7k, kelvin that is, according to the fine article. Very very cold, but better than nothing.

I did a bunch of research on similar Tc superconductors back during my PhD.

7K is considered “warm” from a cryogenics point-of-view because you can just dunk your sample into a dewar of liquid helium at 4.2K. You can even get it cooler, down to about 1K, using evaporative cooling techniques. [1]

It’s getting to lower temperatures than this when things start getting complicated. Eg a closed-cycle evaporative He3 system can get you down to 200 mK, or you can bite the bullet and use a dilution fridge down to around 10mK.

[1] https://en.wikipedia.org/wiki/1-K_pot

Note that liquid Helium boils at 4K, so anything hotter (like 7K) is "easy", where "easy" means "as easy as keeping a NMR working in a Hospital".

(There are probably a lot of other nasty details, but less than 4K is harder.)