Evidence for superconductivity above 260 K at megabar pressures (2018) | Hacker News Reader

Evidence for superconductivity above 260 K at megabar pressures (2018)

arxiv.org

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I've seen this research cited a few times (perhaps even here on HN) and every time the conversation is centered on whether this is practical or the naysayers moaning how this isn't practical.

The research isn't intended for commercial, practical use. There is no point in discussing it in such terms.

The interesting thing is that this behavior was predicted by models, and the experiment confirms the model. That is it. Perhaps with increased confidence in the models it will lead to improvements in practical superconductors in time, but not now.

Have any more papers come out on the graphene based superconductors? There was some based on alkane composites and some based on sheets at irrational angles to one another. Dependin on alkane bond angles and their interaction with graphene, those might be the same thing.

As you seem to understand this topic a bit more: can you explain the model?

What is the current theory how superconductors work?

I don't know anything about superconductors other than what I have read in articles like this one. But even for someone like me, it is obvious that this is a confirmation of theory, not a practical result.

> What is the current theory how superconductors work?

The best general model of superconductivity I'm aware of is still the BCS model:

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

This model basically says a superconductor is a Bose-Einstein condensate, where the Bose "particles" are pairs of electrons in the material that are entangled via an interaction involving low-energy vibration states of the material as a whole. The problem is that this general model doesn't tell us what the critical temperature for the Bose-Einstein condensation is. We have to find that out by a combination of experimentation with different materials, and more detailed extensions of the model that test various hypotheses about what kinds of material properties should affect the critical temperature. This paper is just a recent development in that line of research.

> the conversation is centered on whether this is practical or the naysayers moaning how this isn't practical.

this is par for the course for almost every thread on a research paper. Perhaps it is to be expected, given that this is a website catered to non-scientists

I'd say the topic even exaggerates this response, since a practical superconductor would be revolutionary.

It is funny really. A lot of progress:

1) Is a small incremental improvement in theory. Critics deride it as impractical. Eg, Evidence for superconductivity above 260 K.

2) Is a slightly novel recombination of existing ideas where the ideas have all happened before. Critics deride it as derivative. Eg, the iPhone iterations after the first.

From the big-picture perspective as long as something happens that is new in some sense we will eventually achieve real progress. Consistent small improvements are just as good as one big improvement, except apparently harder to appreciate.

The necessary pressure isn't that important. Obviously using this material under those conditions in the real world won't be realistic.

However, what this shows is that superconductivity is possible at those temperatures, and that pressure is a factor to further investigate when experimenting with other candidate materials.

> pressure is a factor to further investigate when experimenting with other candidate materials

There are crystals which retain their internal strain after release. This is valuable research.

Isnt growing crystal under high pressure in presses also not well understood? Like how are you going to make long traces or intricate structure in a high pressure environment that dont fully know growth and forming process of such crystals?

This occurs at 2 million bar (pressure).

From an engineering perspective, what does it take to contain such pressures, and would it be possible to make 'wire' consisting of a pressure-maintaining jacket surrounding the conductor?

Anyone want to do the maths? How much Kevlar are we talking...?

I believe these experiments are run in a diamond anvil (basically two diamonds pushed together onto a point). I'm skeptical that you could make a wire at this pressure with anything else -- even all the Kevlar in the world.

From what I understand this is required to create the pressure, but a sufficiently strong material may be able to maintain the pressure, although I may be incorrect.

Could the pressure be fairly neutral with current=0 and the current itself create the pressure?

Yep the article itself mentions that the tests are done in the anvil

I think hope is that structures formed at such high pressures would be quasi-stable at atmospheric pressure. Such behavior is no unheard of.

Such pressures is not something our materials can handle on macro scopes. Yet. But they are naturally occurring outside diamond anvils, like deep I gas giants, etc.

It’s facing that such super high pressure chemistry may be more common than our low pressure one, because of the abandonment of raw materials in big planets.

while what you describe is imaginable, I don't think this is the case, as physicists would then speak of "superconductivity at temperature T for a material in phase X formed at pressure P", not "[ ... ] superconductivity at megabar"

Perhaps you also think the claim states room temperature conductivity, and one only needs 260K temperature to form this phase?

When the title is phrased like it is, it usually describes a point or region for the equation of state

Good point on the state. One good thing from such research results even if impractical itself can yield good targets for future research. If it’s a matter of geometry or crystal lattice spacing from the pressure an alternate formulation might be able to make a meta-stable structure that induces superconductance. Perhaps there’s another part of the state diagram that could be modified.

Indeed. It is important to pursue research even if there is no immediate benefit from it. In this case it is even touching on a gateway technology I would call marvellous. Stable high temperature superconductor material would change so much.

What you need is a material with very high ultimate tensile strength. Carbon nanotubes might do it. Let's check.

To compute the burst pressure of a pipe, we can use Barlow's formula:

P_max = S * T /R

where T is wall thickness, R is radius of the outside of the pipe, and S is ultimate tensile strength. Here T=R is the limit where the inside radius is zero. So the best case is

P_max = S/1.0000001

Carbon nanotubes are the highest strength material, with a theoretical S=100 megabar. So not possible, sadly.

But 2 million bars = 2 Mbar. Surely 100 Mbar is enough, or did I miss something?

Bah, I messed up the Pascal-bar conversion in the text. Carbon nanotubes have S = 100 GPa = 1 Mbar. Too late to edit now. Thanks for catching this.

Thanks for these numbers! Can such pressures be attained by other means rather than permanent confinement? For example, the near wall pressure of a collapsing ultrasound bubble reaches about 100 MPa. Three orders of magnitude too little but can this technology be used to achieve much higher pressures? If so I wonder if maintaining an aggregate of material at high pressure (a high pressure foam as it were) would be sufficient to achieve bulk superconductivity.

This is far outside my wheel house, but... Surely it's the pressure differential that dictates the burst pressure? So if you had multiple sleeves, each decreasing in pressure as you get closer to the outside, this could be possible?

I'd expect that for a pipe under high pressure but small pressure differential compressive strength would matter much more than tensile strength.

Yes, it's the pressure differential that matters, but I don't think it helps in practice. Each pipe needs to have a ratio of inner to outer diameter of at least 100. If the innermost radius is 1 mm, at three layers the outside radius of the biggest pipe is 1 kilometer.

> Carbon nanotubes are the highest strength material

That we can produce.

I once asked my chemistry teacher if it's possible to make square carbon lattice, and he didn't have an answer.

      |   |   |
    ─ C ─ C ─ C ─  
      |   |   |
    ─ C ─ C ─ C ─  
      |   |   |
    ─ C ─ C ─ C ─  
      |   |   |

EDIT: looks like both Carbyne and boron nanotubes are stronger than carbon nanotubes.

A square carbon lattice would be very unstable. Carbon can support up to 4 single bonds, but they have to be arranged in a tetrahedral symmetry (sp3). 4-member carbon rings exist in some compounds, but they are unstable and reactive. Flat carbon sheets exist as graphene, which is a hexagonal grid rather than your square one.

Yes, it's possible but it requires a thick jacket. the formula for a thin-shell cylindrical pressure vessel is \sigma = rp/t, where sigma is tensile strength of the material, p is pressure, r is radius, and t is wall thickness. For thick-shell jackets, the formula is slightly more complicated [1] but you get the idea.

For Kevlar, max \sigma is 3.6GPa. So it's possible to maintain an internal pressure of 200GPa (2MBar), but the wall thickness must be much larger than the inside radius.

[1] https://tge.readthedocs.io/en/latest/content/mechanical/thic...

We quoted the same formula, but AFAIK the radius is the outside radius, right? So you cannot go above t/r = 1.0?

Wouldn't that mean that we can get a 1 mm radius almost-room-temperature superconductor with a wall only 55 mm thick? That seems like it would almost be practical.

If you use the formula for thick walled cylindrical pressure vessel, you hit practical limit with outside diameter being twice the inside diameter. When this happens and you also take into account fatigue, usually about half of the yield strength of the material is usable.

Your best bet is using thin shells of maranging steel wrapped around each other with thin layer of pressurized non-compressible fluid in between. The pressure control of the fluid is then absolutely critical. Alternate with 0,5mm thick fluid and 1mm thick steel sections. Rinse and repeat. The total thickness would not be all that crazy, but the pressure control would be incredibly expensive.

EDIT: Did some calculations. Using roughly that system I described earlier you would only hit 10 GPa with roughly 50 meters thick piping system, that would have 75 alternating layers of pressurized oil. And each layer would have different pressure from all the other 74 layers of pressurized oil. So not possible.

If you try using single thick wall in your cylinder, increasing the wall thickness means that your max stress gets closer to the internal pressure reading. So you would need material that can deal with 200 GPa of stress.

Stupid question from a non-physicist: what exactly does the high pressure do here? How does it contribute to the superconductivity?

Stupid answer from an organic chemist with limited inorganic and material chemistry:

In theory it would compress the material to a quasi crystal structure. This compressed material would allow the electrons to freely jump from atom to atom/molecule to molecule without hindrance or loss.

Note: This is pure guessing!

I'm guessing it essentially increases the elastic constants of the material; that is, increases the "spring constants" of the bonds between atoms. As a result, the resonant frequency of acoustic vibrations rises. When these acoustic vibrations are quantized into phonons, the minimum energy of each phonon therefore rises, since it is proportional to this resonant frequency. Since conventional cooper pairs are pairs of electrons bound by a phonon, this increases the binding energy of the pair, and makes it more resilient to being broken up by the thermal energy.

You can have water under high pressure that does not freeze at 0 degrees, as it would at atmospheric pressure. It's because there are phases that water has behave differently under different pressure and temperature. Water behaves very differently at different temperatures and the same goes for Lanthanum (the metal used to create the superhydrate, or whatever its called). At room temperature the metal just does not bond to as many hydrogen atoms as needed to create the superconducting latice (the paper goes of the model that metals that can bond to >6 hyrogen atoms per host molecule are good candidates for room temperature superconductivity) so we have to do it under extreme pressure.

The traditional way that superconductors have been made has been to lower the temperature near absolute zero to get the metals into that a superconducting phase. What these researchers are trying to do is prove a model that suggests that there are metal compounds with superconductivity at near room temperature.

The material forms certain crystal structures at those pressures which were predicted to be superconducting.

Putting P-T into context, the conditions involved in synthesizing some forms of lanthanum decahydride are enough to create diamonds.[1]

The cubic phase of LaH10 is synthesized around 170 GPa and 1000 K.[2] The SC properties arise ~10 centigrade below the freezing point of water, but still under enormous pressure (190 GPa).

So if a sample of graphite were cooked aside during the synthesis of cubic LaH10, it would have become a diamond by the end of the experiment.

[1] https://en.wikipedia.org/wiki/Diamond#Thermodynamics

[2] https://sci-hub.se/10.1002%2Fanie.201709970 (This paper shares some authors with the paper to which this post links.)

That's over one million atmospheres of pressure we are talking about here. Are there any predictions of how this is going to affect modern-day electronics that will be unable to undergo such massive pressure?

Perhaps it would be possible to contain materials at such high pressures inside other materials like carbon nanotubes or diamonoid structures?

Maybe we can get to some happy medium where it's a practical level of pressure and cooling, not quite room temperature but still something we can use.

Liquid nitrogen is almost that already. It's readily available, reasonably cheap, and you just need Styrofoam for the bulk of the insulation.

The superconducting material itself is expensive, brittle, hard to fabricate, and hard to work with.

Standard "high-temperature" superconducting wire (e.g. YBCO) isn't actually all that bad to handle or wind. You buy it extruded in a metal matrix (usually copper), and treat it only a little bit more gently than standard copper wire.

let's all move to megabar!

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