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.
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.)