Well, some types of superconductors work fine at the simply low temperatures provided by liquid nitrogen (77 K). Though there are other factors like ability to

Well, some types of superconductors work fine at the simply low temperatures provided by liquid nitrogen (77 K). Though there are other factors like ability to be made into wires and how much current they can carry before switching out of superconductivity you might be having in mind with your single Kelvins number.