn't entirely stable, as the authors note that leaving it in the anvil overnight would cause it to decay again. But the unidentified chemical was stable enough to characterize.

At pressures of 175 GigaPascals, a transition to superconductivity took place once the sample was cooled to about 180 Kelvin, which corresponds to -93°C. But increasing the pressure caused this critical temperature to shift to higher temperatures. By 240 GigaPascals, the critical temperature had risen to just -28°C. By the maximum pressure tested, electrical resistance vanished at 288K—that's 15°C, or 60°F. In other words, these are temperatures you might easily find if you step out your front door, albeit at pressures only found deep inside Jupiter.

To confirm this was truly superconductivity, the researchers tested the magnetic properties of the material, showing that they change at the same temperature. High external magnetic fields also reduced the critical temperature, as expected for something that interacts poorly with a superconducting material.

What is this stuff?

In an ideal world, the researchers note, we'd look at the structure of the superconducting material and try to figure out what materials can form a similar structure at less extreme pressures. Unfortunately, as noted above, we don't know what the superconducting material is.

Normally, we'd look at a chemical using some form of X-ray imaging. But X-ray equipment isn't physically compatible with the diamond anvil needed to reach these pressures. The researchers use a simpler form of spectroscopy and confirm that there are both carbon-hydrogen and sulfur-hydrogen bonds present in the superconducting material. But the readings they do have are from relatively low pressures (under 50 GigaPascal), and we already know that increasing pressures can cause atoms to rearrange and form bonds they wouldn't normally consider under lower pressures.

Even if we were to figure out how to use existing technologies on these samples, they're not great at identifying the locations of hydrogens because they're so small. So a lot of the paper's discussion involves describing the team's efforts to find a way to characterize their record-setting material. Until they can, there's no way of understanding whether this work might provide a model for how to achieve something similar under less extreme conditions.

But beyond this specific chemical, it's clear that we've latched on to something—hydrogen rich materials—that is clearly leading to rapid advances in superconductors. After years of relative stagnation, superconductivity has gotten very exciting again.

Nature, 2020. DOI: 10.1038/s41586-020-2801-z  (About DOIs).