In the period following the discovery of high-temperature superconductors, there was no good conceptual understanding of why those compounds worked. While there was a burst of progress towards higher temperatures, it quickly ground to a halt, largely because it was fueled by trial and error. In recent years we have gained a better understanding of the mechanisms that enable superconductivity, and we are seeing a second burst of rapidly rising temperatures.
Key to the progress was a new focus on hydrogen-rich compounds, based on the knowledge that the vibrations of hydrogen in a solid encourage the formation of superconducting electron pairs. By using ultra-high pressures, researchers have been able to force hydrogen into solids that have been shown to be superconducting at temperatures that could be achieved without resorting to liquid nitrogen.
Now, researchers have broken down an important psychological barrier by demonstrating the first chemical to be superconducting at room temperature. There are only two catches: we’re not quite sure what the chemical is, and it only works at 2.5 million atmospheres of pressure.
In a normal material, electrons travel as individuals, colliding with atoms and defects, thereby creating electrical resistance. To become superconducted in these materials, the electrons must find a partner to form what is called a Cooper pair. Once coupled, these electrons can travel through the material like a quantum liquid, taking essentially any path available to them. This allows them to avoid all the imperfections that would otherwise slow them down. You have superconductivity.
To encourage superconductivity, you have to encourage the formation of Cooper pairs, which involves overcoming the natural repulsion of electrons. The physics that allow this repulsion to be overcome involves interactions between electrons and the vibrations — called phonons — in the material in which they reside.
High-frequency phonons are most effective at encouraging Cooper pairing. And that brings us to hydrogen. The low mass of a hydrogen atom makes high-frequency vibrations easier to achieve, meaning hydrogen-rich materials are good candidates for superconductors at high temperatures.
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 high hydrogen content.
So far, the two most prominent examples of these have both used ultra-high pressures to make these materials. In one instance, the pressure was applied to hydrogen sulfide, causing it to disintegrate, squeezing out a mixture of pure sulfur and a chemical with an unusual and unclear ratio of hydrogen to sulfur. In another, a mixture of hydrogen and lanthanum was compressed to form a lanthanum hydride, which contained a lot of hydrogen in its crystal structure. Both reached record temperatures for that time.
While many hydrogen-rich materials are predicted to exist, most of the disappointments were in superconductivity testing. The authors of the new paper tried a slightly different approach: take two elements that naturally form hydrogen-rich chemicals, mix them together with some hydrogen, and mash the entire mix under ultra-high pressure. The chemicals used for this were sulfur (which forms H.2S) 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 equivalent to 10,000 atmospheres of pressure), the researchers exposed it to a green laser for several hours. The wavelength of the laser was chosen to break sulfur-sulfur bonds, allowing the formation of hydrogen-sulfur bonds. Whatever formed is not entirely stable, as the authors note that if left overnight in the anvil, it will perish again. But the unidentified chemical was stable enough to characterize.
At pressures of 175 GigaPascals, a transition to superconductivity occurred after the sample was cooled to about 180 Kelvin, which corresponds to -93°C. But by increasing the pressure, this critical temperature shifts to higher temperatures. With 240 GigaPascals, the critical temperature had risen to only -28°C. At the maximum pressure tested, the electrical resistance disappeared at 288K – that’s 15°C or 60°F. In other words, these are temperatures you could easily find when you step out your front door, albeit at pressures only deep in Jupiter. can be found.
To confirm that this was really superconducting, the researchers tested the magnetic properties of the material, which showed that they change at the same temperature. High external magnetic fields also lowered the critical temperature, as expected for something that interacts poorly with a superconducting material.
What kind of stuff is this?
In an ideal world, the researchers note, we would look at the structure of the superconducting material and try to figure out which materials can form a similar structure at less extreme pressure. Unfortunately, as mentioned above, we don’t know what the superconducting material is.
Normally we would look at a chemical using some form of X-ray imaging. But X-ray equipment is not physically compatible with the diamond anvil needed to achieve this pressure. Using a simpler form of spectroscopy, the researchers confirm that both carbon-hydrogen and hydrogen sulfide bonds are present in the superconducting material. But the measurements they have are of relatively low pressures (less than 50 GigaPascals), and we already know that increasing pressures can cause atoms to rearrange and form bonds that they wouldn’t normally consider at lower pressures.
Even if we were to figure out how to use existing technologies on these samples, they’re not very good at identifying the locations of hydrogen atoms because they’re so small. So much of the discussion in the paper is about describing the team’s efforts to find a way to characterize their record-setting material. Until they can, there’s no way to understand whether this work could provide a model for how to achieve something similar under less extreme conditions.
But other than this particular chemical, it’s clear that we’ve stuck with something — hydrogen-rich materials — that’s clearly leading to rapid advances in superconductors. After years of relative stagnation, superconductivity has become very exciting again.
Nature2020. DOI: 10.1038/s41586-020-2801-z (About DOIs).