Thread by @MichaelSFuhrer

There's been quite a lot of interest in recent reports of room-temperature superconductivity in the material dubbed LK-99.

Setting those aside for the moment, how likely is it that room temperature superconductivity, superfluidity, or zero resistance might be achieved?

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Superconductivity refers specifically to superfluidity of electron pairs in a solid. Superfluids can flow without friction or dissipation (heat generation), and charged superfluids (such as fluids made of electron pairs) can carry currents (flow of charge) with resistance.

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Superconductivity requires an attractive interaction between electrons, which isn’t easy to obtain, as electrons are negatively charged and repel each other!

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But weak effective attractions in a metal due to the electrons interacting with the positive ions of the solid (electron-phonon coupling) can produce an attractive force and bind the electrons into Cooper pairs.

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These Cooper pairs are bosons, and can occupy a macroscopically quantum coherent state in the solid which is a superfluid and superconductor.

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We have some ideas about how strong electron-phonon coupling can be, and how high a temperature electron-phonon coupled superconductivity can happen. In materials at ambient pressure it’s probably limited to cryogenic temperatures...

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...though high pressure generally enhances electron-phonon superconductivity, possibly up to around room temperature.

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en.wikipedia.org/wiki/Metallic_…

But the highest temperature ambient-pressure superconductors known probably don’t superconduct by the electron-phonon mechanism. Likely some type of magnetic fluctuation causes the attractive interaction, though this is still a topic of active research.

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And it's an open question whether superconductors can be found which superconduct at room temperature and ambient pressure. The highest superconducting transition temperature at ambient pressure is about 133 K, which is a little less than half room temperature (around 294 K).

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So a factor of two improvement is all that’s needed – this seems quite possible though it’s not known how to do it.

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What about other ways to achieve superfluidity or zero resistance?

Excitons are electrons bound to holes (missing electrons in valence band) in a semiconductor. Unlike electron pairs, electrons and holes attract, and their binding energy can be much larger than room temp.
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Excitons can be made to couple strongly to light confined in a cavity (e.g. by two mirrors), creating exciton-polaritons which are part-matter, part-light particles. These particles have Bose statistics and can undergo Bose-Einstein condensation to form a superfluid.

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And superfluidity at *room-temperature*, including frictionless flow, has been recently demonstrated for an exciton-polariton condensate in a perovskite semiconductor!

So room temperature superfluids are already a thing!

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doi.org/10.1038/s41467…

Generally the exciton-polariton superfluid requires some energy input as light tends to leak out of the system, or get absorbed. But that energy input isn’t critical for forming a condensate, and could be made small.

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It is also possible to remove the light part altogether and condense excitons into a superfluid. But generally excitons are metastable to recombination and light emission.

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In some materials (narrow-gap semiconductors or semimetals), excitons may become energetically stable and spontaneously form and condense into a coherent superfluid.

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nature.com/articles/s4156…

However this condensed exciton state is generally referred to as an “exciton insulator” as the neutral exciton do not carry a supercurrent.😥

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Another approach is to spatially separate the electrons & holes in separate semiconductor layers, close enough that they attract each other electrostatically and bind into excitons. This prevents recombination and can realize superfluid condensates.


18/nature.com/articles/natur…

These spatially separated electron and hole layers can also carry supercurrents!


19/nature.com/articles/nphys…

There’s some exciting evidence that the transition temperature of these condensates can be made quite high (100 K) in two-dimensional semiconductor layers.



But no zero resistance state at such elevated temperatures has been observed (yet).

20/nature.com/articles/natur…

Do we need a superfluid to have zero (i.e. exponentially small) resistance?

Surprisingly, no! Room temperature zero resistance has also been achieved!

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The quantum Hall effect is an example of zero longitudinal resistance, and can be achieved in graphene at room temperature and without high pressure (though in a magnetic field of 29 Tesla!)


22/ https://t.co/f1KZSNXqT1doi.org/10.1126/scienc…

In the quantum Hall effect currents are carried in only one direction by chiral edge modes, which are truly dissipationless (exponentially protected from backscattering by the gapped insulating bulk of the sample).

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High magnetic fields might not be necessary. Magnetic topological insulators can achieve a similar state (quantum anomalous Hall effect, see first observation below) and have been demonstrated to work at temperatures of a few Kelvins...



24/doi.org/10.1126/scienc…

...but there’s no physical reason why the right material couldn’t exhibit the quantum anomalous Hall effect at room temperature.

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We formed a Centre of Excellence @FLEETCentre funded by @arc_gov_au to study these new kinds of dissipationless flow (exciton & exciton-polariton condensates, and edges of topological insulators) in Australia in 2017. You can read more about it here.



26/fleet.org.au

Also check out this short video about @FLEETCentre which describes these new kinds of dissipationless flow, and how they might revolutionize electronic devices.



27/27

@FLEETCentre Whoops, the link in tweet 20 should have been this one.



28/27nature.com/articles/s4158…

@MesonMatt As far as we know, the only way to make a superconductor is to make an interacting charged composite boson with a binding energy greater than the desired transition temperature. You have to propose something to bind it that strongly.

@MesonMatt Just saying the word "Majorana" doesn't make the binding energy any larger. What's the mechanism?

And in tweet #2, should be "without resistance" rather than "with resistance"!

29/27

https://twitter.com/zornsllama/status/1688450628840763393