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At the moment, the highest critical temperature superconductor known to science (or myself, at least) is mercury barium calcium copper oxide. With a $T_{c}$ of roughly 133 K, that's well above the boiling point of nitrogen, and even well above the boiling point of oxygen, though using liquid oxygen to cool down anything probably wouldn't be the brightest idea. However, it's nowhere near the type of temperature that can cheaply be maintained, and far further still from the temperatures found naturally.

Are room-temperature superconductors forbidden by any known theory? If not, is there any known theory stating a mechanism by which they could operate, and what is the mechanism?

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    $\begingroup$ One complication is that the mechanism behind even the current high-Tc superconductors is a little obscure, so it's hard to say how far it could be pushed. $\endgroup$
    – zeldredge
    Commented Nov 23, 2016 at 3:13
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    $\begingroup$ There's nothing special about room-temperature - that's just anthropocentric thinking. But just think about what heat is - "random" movements of matter; this means that higher temperature also means more noise, and noise means losses, and losses mean you no longer have a superconductor. It's pretty amazing we can get as high as 133 K - that's already quite a bit of thermal noise. There might be some physical limit, but I don't think we have a comprehensive enough theory of superconductivity yet. Note that this is similar to other quantum states, like superfluidity. $\endgroup$
    – Luaan
    Commented Nov 23, 2016 at 9:36
  • $\begingroup$ TLDR answer: Yes, through the mechanism of discovering/engineering a new substance that's currently unknown to modern materials science. $\endgroup$
    – Mason Wheeler
    Commented Nov 23, 2016 at 12:37
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    $\begingroup$ @Luaan The question doesn't seem to suggest that there's anything physically special about room temperature. I'd imagine that the question is about room temperature because that's interesting to the asker, not because it's interesting to the superconductor. $\endgroup$
    – David Richerby
    Commented Nov 23, 2016 at 14:28
  • $\begingroup$ Could graphene count? I'm not certain myself since carbon isn't a metal, but since I've heard reports about amazing efficiency I figure it may be worth considering for any application one may usually consider a superconductor for. $\endgroup$
    – Raven
    Commented Nov 23, 2016 at 16:31

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Room-temperature superconductors are not forbidden by any known theory. However, discovery is difficult, while engineering is possible. One thing about superconductors is that they do not give off any heat. So cooling is just a function of fighting the insulation. With the discovery of super-insulators, the rest is just engineering!

Here's an interesting wikipedia article on the subject.

It should, however, be noted that these kinds of quantum states may be more common than non-quantum states. For instance, it is believed that neutron stars may be in a quantum state of some sort (perhaps super fluidity - I'm going from memory, so the details are a little hazy).

One thing that is clear is that with the application of extraordinary pressures, the transition temperature generally goes up. The highest pressures are supplied by diamond anvils, where the pressure chamber is formed between the points of two diamonds. Researchers generalise to other control parameters (Temperature, Pressure, Magnetic Field, ...), but generally speaking the control parameter is inimical to superconductivity (with pressure being the notable exception).

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  • $\begingroup$ From your link: The highest temperature known superconducting material is hydrogen sulfide, whose critical temperature reaches 203 K (−70 °C) $\endgroup$
    – Roman
    Commented Nov 23, 2016 at 14:19
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    $\begingroup$ Also, The lowest natural temperature ever directly recorded at ground level on Earth is −89.2 °C (−128.6 °F; 184.0 K), which was at the Soviet Vostok Station in Antarctica, on July 21, 1983..... So when the weather is truly terrible, then at least you could float a magnet. At least you could until the magnet... and you get blown away $\endgroup$
    – Roman
    Commented Nov 23, 2016 at 14:21
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    $\begingroup$ Oh, you need 1.5 million times atmospheric pressure to create H3S. So your experiment would also explode. This doesn't sound like all that much fun anymore $\endgroup$
    – Roman
    Commented Nov 23, 2016 at 14:26
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    $\begingroup$ @Roman Why? If you find a way to contain it, the explosion would create a nice heater... which is presumably what you were wanting in the first place. ;-) $\endgroup$
    – jpaugh
    Commented Nov 23, 2016 at 14:46
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    $\begingroup$ @DrXorile I'm suprised that you left this bit out: From one of the sources of that article, "[Fan] Zhang and Yugui Yao predict that substituting 7.5% of the sulfur atoms in hydrogen sulfide with phosphorus and upping the pressure to 2.5 million atmospheres (250 GPa) could raise the superconducting transition temperature all the way to 280 K, which is above water's freezing point." $\endgroup$
    – jpaugh
    Commented Nov 23, 2016 at 14:52
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As mentioned here, metallic hydrogen may be a conventional superconductor up to about 290 K. This is then due to the low mass of the metal ions, this leads to a strong coupling of the electrons with the lattice vibrations.

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    $\begingroup$ Maintaining the pressure for metallic hydrogen is probably harder than maintaining the cooling for something ordinary. $\endgroup$
    – Loren Pechtel
    Commented Nov 23, 2016 at 7:53
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    $\begingroup$ @LorenPechtel Yes, but that's not the point here. The OP isn't asking for the cheapest superconductor, just the one with the highest critical temperature - or really, if there even is a physical limit or not. Even if it took enough pressure to form a neutron star, it would be a great answer :) $\endgroup$
    – Luaan
    Commented Nov 23, 2016 at 9:38
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    $\begingroup$ Neutron star material is expected to be superconducting (and superfluid) even at typical neutron star temperatures, of order a billion degrees. There are engineering problems with using this for some applications. $\endgroup$
    – DMPalmer
    Commented Nov 23, 2016 at 16:15
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The proton gas in the interiors of neutron stars$^1$ is likely to be both superfluid and superconducting at temperatures way above 300 K - with critical temperatures as high as $10^9$ K (e.g., Haskell & Sedrakian 2017).

This is because the behaviours of the "Cooper pairs" in this case are due to long range strong force interactions with a potential energy of $\sim$ MeV.

Thus there is nothing fundamental about low temperatures, they just need to be low enough that you can make the Cooper pairs with whatever long range interaction is being considered.

$^1$ There must be protons (and electrons) present in a neutron gas. The neutrons decay until the densities and hence Fermi energies of the protons and electrons are high enough to block further decay. Typically this means that $n_p/n_n$ is in the range 0.01-0.1.

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Room temp superconductors are not forbidden fruit. It's simply finding right chemistry. Room temperature also implies they can exist in standard atmospheric pressures. So materials that can be used a RTDC don't require pressurization which in and of itself is energy intensive habit. Such a material would be revolutionary though would still require some modest amount of cooling to handle enormous energies transmitted thru it as a medium so as to reduce its resistance.

California is beginning massive program of burying major utility power lines underground because underground soil is ideal insulation and thermal sink for cables. More importantly cables invulnerable to heatwave, rain, snow, Wildfire, floods.

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