Nickel superconductor works above -233°C threshold at normal pressure

From the article:

> The -233°C threshold (40 K), often associated with the McMillan limit, marks a boundary beyond which conventional superconductivity theories become less predictive.

So it’s like finding a gas giant exoplanet that’s in the “wrong” place. We understand Jupiter and Saturn. We don’t understand a gas giant orbiting its star in ten days. Thats weird. And science can’t progress when there is nothing “weird”. It means all the data matches our models. It’s boring. It has nothing to teach.

A ten day orbit or a normal metal group superconductor are outside our comfort zone and may mean new physics.

If we're trying to understand how superconductors work, then it's useful to have more examples.

a recent work claims that in principle the upper bound to the critical temp of conventional sc at ambient p is ~100K, so this is a valid empirical step in that direction

https://arxiv.org/abs/2406.08129

The McMillan limit has been challenged before

https://www.nosta.gov.cn/pc/en/awards/prize1/1460.shtml

Nature has a much better news article than phys.org: https://www.nature.com/articles/d41586-025-00450-3

We have two classes now, copper and iron based superconductors that work at room temperature above 40K or so. Below 40K, there are countless superconductors and the mechanism they use breaks down at around 40K. There's no reason to think that a material which is a superconductor below 40K will have some variant that is one above 40K.

Once we have evidence of superconductivity above 40K or so (at normal pressures) things change. Those materials follow different rules that we only partially understand. And there is reason to hope that variants exist which will be superconductors at even higher temperatures. There's no set barrier or upper limit once you cross the 40K threshold. One may exist, but we don't know it.

What happened with copper, and iron (to a much lesser extent), is that we marched up the curve and eventually made superconductors with many real-world. Once you reach 77K (-195C) we're in business. The hope is that maybe we can do the same with nickel. This has traditionally only really worked well with copper-based compounds.

Having a 3rd class of materials gives us a lot more options. And the more examples we have, the more likely we are to come up with some useful theory.

Frame of reference. Kelvin immediately tells you the distance from absolute zero, which is at least somewhat relevant in this context. Celsius tells you the distance from liquid water which isn't very helpful in understanding the figure.

I think fewer people know the offset between K and C than the fact that 0K represents absolute zero.

> I'm curious by what measure you suggest it's "too cold".

I'm accustomed to thinking about superconductors in an environment on the order of around ~0.01K. And, it's worth spending a little time understanding absolute temperature. Take, say, Zirconium -- its critical temperature is around 0.55K. This new material's Tc is around 40/.55 = 73 times hotter. This perspective is useful if you're thinking about how much work it'll be to get something down to a given temperature. So you've kinda hit my complaint on the head -- it's precisely because temperatures of that magnitude don't make sense to me. And I'd expect folks reading phys.org to be unsurprised by temperatures reported in K.

There’s another more fundamental reason to use Kelvin here, linearity.

100K is twice as hot as 50K, while the idea of twice as hot is meaningless in C or F.