Thread by @MichaelSFuhrer

I am seeing a lot of newcomers lately to the room-temperature superconductor rodeo.

They might not be aware of the long history of these events, and I think there’s some cross-cultural communications difficulties going on because of that.

1/

There’s no reason (that we know) that a room-temperature superconductor can’t exist.

But we also don’t know how to make one by design.

It almost certainly won’t superconduct by a “conventional” (i.e. phonon-mediated BCS) mechanism.

2/

So it’ll be a serendipitous discovery in some unexpected strange material.

But not every serendipitously discovered unexpected apparent very low resistance state in a strange material is superconductivity!

3/

You’d think superconductivity would be easy to detect; it comes with zero electrical resistance, so if you measure resistance, and it’s zero, you’re done. Unfortunately there are many ways to get fooled (too many for one thread!)

4/

So generally you’ll see multiple pieces of evidence for superconductivity in a new report: Meissner effect, AC susceptibility, temperature-dependent critical field and critical current, single-particle tunnelling gap, jump in specific heat at T_c, Josephson tunnelling...
5/

... AC Josephson effect, etc. (Probably not all of these in one paper, but usually at least a couple in addition to zero resistance.)

6/

Even then, nature sometimes throws good scientists a curve ball, and can fool on multiple counts. So there is a steady trickle of difficult-to-explain results that look a lot like superconductivity, sometimes at unexpectedly high temperatures.

“Tantalizing” is often used.

7/

These are colloquially called Unidentified Superconducting Objects.

(I’ve heard a few scientists credited with originating that phrase; Bob Cava credits Koichi Kitazawa in the linked paper).



8/sciencedirect.com/science/articl…

Just a few examples (the ones I could think of off the top of my head – I am sure every condensed matter physicist has their favourites!)

1) palladium hydride


9/sciencedirect.com/science/articl…

2) multi-walled carbon nanotubes


3) doped graphite
https://t.co/UWQmNZF5lu

4) silver nanoparticles in a gold matrix
https://t.co/I6ZX8zwGpg

10/arxiv.org/abs/cond-mat/0…
onlinelibrary.wiley.com/doi/10.1002/ad…
arxiv.org/abs/1807.08572

The last one comes with a nice video showing diamagnetism at room temperature!



11/

There are also some more scandalous cases where fraud was known to occur or strongly suspected. But AFAIK the examples above aren’t scandals*, and reputable scientists were involved.

*Some may disagree. Let’s just say that there are probably many genuine reports out there.

12/

Also notable is that there’s no clear end to each of these stories; in many cases if you look into these past examples, you’ll find them just as credible as the most recent example. It’s just that, after a while, with no news of experimental replications in other labs...

13/

...interest fizzles out.

Unfortunately many mysteries in science remain unsolved!

14/

New superconductors *are* discovered of course, sometimes w/ unexpectedly high (but well below RT) transitions, in unexpected places (doped C60, MgB2, and pnictides are a few during my career). For these, experimental replications are numerous and they're widely accepted.

15/

I hope this goes some way towards explaining how people in the field view reports of superconductivity at unexpected high temperatures. They are exciting! And worthy of discussion. They’re science, and they also inspire more great science...

16/

...trying to figure out if there’s really a new superconductor there, and how it might work.

But there’s also a healthy scepticism and a wait-and-see attitude among those who’ve been to the rodeo before.

17/17

PS, in tweet 2, I should have specified that a room temperature *ambient pressure* superconductor almost certainly won't have a phonon-mediated BCS mechanism.

(It could happen at high pressure, though recent reports are also colourful.)

18/17