Only a H2 in A Gilded Cage

“OK, Susan, you’ve led us through doing high-pressure experiments with the Diamond Anvil Cell and you’ve talked about superconductivity and supermagnetism. How do they play together?”

“It’s early days yet, Sy, but Dias and a couple of other research groups may have brought us a new kind of superconductivity.”

“Another? You talked like there’s only one.”

“It’s one of those ‘depends on how you look at it‘ things, Al. We’ve got ‘conventional‘ superconductors and then there are the others. The conventional ones — elements like mercury or lead, alloys like vanadium‑silicon — are the model we’ve had for a century. Their critical temperatures are generally below 30 kelvins, really cold. We have a 60‑year‑old Nobel‑winning theory called ‘BCS‘ that’s so good it essentially defines conventional superconductivity. BCS theory is based on quantum‑entangled valence electrons.”

“So I guess the unconventional ones aren’t like that, huh?”

“Actually, there seem to be several groups of unconventionals, none of which quite fit the BCS theory. Most of the groups have critical temperatures way above what BCS says should be the upper limit. There are iron‑based and heavy‑metals‑based groups that use non‑valence electrons. There are a couple of different carbon‑based preparations that are just mystical. There’s a crazy collection of copper oxide ceramics that can contain five or more elements. Researchers have come up with theories for each of them, but the theories aren’t predictive — they don’t give dependable optimization guidelines.”

“Then how do they know how to make one of these?”

“Old motto — ‘Intuition guided by experience.’ There are so many variables in these complex systems — add how much of each ingredient, cook for how long at what temperature and pressure, chill the mix quickly or anneal it slowly, bathe it in an electrical or magnetic field and if so, how strong and at what point in the process… Other chemists refer to the whole enterprise as witch’s‑brew chemistry. But the researchers do find the occasional acorn in the grass.”

“I guess the high‑pressure ploy is just another variable then?”

“It’s a little less random than that, Sy. If you make two samples of a conventional superconductor, using different isotopes of the same element, the sample with the lighter isotope has the higher critical temperature. That’s part of the evidence for BCS theory, which says that electrons get entangled when they interact with vibrations in a superconductor. At a given temperature light atoms vibrate at higher frequency than heavy ones so there’s more opportunity for entanglement to get started . That set some researchers thinking, ‘We’d get the highest‑frequency vibrations from the lightest atom, hydrogen. Let’s pack hydrogens to high density and see what happens.'”

“Sounds like a great idea, Susan.”

“Indeed, Al, but not an easy one to achieve. Solid metallic hydrogen should be the perfect case. Dias and his group reported on a sample of metallic hydrogen a couple of years ago but they couldn’t tell if it was solid or liquid. This was at 5 megabars pressure and their diamonds broke before they could finish working up the sample. Recent work has aimed at using other elements to produce a ‘hydrogen‑rich’ environment. When Dias tested H2S at 1.5 megabar pressure, they found superconductivity at 203 kelvins. Knocked everyone’s socks off.”

“Gold rush! Just squeeze and chill every hydrogen‑rich compound you can get hold of.”

“It’s a little more complicated than that, Sy. Extreme pressures can force weird chemistry. Dias reported that shining a green laser on a pressurized mix of hydrogen gas with powdered sulfur and carbon gave them a clear crystalline material whose critical temperature was 287 kelvins. Wow! A winner, for sure, but who knows what the stuff is? Another example — the H2S that Dias loaded into the DAC became H3S under pressure.”

“Wait, three hydrogens per sulfur? But the valency rules—”

“I know, Sy, the rules say two per sulfur. Under pressure, though, you get one unattached molecule of H2 crammed into the space inside a cage of H2S molecules. It’s called a clathrate or guest‑host structure. The final formula is H2(H2S)2 or H3S. Weird, huh? Really loads in the hydrogen, though.”

“Jupiter has a humungous magnetic field and deep‑down it’s got high‑density hydrogen, probably metallic. Hmmm….”

~~ Rich Olcott

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