High Temperature Superconductivity Record Smashed by Sulphur Hydride | Hacker News Reader

High Temperature Superconductivity Record Smashed by Sulphur Hydride

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118 pointsjonbaer34 comments

Tangentially, the metallic hydrogen mentioned is one of the craziest chemical substances to possibly-exist [0].

Besides likely being a room-temperature superconductor (at ridiculous pressures, like 500 GPa), it's postulated to be metastable -- like diamond, you could create it at pressure, and it might stay a solid metal at STP conditions. It's postulated to be made of atomic hydrogen -- lone H atoms, without the molecular bonds of H_2. The recombination energy H + H -> H_2 suggests [1] it's the most energy-dense chemical fuel that exists, with 20 times the specific energy of {H2 + O2}. It could allow [1] rocket engines with I_sp of 1,700 seconds -- four times higher than LH2/LOX. It's thought to be the main phase of hydrogen inside the planet Jupiter [2] and responsible for its dynamo [3] (but as an ordinary conductor, not a superconductor). It's also speculatively a structural material, one that's less dense than water [4].

It might have been created in a lab, in 2011 [4], but it's not clear.

[0] https://en.wikipedia.org/wiki/Metallic_hydrogen

[1] http://www.nasa.gov/pdf/637123main_Silvera_Presentation.pdf

[2] https://en.wikipedia.org/wiki/Jupiter#Internal_structure

[3] https://en.wikipedia.org/wiki/Magnetosphere_of_Jupiter

[4] http://www.nature.com/news/metallic-hydrogen-hard-pressed-1....

The metastability at STP is particularly intriguing. I wonder what quantum chemistry calculations have been performed on this material. I need to look this up, and maybe perform a few of my own!

Supposing that we can actually generate enough pressure to generate metallic hydrogen at some point, I wonder what the downsides of the material would be.

You might find a few of these papers interesting. The first challenge is working out the favoured crystal structure, including quantum corrections for the nuclear motion:

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.87.184...

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.110...

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112...

I had just looked those up actually. The last one is the most interesting to me. I don't like the idea of using DFT for a material whose properties are so unknown experimentally. (d, v)-QMC is much better; however, I think metallic hydrogen could possibly be a prime candidate for the new FCIQMC method. Of course, this is my rough assessment over an hour. I haven't actually read the first two papers in detail.

Indeed, though there are reasonable arguments that DFT functionals are ok in high pressure regimes, it's obviously best to use the most physically accurate methods when you're in unknown regimes. It's just going to be ridiculously expensive :)

I'm not an expert in QMC and haven't really kept up with Alavi's FCIQMC, but that'd be the gold standard if you can also afford to include quantum nuclear motion, which seems to be important here.

> but that'd be the gold standard if you can also afford to include quantum nuclear motion

Yeah, which I haven't seen for FCIQMC. (Alavi's group really seems to be the only ones actually using the method so far. I imagine it's still too new for someone else to want to code up unless he distributes the code.)

Also, I was perusing your post history (hope you don't mind), and noticed you do DFT calculations. You probably know much more about that than me; I'm primarily MD but keep up with the quantum chemical methods more as a side research project.

What are your thoughts using DFT for metallic hydrogen? Is there an exchange-correlation functional that could be good enough?

(Disclaimer, I'm working on FCIQMC right now)

One of the reasons that FCIQMC doesn't have nuclear motion is that the gradients from FCIQMC, and actually most QMC techniques, are really computationally intensive, so this means that creating the ab-initio surface for the nuclei to roll over is really hard. Perhaps you were considering some sort of FCIQMC approximation to the path integral, but it's not entirely obvious to me how this would work.

As for how DFT would work for this... It should work quite well for qualitative predictions. Actually, DFT does remarkably well for metals and functionals like asymptotically corrected PBE0 are providing remarkable physical insight. While I wouldn't trust the numbers that come from any DFT simulation to three decimal points, I'd certainly trust the physics that's captured.

That being said, metallic hydrogen should be a strongly multireference system, so I'd be interested in seeing how a green's function approach based in many-body perturbation theory (see GF2 from Zgid at U Michigan) would do, as it doesn't struggle with issues of references while still giving you coupled cluster level accuracy.

Very interesting. I hadn't heard of GF2 before; I'll have to look it up.

As a side note, I find it interesting how I'm always running into people working in such specialized fields on HN. I wouldn't have imagined I'd find someone working on FCIQMC posting on here, but I'm always surprised. Sounds like fun research.

Yep. Personally, I trust DFT about as far as I can throw it, and think that spending time worrying about functionals is fairly pointless and likely to end up over-fitting data, especially if they aren't incorporating new physics :)

That last paper does say, however:

> We used the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation density functional, which is well suited for very high-pressure studies, as the charge density is more uniform than at low densities, and it obeys the uniform limit and gives a good account of the linear response of the electron gas to an external potential

but also

> DFT studies of high-pressure phases of hydrogen have been performed using several approximate density functionals, and a significant dependence of the results on the functional has been noted. The enthalpy differences between phases are so small that changes of only a few meV per proton can make a noticeable difference to the phase diagram

so that's some physics. They seem to end up using DMC for the static lattice and DFT for the vibrational corrections on top of that.

> the downsides of the material would be

Well, considering how energy-dense it is... As a construction material, wouldn't it be more than a tad dangerous if it ever caught on fire?

It wouldn't so much catch on fire as revert to gas, straight from solid to H2. Otherwise known as an explosion. Then maybe you get burning hydrogen.

Oh, there will be fire. The amount of energy going from H to H2 is more than there is from H2 to H2O.

If this really is atomic hydrogen it holds a ferocious amount of energy.

https://en.wikipedia.org/wiki/Atomic_hydrogen_welding

Yes, my point was that it doesn't make any sense to call that a fire. H(s) to H2(g) is a highly exothermic phase change. Fire is oxidation.

That's not a phase change (despite the solid to gas), that's a chemical reaction.

Fire is oxidation yes, but despite the name oxidation does not require oxygen, it just requires electron transfer, which happens here.

Given temperature and pressure it formed at I am not sure oxidation would be a problem at all.

That sounds like a fun weekend hack.

I mean, it would have to be its own research project haha. There's no way I could begin to get a simulation like that going in a weekend.

It causes cancer in rats.

Wow, this reads like technobable from bad sci-fi :) I mean too good to be true kind.

Wow, I never considered the concept of a planet with a superconducting dynamo. That's a fascinating possibility.

Even cooler: there's articles that claim the interiors of neutron stars are superconducting -- the current carried by proton pairs, rather than electron pairs. Maybe this is the source of magnetar dynamos.

http://www.ohio.edu/research/communications/neutronstar.cfm

https://en.wikipedia.org/wiki/Magnetar#Origins_of_magnetic_f...

If it were stable at STP it would make for a hell of an explosive. Simply cracking a small piece and blam! Instant over pressure and frost bite as the phase changed sucks all that energy out of the surrounding environment.

Well, the frost bite might end when the hydrogen meet some oxygen.

It would be interesting to see if it would go exothermic or not. On the one hand you would suddenly have a bunch of free hydrogen atoms, presumably becoming H2 as they expanded. The pressure wave would be caused by sudden decrease in density (phase shift from solid to gas) but and in the process the new molecules would probably briefly become liquid and then gas. There would not be enough oxygen in the center of that expansion to convert to water, but on the edges of the expanding pressure wave their would be. One possible outcome would be that on the surface of the pressure wave, oxygen would combine with hydrogen to form water, but that water would then insulate the hydrogen behind it from any oxygen, stopping any further reaction. At some point thought he heat going into the phase shift is going to come out of the water, which will force it to freeze, and the as the surface area of the pressure bubble expands, its pressure will decrease until it is in equlibrium with the frozen water's surface tension.

One possible outcome then is than a conversion would result in a 'pop', and the sudden appearance of an ice sphere which is filled with pure hydrogen.

Fee hydrogen (atomic hydrogen) releases a tremendous amount of energy when it reacts to make molecular hydrogen (H2). Far far more than reacting with oxygen.

So the oxygen part is pretty irrelevant when analyzing this.

This article is science writing done right.

All the information is there, but written clearly, without much jargon. A layman could understand it well enough, a physicist can understand it as well (the 'detailed' information hasn't been deleted or mangled by a journalist not understanding what they're writing, just presented clearly). And the graphs they show actually tell you what you want to know. Fantastic.

Yes, it's very, very good - a joy to read.

"[...] have measured sulphur hydride superconducting at a temperature of 190 Kelvin (-83 degrees Celsius). There is a caveat, of course. The material has to be squeezed at pressures greater than 150 gigapascals — that’s about half the pressure at the centre of the Earth."

And 1.5 million times the atmospheric pressure at sea level ...

Put in a different, less unreachable comparison, the yield strength of kevlar is 3.6 GPa (tension) and diamond forms between 4.5 and 6 GPa, w (compression)

It's been 60 years since we first synthesized diamonds, and back then they were capable of generating 10 GPa, that's only a factor of 15.

Another comparison: the highest pressure we can create without shock waves is 300-400 GPa with a diamond anvil cell [1]. With shock waves we can create momentary bursts of up to 100 TPa

[1] http://en.wikipedia.org/wiki/Diamond_anvil_cell

When I studied physics five years ago, my impression was that nobody really understood high-temperature superconductivity.

There were some mathematical models, and some professors who claimed to understand it, but nobody was able to give a coherent explanation to the (mostly nearly finished) students, much less predict which materials would exhibit hight-temp superconductivity.

Does anybody know if that changed significantly? The article reads as though the measurements were inspired by the theory, which is always a good sign.

Although this is a high-temperature superconductor in the literal sense, it isn't the kind of high-temperature superconductor that no one understands. Those are the cuprates (and now iron based exotica).

What's really interesting about this potential discovery isn't just that it exhibits superconductivity at a relatively high temperature but that it seems to be a conventional superconductor. That should give some insight that a slightly better cuprate might not.

That's a type I (low temperature) superconductor. This type is well understood, and explaining in lots of introduction to materials science and quantum mechanics books.

High temperature supercondutors are still not understood.

Completely off topic, but I really hate articles where all I initially see is a headline and a large, unrelated image.

I hope the trend dies quickly...

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